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Context Predicting the nitrogen (N) mineralisation from soil organic matter is a key aid to fertiliser decision-making and improving the N fertiliser use efficiency of a crop. Aims and methods Field experiments were conducted to assess the amount of inorganic N derived from soil organic matter mineralisation over two seasons (2017–2018 and 2018–2019) across treatments differing in irrigation frequency and amount. During both seasons, the plant line soil in each treatment was sequentially sampled at each irrigation event. Key results There was an effect of the soil water deficit on the measured accumulated soil inorganic N derived from mineralisation in both measurement years. It was observed that soil inorganic N accumulated in the plant line rather than in other hillside and furrow positions for all soil moisture deficit treatments in both years. In 2017–2018, N accumulated in the plant was significantly greater than the measured accumulated inorganic N (0–300 mm). Conclusions and implications The sequential soil sampling approach was challenging in irrigated systems and we propose a hybrid measurement of pre-season available soil N and/or plant N uptake in nil N fertiliser plots as a means of estimating N derived from soil organic matter mineralisation.

Current estimates of global indirect N2O emissions are based on a relatively small dataset and remain a major source of uncertainly in the global N2O budget. Nitrogen (N)-enriched groundwater from agricultural fields may act as an important source of indirect N2O emissions as it discharges to adjacent watershed areas. During 2015–2017, dissolved N2O concentrations in groundwater were measured and indirect N2O emission factors (EF5g) calculated under three typical high-N land-use types (vineyard, vegetable field and paddy field) in eastern China. The average dissolved N2O concentrations in groundwater were 58.1 ± 40.4, 18.5 ± 11.5 and 0.72 ± 0.27 μg N L−1 for vineyard, vegetable field and paddy field, respectively. The dissolved N2O was over-saturated and was therefore a net source of N2O to the atmosphere. The indirect N2O emission factors (EF5g) of vineyard (0.0091) and vegetable (0.0092) fields were much higher than the current Intergovernmental Panel on Climate Change (IPCC) default value of 0.0025 which indicated that these land-uses may have led to indirect N2O emissions from the underlying groundwater. In contrast, the EF5g of the paddy field (0.0019) was slightly lower than the default EF5g proposed by IPCC (2006) and contributed minimal indirect N2O emissions to the atmosphere. However, the current IPCC method may have overestimated the contribution of groundwater N2O to the global N cycle because it took residual but not initial groundwater NO3−-N concentration into account when calculating EF5g. Therefore, we proposed the adoption of an improved method for calculating the EF5g and compared it to the current IPCC (2006) method using data from the present study and other published data. The results of the comparison showed that the improved method was more scientifically appropriate measurement for calculating EF5g.

The Australian grains industry relies on mineralized nitrogen (N) from soil organic matter and plant residues, but fertilizer N is increasingly needed to optimize yields. Most farmers are guided on N fertilizer requirements by commercial crop advisors. We surveyed (n = 132) and interviewed (n = 11) New South Wales grains advisors to gauge the usage of soil process understanding, soil data and decision support systems (DSSs) when developing N recommendations. Soil moisture at sowing, seasonal forecasts, crop rotation, soil mineral N, financial risk profiles and paddock history were all used to prepare N fertilizer advice, but stored soil moisture was most important. Farmer confidence in soil N testing was low due to high spatial variability. Most advisors calculated N fertilizer required for yields within 10%–15% of crop potential, but clients’ attitude to financial risk guided final N recommendations. Conservative growers preferred a low input system, while more reliable rainfall or greater reliance on stored soil water led growers to apply higher N rates to maximize long-term profits. Advisors preferred “rules-of-thumb,” simple DSSs and knowledge of crop growth, to elaborate DSSs requiring detailed inputs and soil characterization. Few used in-crop N sensing. N decision methodologies need to be updated to account for changes in soil fertility, cropping systems and farming practices. New research is needed to answer practical questions regarding soil N mineralization and N losses associated with alternative N application practices and extreme weather events. Training of new advisors in N processes and DSS use needs to be ongoing.

For many agricultural commodity sectors, efforts to meet international obligations regarding emissions reduction are increasing. The Australian cotton industry has already made advances in this regard and the high yields associated with Australian cotton production dramatically minimise greenhouse gas emissions intensity. However, certainty about the quantum of greenhouse gas emissions and the relative contribution of different components of the emissions profile is somewhat unclear; and opportunities for farm-level practice change have not been fully explored. The objectives of this paper were to, (a) build a robust greenhouse gas emissions profile for the product-chain of Australian cotton fibre (lint) grown in northwest New South Wales, (b) using Life Cycle Assessment (LCA), compare the relative contributions of the different industries involved in the product-chain of cotton fibre (e.g. fertiliser producers, cotton farmers, cotton ginning plants) and (c) assess the effects of an array of on-farm mitigation options. Testing the various management options provides information for growers and policy-makers about the relative emissions reduction benefits possible. Additionally, we compared results of previous Australian cotton production studies using similar assumptions and extrapolated the emissions profile to a national scale, with the intention of informing commodity and carbon markets. The foreground data for the study were for the three production seasons from 2011-2012 to 2013–2014 in northwest New South Wales, with a functional unit of one tonne of cotton lint at port. We also drew upon published data, survey data, scientific literature and Australian and international databases. To ensure consistency between our approach and that applied to meet international emissions reporting obligations, we applied emissions formulae and factors from the Australian National Inventory Report, except where more specific published data were available. We assumed that 96% of production was from irrigated systems, with 85% of irrigation water pumped by diesel-powered irrigation pumps and a median irrigated yield of 10.3 bales per ha. We tested the sensitivity of the resulting emissions profile to a wide array of enterprise assumptions and calculation variables. The climate change impact of cotton lint on a cradle-to-port basis was 1601 kg CO2e per tonne of cotton lint. The ‘hot-spots’ within the emissions profile included the production and use of synthetic nitrogen (N) fertilisers (46% of emissions), the production and use of electricity and diesel used for irrigation (10%) and the production and use of diesel for farm machinery (9%). Farm level management options with potential to minimise life cycle GHG emissions were: reducing N fertiliser rate from a commercial rate of 255 kg N/ha to 240 kg N/ha or 180 kg N/ha (2.6% and 13.2% emissions reduction); use of controlled-release and stabilised N fertilisers (5.9% reduction), changing from diesel to solar-powered irrigation pumps (8.1% reduction), changing from diesel to biofuel-powered farm machinery (3.4% reduction), changing from continuous cotton to a cotton-legume crop rotation (3.9% reduction) and use of N fertigation (2.1% reduction). Whilst we focused on farm-level mitigation strategies, these changes were placed in the context of the cradle-to-gate system, to account for associated changes in pre-farm and post-farm emissions.

Reactive N in the runoff from paddy fields is considered to be the dominant contributor to the deterioration of river and lake water quality in China. However, there has been little researches on the interaction between N fertilizer rates and N losses via runoff from the paddy water during the summer rice-growing seasons. In the present study, we monitored the rice grain yield, N agronomic efficiency (AEN), and N losses via runoff from fertilizer N rate-response experiments applied to rice in the Taihu Lake region over 6 years. Total N loss via runoff averaged 19.3 kg ha−1 per rice season, corresponding to 7.1% of fertilizer N input at the regional N rate of 270 kg ha−1. Seasonal cumulative ammonium-N, organic N, and total N losses via runoff were all positively correlated with the fertilizer N rates (n = 1141, P

Black Vertosols of subtropical Australia emit large amounts of nitrous oxide (N2O) to the atmosphere under fertilizer-applied grain cropping compared to other Australian cropping soils. N2O emissions can be mitigated by either reducing fertilizer N inputs or altering crop rotation systems. In this study, the WNMM agroecosystem model was used to investigate the responses of N2O emissions from four different crop rotation systems including canola-wheat-barley (T1CaWB), chickpea-wheat-barley (T3CpWB), chickpea-wheat-chickpea (T4CpWCp), and chickpea-Sorghum (T5CpS) under projected future climate change scenarios on a black Vertosol at Tamworth, New South Wales, Australia. In simulations of the twenty-first century under four different scenarios for atmospheric greenhouse gas concentrations, the annual N2O emissions from the four cropping systems increased with greenhouse gas forcing of the climate. The annual N2O emissions from T4CpWCp (with no fertilizer N application) were the most sensitive to climate change, with 14.3–61.9% increase compared with historic simulations of 1952–2014. The simulated T5CpS treatment (with a long fallow) kept the gross margin-scaled N2O emissions below 1 g N per Australian dollar under all climate change scenarios. This suggests that the inclusion of a long fallow in a crop rotation system can slow down the pace of increasing gross margin-scaled N2O emissions in response to climate change. Our simulation results also imply that legume rotations as mitigation options on N2O emissions may not be resilient to the future changing climate even though they can greatly reduce N2O emissions under the current climate.

The impacts of soil properties and urea fertigation on nitrous oxide (N2 O) emissions from uncropped areas of furrow-irrigated Vertisol paddocks are unknown. We sampled soils from the head-ditch end (upslope) and sediments from the tail-drain end (downslope) of 10 Vertisols under irrigated cotton (Gossypium hirsutum L.) production in northeastern Australia. Four replicates of each sample were incubated in open-top polyvinyl chloride (PVC) chambers at 25 ± 2°C for 25 d. Nitrous oxide emissions were measured periodically after simulated irrigations on Days 0 and 15 with either water or, for soils, urea solution. Compared with the soils, sediments were enriched in silt, total organic carbon (TOC), total nitrogen (TN), ammonium N, and dissolved organic C (DOC) but had lower pH and sand content. Sediments emitted more N2 O than soils from the same paddocks after water irrigations. Nitrous oxide fluxes varied by two orders of magnitude between paddocks, with most variation explained by baseline nitrate N, TOC, pH (inversely), and sand content. Urea solution applied to soils at 30 kg N ha-1 irrigation-1 increased N2 O emitted, but more so after the second irrigation. In irrigated cotton systems, tail-drain sediments are a potential hotspot for N2 O emissions that has not previously been documented.

Straw-return methods that neither negatively impact yield nor bring environmental risk are ideal patterns. To attain this goal, it is necessary to conduct field observation to evaluate the environmental influence of different straw-return methods. Therefore, we conducted a 2-year field study in 2015–2017 to investigate the emissions of methane (CH4) and nitrous oxide (N2O) and the changes in topsoil (0–20 cm) organic carbon (SOC) density in a typical Chinese rice-wheat rotation in the Eastern China. These measurements allowed a complete greenhouse gas accounting (net GWP and GHGI) of five treatments including: FP (no straw, plus fertilizer), FS (wheat straw plus fertilizer), FB (straw-derived biochar plus fertilizer), FSDI (wheat straw with straw-decomposing microbial inoculants plus fertilizer) and CK (control: no straw, no fertilizer). Average annual SOC sequestration rates were estimated to be 0.20, 0.97, 1.97 and 1.87 t C ha−1 yr−1 (0–20 cm) for the FP, FS, FB and FSDI treatments respectively. Relative to the FP treatment, the FS and FSDI treatments increased CH4 emissions by 12.4 and 17.9% respectively, but decreased N2O emissions by 19.1 and 26.6%. Conversely, the FB treatment decreased CH4 emission by 7.2% and increased N2O emission by 10.9% compared to FP. FB increased grain yield, but FS and FSDI did not. Compared to the net GWP (11.6 t CO2-eq ha−1 yr−1) and GHGI (1.20 kg CO2-eq kg−1 grain) of FP, the FS, FB and FSDI treatments reduced net GWP by 12.6, 59.9 and 34.6% and GHGI by 10.5, 65.8 and 37.7% respectively. In rice-wheat systems of eastern China, the environmentally beneficial effects of returning wheat straw can be greatly enhanced by application of straw-decomposing microbial inoculants or by applying straw-derived biochar.

Fifty years of sustained investment in research and development has left the Australian cotton industry well placed to manage nitrogen (N) fertiliser. The average production in the Australian cotton industry today is greater than two tonnes of lint per hectare due to improved plant genetics and crop management. However, this average yield is well below the yield that would be expected from the amount of N fertiliser used. It is clear from the recent studies that across all growing regions, conversion of fertiliser N into lint is not uniformly occurring at application rates greater than 200–240 kg·hm− 2 of N. This indicates that factors other than N availability are limiting yield, and that the observed nitrogen fertiliser use efficiency (NFUE) values may be caused by subsoil constraints such as sodicity and compaction. There is a need to investigate the impact of subsoil constraints on yield and NFUE. Gains in NFUE will be made through improved N fertiliser application timing, better targeting the amount of fertiliser applied for the expected yield, and improved soil N management. There is also a need to improve the ability and confidence of growers to estimate the contribution of soil N mineralisation to the crop N budget. Many Australian studies including data that could theoretically be collated in a meta-analysis suggest relative NFUE values as a function of irrigation technique; however, with the extensive list of uncontrolled variables and few studies using non-furrow irrigation, this would be a poor substitute for a single field-based study directly measuring their efficacies. In irrigated cotton, a re-examination of optimal NFUE is due because of the availability of new varieties and the potential management and long-term soil resilience implications of the continued removal of mineralised soil N suggested by high NFUE values. NFUE critical limits still need to be derived for dryland systems.

Soil emissions of nitrous oxide (N2O) are generally low in Australian semi-arid cropping systems, but can be reduced further by incorporating legumes into cereal-based rotations. We used automated and manual chambers to compare N2O emissions throughout a two-year legume-wheat field experiment. Two pulse crops [lupin (Lupinus angustifolius L.) and field pea (Pisum sativum L.)] and two forage legumes [vetch (Vicia sativa L.) and clover (a mixture of four Trifolium spp.)] were grown in the first year, followed by a wheat (Triticum aestivum L.) crop in the second year without additional nitrogen (N) fertilizer under either no-till or tillage systems. All legume crops were either (a) chemically terminated at anthesis (brown-manured, BM), or (b) cut for hay (forage legumes) or harvested for grain (pulses) (product-removed, PR). The fate of legume N in the BM treatment was traced using 15N labelled urea applied to soil in micro-plots. Results showed that N2O emissions during the legume and wheat phases were mostly unaffected by legume species, the exception being lower emissions from the BM clover pasture under the no-till treatment. Under the PR treatment, tillage tended to increase N2O emissions compared with the no-till treatment during both legume (121 vs. 104 g N2O-N ha−1 year−1) and wheat phases (91 vs. 79 g N2O-N ha−1 year−1). In contrast, a mixed result was found under the BM treatment with a significant tillage × crop management interaction during the legume phase, but no effect of tillage during the wheat phase. In general, the BM treatment promoted more N2O emissions due to more N input into soil compared to the PR treatment, with the impact extending into the subsequent wheat crop. The N2O emissions from the BM treatment during the legume (195 g N2O-N ha−1 year−1) and wheat phase (181 g N2O-N ha−1 year−1) were greater than those from the PR treatment for the corresponding phases (113 vs. 85 g N2O-N ha−1 year−1). The 15N study showed that the majority of legume-derived N was retained in the soil at the end of the legume phase and likely to be readily available for the subsequent cereal crop. The mitigation of N2O emissions derived from legumes in a semi-arid cereal-based cropping system can be optimized in a no-till system where product removal is practiced (cut for hay or harvested for grain) with legume species choice having little or no additional impact.

Ammonia (NH3) volatilization from fertilised agricultural soils is driven by complex interactions between edaphic, climatic and plant canopy factors that can be difficult to measure or predict. We developed a simplified approach using default parameters in the DairyMod model to predict daily NH3 volatilization from urea applied to grazed dairy pastures. Several published datasets were used to validate the reliability of the model to reproduce key related soil processes in a whole farming systems framework. For the sites where monitoring for the experimental duration occurred, DairyMod simulated the main features of the observed NH3 emissions, with an overall predicted median of 4.1 kg/ha or 7% of applied N, compared to the measured median of 6.1 kg/ha for 12% of applied N fertilizer. There was an overall root mean square error (RMSE) of 0.9 kg NH3 N/ha/d and an overall mean prediction error (MPE) of 0.5 kg NH3 N /ha/d. However, there was high uncertainty in several of the datasets used which made it difficult to be conclusive about the validation. The simulation accuracy was improved using daily wind speed (collected on-site in field campaigns) as input to the evapotranspiration calculations. In cases of high certainty in the volatilization data, it was concluded that the model was useful for the analysis of N cycling in situations used for dairy farming without the need for a more complex mechanistic method with difficult-to-obtain parameters. DairyMod presents a simple but readily reproducible prediction of NH3 volatilization from urea application on pasture in intensive livestock farming systems compatible with the certainty of the model inputs and scale of model application. However, the collective understanding of NH3 volatilization in pasture based dairy systems is currently based on a limited number of often uncertain, short term plot studies in the absence of animals.

Current estimates of global indirect N

Inland aquaculture ponds in China collectively cover 2.57 million ha, so emissions of the greenhouse gases methane (CH4) and nitrous oxide (N2O) from these ponds may constitute a significant contribution to global warming. During 2016 and 2017, CH4 and N2O fluxes and a range of pond-water and sediment properties were measured in replicated (n = 4) “mixed-fish” and “crab” aquaculture ponds in southeast China. Annual CH4 and N2O emissions were 64.4 kg C ha−1 and 2.99 kg N ha−1, respectively, from the “mixed-fish” ponds, and 51.6 kg C ha−1 and 3.32 kg N ha−1, respectively, from the “crab” ponds. Emission differences between pond types were significant (p

Introducing nitrogen (N)-fixing legumes into cereal-based crop rotations reduces synthetic fertiliser-N use and may mitigate soil emissions of nitrous oxide (N2O). Current IPCC calculations assume 100% of legume biomass N as the anthropogenic N input and use 1% of this as an emission factor (EF)—the percentage of input N emitted as N2O. However, legumes also utilise soil inorganic N, so legume-fixed N is typically less than 100% of legume biomass N. In two field experiments, we measured soil N2O emissions from a black Vertosol in sub-tropical Australia for 12 months after sowing of chickpea (Cicer arietinum L.), canola (Brassica napus L.), faba bean (Vicia faba L.), and field pea (Pisum sativum L.). Cumulative N2O emissions from N-fertilised canola (624 g N2O-N ha−1) greatly exceeded those from chickpea (127 g N2O-N ha−1) in Experiment 1. Similarly, N2O emitted from canola (385 g N2O-N ha−1) in Experiment 2 was significantly greater than chickpea (166 g N2O-N ha−1), faba bean (166 g N2O-N ha−1) or field pea (135 g N2O-N ha−1). Highest losses from canola were recorded during the growing season, whereas 75% of the annual N2O losses from the legumes occurred post-harvest. Legume N2-fixation provided 37–43% (chickpea), 54% (field pea) and 64% (faba bean) of total plant biomass N. Using only fixed-N inputs, we calculated EFs for chickpea (0.13–0.31%), field pea (0.18%) and faba bean (0.04%) that were significantly less than N-fertilised canola (0.48–0.78%) (P Inputs of legume-fixed N should be more accurately quantified to properly gauge the potential for legumes to mitigate soil N2O emissions. EF’s from legume crops need to be revised and should include a factor for the proportion of the legume’s N derived from the atmosphere.

This project aimed to identify land uses and soil management practices that have significant associations with soil organic carbon (SOC) stocks (0–0.3 m) in New South Wales (NSW), Australia. The work presented in this paper is based on a one-off survey targeting key land uses and management practices of eastern NSW. Because of the nature of the work, the land uses and management combinations surveyed in different soils and climatic conditions were significantly unbalanced, and separately analyzing associations after breaking the dataset into different land uses may lead to significant increases in Type errors. Therefore, redundancy analysis (RDA) was undertaken to explore the association between explanatory variables (i.e., land uses, soil management, soil properties and environmental variables) and the variation in stocks (mass per unit area) of particulate organic carbon (POC), humic organic carbon (HOC) and resistant organic carbon (ROC) across 780 sites in eastern NSW, south eastern Australia. Results indicated that soil properties, land uses, soil management and environmental variables together could explain 52% of total variation in stocks of the SOC fractions. Specifically soil properties and environmental variables explained 42.8%, whereas land uses and management practices together explained 9.2% of the total variation in SOC fractions. A forward selection RDA was also undertaken considering soil properties and environmental variables as covariates to assess the statistical significance of land uses and management practices on stocks of POC, HOC and ROC. We found that pasture had significant positive associations on stocks of carbon fractions. Among the soil properties and environmental variables rainfall, longitude and elevation had a significant positive influence while pH and bulk density had a significantly negative influence on the HOC, POC and ROC stocks. Using a novel multivariate technique, the current work identified the land uses and soil management that had significant impact on carbon stocks in soil after accounting for influences soil properties and environmental variables.

Limited information exists on potential impacts of climate change on nitrous oxide (N2O) emissions by including N2-fixing legumes in crop rotations from rain-fed cropping systems. Data from two 3-yr crop rotations in northern NSW, Australia, viz. chickpea-wheat-barley (CpWB) and canola-wheat-barley (CaWB), were used to gain an insight on the role of legumes in mitigation of N2O emissions. High-frequency N2O fluxes measured with an automated system of static chambers were utilized to test the applicability of Denitrification and Decomposition model. The DNDC model was run using the on-site observed weather, soil and farming management conditions as well as the representative concentration pathways adopted by the Intergovernmental Panel on Climate Change in its Fifth Assessment Report. The DNDC model captured the cumulative N2O emissions with variations falling within the deviation ranges of observations (0.88±0.31kgNha-1rotation-1 for CpWB, 1.44±0.02kgNha-1rotation-1 for CaWB). The DNDC model can be used to predict between modeled and measured N2O flux values for CpWB (n=390, RSR=0.45) and CaWB (n=390, RSR=0.51). Long-term (80-yr) simulations were conducted with RCP 4.5 representing a global greenhouse gas stabilization scenario, as well RCP 8.5 representing a very high greenhouse gas emission scenario based on RCP scenarios. Compared with the baseline scenarios for CpWB and CaWB, the long-term simulation results under RCP scenarios showed that, (1) N2O emissions would increase by 35-44% for CpWB and 72-76% for CaWB under two climate scenarios; (2) grain yields would increase by 9% and 18% under RCP 4.5, and 2% and 14% under RCP 8.5 for CpWB and CaWB, respectively; and (3) yield-scaled N2O-N emission would increase by 24-42% for CpWB and 46-54% for CaWB under climate scenarios, respectively. Our results suggest that 25% of the yield-scaled N2O-N emission would be saved by switching to a legume rotation under climate change conditions.

Most soil nitrous oxide (N2O) emissions from rain-fed grain sorghum grown on sub-tropical Vertosols in north-west New South Wales, Australia, occur between fertiliser nitrogen (N) application at sowing and booting growth stage. At three experiments, we investigated the potential for deferring some (split-N) or all (delayed) fertiliser N until booting to mitigate N2O produced without compromising optimum crop yields. N products included urea, 3,4-dimethyl pyrazole phosphate (DMPP)-urea, polymer-coated urea (PCU) and N-(n-butyl)thiophosphoric triamide (NBPT)-urea. For a fourth experiment, the N fertiliser rate was varied according to pre-sowing soil mineral N stocks left by different previous crops. All experiments incorporated 15N mini-plots to determine whether delayed or split-N affected crop N uptake or residual soil N. Compared to urea applied at-sowing, delayed applications of urea, DMPP-urea or NBPT-urea at booting reduced the N2O emission factor (EF, percentage of applied N emitted) by 67–81%. Crop N uptake, grain yield and protein tended to be lower with delayed N than N at-sowing due to dry mid-season conditions. Much of the unused N remained in the soil at harvest. Split-N (33% sowing:67% booting) using urea, reduced EF by 59% compared to at-sowing urea, but maintained crop N uptake, grain yield and protein. Using DMPP-urea or PCU for the at-sowing portion of the split reduced EF by 84–86%. Grain yield was maintained using PCU, but was lower with DMPP-urea, which had more N in vegetative biomass. Using NBPT-urea for the in-crop portion of the split did not affect N2O emissions or crop productivity. Nitrogen budgeting to account for high pre-sowing soil mineral N nullified urea-induced N2O emissions. An N-budgeted, split-N strategy using urea offers the best balance between N2O mitigation, grain productivity and provision of a soil mineral N buffer against dry mid-season conditions. Split-N using DMPP-urea or PCU further enhanced N2O mitigation but there was no yield response to justify the extra expense.

Delaying the accumulation of soil nitrate from urea applied at sowing should mitigate nitrous oxide (N2O) emissions without compromising optimum crop production. This delay may be achieved chemically using a nitrification inhibitor such as 3,4 dimethylpyrazole phosphate (DMPP), or physically by coating urea with a degradable polymer (PCU). In five field experiments across three summers, the impact of DMPP-coated urea applied at sowing on soil mineral nitrogen (N), N2O emissions and yields of grain sorghum or sunflower grown on sub-tropical Vertosols was assessed. At two experiments, DMPP effects on plant N uptake, soil N movement and total N loss were determined with 15N. One experiment included PCU and several blends: urea+DMPP-urea; urea+PCU; urea+DMPP-urea+PCU. Averaged across all experiments, DMPP reduced cumulative N2O emitted by 92% (range: 65–123%) and N2O emission factor (EF: percent of applied N emitted) by 88%. There was no statistical difference in N2O emitted between the 0N control and DMPP-urea. PCU reduced N2O emitted by 27% and EF by 34%. The urea+DMPP-urea blend also nullified urea-induced N2O, but urea+PCU increased N2O emissions and decreased grain yield due to a mismatch between soil N availability and plant N demand. DMPP arrested 15N movement in soil and reduced total 15N loss from 35% to 15% at one of the two 15N experiments. Applying DMPP-urea at sowing is an effective N strategy that nullifies urea-induced N2O emissions, maintains crop yield, and retains N in the soil–plant system. Negative impacts of the PCU+urea blend highlight the influence of growing season conditions on fertiliser N release.

Few studies have compared emissions of nitrous oxide (N2O), the potent greenhouse gas associated with decomposition of both below-ground (root) and above-ground (shoot) residues. We report a laboratory incubation experiment to evaluate effects of root and shoot residues from wheat, canola, soybean, and sorghum, incorporated into a naturally fertile acidic Black Vertisol, on N2O and carbon dioxide (CO2) emissions. The residue-amended Vertisol samples were incubated at 25 °C and 70 % water-filled pore space (WFPS) to facilitate denitrification activity for a total period of 56 days. The incubated soils were periodically sampled for N2O, CO2, mineral N, and dissolved organic carbon (DOC). In general, shoot residues emitted more CO2 than roots, while N2O emissions were 50–70 % higher in cereal root residues than those in shoots. Surprisingly, the highest N2O emissions were associated with soils amended with the more inert high C/N ratio residues (wheat and sorghum roots), and to some extent, lowest emissions were associated with low C/N ratio (more labile) residues, particularly during the early stages of incubation (0–22 days). During this stage, there was a significant (p

Australia’s “Direct Action” climate change policy relies on purchasing greenhouse gas abatement from projects undertaking approved abatement activities. Management of soil organic carbon (SOC) in agricultural soils is an approved activity, based on the expectation that land use change can deliver significant changes in SOC. However, there are concerns that climate, topography and soil texture will limit changes in SOC stocks. This work analyses data from 1482 sites surveyed across the major agricultural regions of Eastern Australia to determine the relative importance of land use vs. other drivers of SOC. Variation in land use explained only 1.4% of the total variation in SOC, with aridity and soil texture the main regulators of SOC stock under different land uses. Results suggest the greatest potential for increasing SOC stocks in Eastern Australian agricultural regions lies in converting from cropping to pasture on heavy textured soils in the humid regions.

Nitrification and urease inhibitors have been used to reduce nitrous oxide (N2O) emissions and increase nitrogen use efficiency in many agricultural systems. However, their agronomic benefits, such as the improvement of grain yield, is uncertain. A two-year field experiment was conducted to (1) investigate whether the use of 3,4-dimethylpyrazole phosphate (DMPP) or N-(n-butyl) thiophosphoric triamide (NBPT) can reduce N2O emissions and increase grain yield and (2) explore the financial benefit of using DMPP or NBPT in a rain-fed cropping system in south-eastern Australia. The experiment was conducted at Wagga Wagga, New South Wales, Australia with wheat (Triticum aestivum L.) in 2012 and canola (Brassica napus L.) in 2013. Results showed that urea coated with DMPP reduced the cumulative N2O emission by 34% for a wheat crop in 2012 (P

Nitrogen (N) fertiliser inputs for irrigated cotton production are rapidly increasing to support ever-increasing yields, but much of the applied N may be lost as N gases, including nitrous oxide (N2O), via denitrification in medium–heavy clay soils. The addition of a nitrification inhibitor can reduce overall N loss and N2O emissions. Currently, nitrapyrin (2-chloro-6-trichloro methyl pyridine) is the only inhibitor used with anhydrous ammonia (AA), whereas 3,4-dimethyl pyrazole phosphate (DMPP) has potentially greater stability and longevity in soil, but is not compatible with AA. A newly-developed formulation based on DMPP, 3,4-dimethyl pyrazole tetra-methylene sulfone (DMPS), can be direct-injected with AA. We compared N2O emissions from DMPS- and nitrapyrin-treated AA from two Vertosols used for irrigated cotton. At Emerald (Queensland), both inhibitors reduced N2O emitted by 77% over 2 months. At Gunnedah (New South Wales), DMPS was active in the soil for 3 months, reducing N2O by 86%, whereas nitrapyrin activity lasted for 2 months and reduced N2O by 65%. Realising the potential for improved environmental benefits from directly injecting DMPS with AA requires an agronomic benefit justifying its additional cost to the farmer. Future research needs to investigate the potential for reduced N rates when using these inhibitors – without compromising high yields.

To contribute to national greenhouse gas emissions (GHG) reduction targets, grain growers need strategies that minimise emissions associated with grain production. We used life cycle assessments (LCAs) with field-measured production inputs, grain yields and proteins, legume nitrogen (N2) fixation, and soil nitrous oxide (N2O) and methane (CH4) emissions, to explore mitigation strategies in 3-year crop sequences in subtropical Australia. The sequences were: canola plus 80 kg/ha fertiliser nitrogen (80N)–wheat 85N–barley 65N (CaNWtNBaN), chickpea 0N–wheat 85N–barley 5N (CpWtNBa), chickpea 0N–wheat 5N–chickpea 5N (CpWtCp), and chickpea 0N–sorghum 45N (CpSgN). We also assessed the impacts of split fertiliser N application and urea coated with DMPP, a nitrification inhibitor, on the LCA for the CaNWtNBaN sequence. Total pre-farm plus on-farm GHG emissions varied between 915 CO2-e/ha (CpSgN) and 1890 CO2-e/ha (CaNWtNBaN). Cumulative N2O emitted over the 3-year study varied between 0.479 kg N2O-N/ha (CpWtCp) and 1.400 kg N2O-N/ha (CaNWtNBaN), which constituted 24–44% of total GHG emissions. Fertiliser production accounted for 20% (CpSgN) to 30% (CaNWtNBaN) of total emissions. An extra 4.7 kg CO2-e/ha was emitted for each additional kg N/ha of applied N fertiliser. Three-year CH4 emissions ranged from −1.04 to −0.98 kg CH4-C/ha. Split N and DMPP strategies could reduce total GHG emissions of CaNWtNBaN by 17 and 28% respectively. Results of the study indicate considerable scope for reducing the carbon footprint of subtropical, dryland grains cropping in Australia.

Nitrogen balances, i.e. the difference between N2 fixation inputs and N in harvested products (outputs), and rotational benefits of chickpea (Cicer arietinum) on soil organic fertility and wheat (Triticum aestivum) yields were quantified for rain-fed systems in the northern Punjab, Pakistan. The experiments were conducted during 1995–2000 at three sites. The four treatments were continuous wheat (0 N), continuous wheat (+N), chickpea-wheat (0 N) and chickpea-wheat (+N). The +N fertiliser rate was 100 kg N ha-1 applied to the wheat. Grain yields of the wheat with 0 N varied in the range 1.0–3.0 t ha-1, compared with 2.0–3.2 t ha-1 for the N-fertilised wheat. Chickpea grain yields were in the range 0.6–2.0 t ha-1. Chickpea N2 fixation was assessed using the natural 15N abundance method. Percentage of chickpea N derived from N2 fixation (%Ndfa) estimates were 58% (Mandra), 65% (Taxila) and 86% (Islamabad). The overall mean %Ndfa was 78%. Crop N fixed by the chickpea varied between sites (87–186 kg N ha-1) and essentially reflected crop biomass. The overall mean N balance for chickpea (crop N fixed minus N removed in grain and above-ground residues) was +28 kg N ha-1. Wheat grain yields responded to chickpea (19–73% increase for the three sites), to fertiliser N (99–136% increase) and to the combination of chickpea and fertiliser N (106–145% increase). Chickpea in the rotation increased soil organic C by 30% and soil N by 38%, relative to the continuous wheat with 0 N. These experiments indicated that chickpea could have a positive N balance, even when shoot residues were removed, and confirmed the rotational benefits of chickpea for improving soil organic fertility and yield of a following wheat crop.

Improved management of nitrogen (N) in low N soils is critical for increased land productivity and economic sustainability. We report results of a rainfed rotation experiment, conducted in the Northwest Frontier Province (NWFP), Pakistan, during 1995–1999 to evaluate effects of residue retention and fertiliser N on N2 fixation inputs and yields of a mungbean (Vigna radiata)–wheat (Triticum aestivum) sequence, and a lentil (Lens culinaris)–summer cereal sequence. Mungbean and sorghum (Sorghum bicolor) or maize (Zea mays) were grown in the summers and lentil and wheat in the winters. Immediately after grain harvest, above-ground residues of all crops were either completely removed (−residue), or chopped into 5–20 cm pieces, spread across the plots and incorporated by chisel plough (+residue). Fertiliser N rates were nil or 120 kg N/ha for wheat and nil or 150 kg N/ha for sorghum/maize. The percentage of mungbean N derived from N2 fixation (%Ndfa) ranged from 47% to almost 100% (mean of 75%). On average, mungbean fixed 112 kg N/ha (+residues) and 74 kg N/ha (−residues), with N balances of +64 kg N/ha (+residues) and +9 kg N/ha (−residues). Lentil %Ndfa ranged from 50 to 87% (mean of 73%). Values for crop N fixed were 42–85 kg N/ha, with a mean of 68 kg N/ha. Average N balances for lentil were +27 kg N/ha (+residues) and +16 kg N/ha (−residues). Grain yields of the 0N wheat responded to the previous mungbean (36% increase over the 0N sorghum), but showed an even greater response to fertiliser N applied to the previous sorghum (150% increase). Highest yields were recorded for the N-fertilised wheat (average of 2.27 t/ha). Shoot biomass yields of the 0N sorghum and maize responded strongly to the previous lentil crop (49% average increase over the 0N wheat) and fertiliser N, applied either to the crop itself (140%) or to the previous wheat crop (32%). Residue retention increased shoot biomass yields of both the summer (average of 20%) and winter crops (average of 9%). Grain yield benefits of residues were 13% for mungbean, and 8% for wheat and lentil. Soil organic N and total organic C, labile C and C management index (CMI), were all increased by N inputs, from both fertiliser and N2 fixation, and by retention of residues We concluded that retention of residues improves the N economy of the cropping system and enhances crop productivity through the additional N and other soil effects. The question of whether farmers who traditionally remove residues for fodder and fuel would change practices and return the residues to the soil will depend to a large extent on the relative profitability of both options.

On-farm and experimental measures of the proportion (%Ndfa) and amounts of N2 fixed were undertaken for 158 pastures either based on annual legume species (annual medics, clovers or vetch), or lucerne (alfalfa), and 170 winter pulse crops (chickpea, faba bean, field pea, lentil, lupin) over a 1200 km north-south transect of eastern Australia. The average annual amounts of N2 fixed ranged from 30 to 160 kg shoot N fixed ha−1 yr−1 for annual pasture species, 37–128 kg N ha−1 yr−1 for lucerne, and 14 to 160 kg N ha−1 yr−1 by pulses. These data have provided new insights into differences in factors controlling N2 fixation in the main agricultural systems. Mean levels of %Ndfa were uniformly high (65–94%) for legumes growing at different locations under dryland (rainfed) conditions in the winter-dominant rainfall areas of the cereal-livestock belt of Victoria and southern New South Wales, and under irrigation in the main cotton-growing areas of northern New South Wales. Consequently N2 fixation was primarily regulated by biomass production in these areas and both pasture and crop legumes fixed between 20 and 25 kg shoot N for every tonne of shoot dry matter (DM) produced. Nitrogen fixation by legumes in the dryland systems of the summer-dominant rainfall regions of central and northern New South Wales on the other hand was greatly influenced by large variations in %Ndfa (0–81%) caused by yearly fluctuations in growing season (April–October) rainfall and common farmer practice which resulted in a build up of soil mineral-N prior to sowing. The net result was a lower average reliance of legumes upon N2 fixation for growth (19–74%) and more variable relationships between N2 fixation and DM accumulation (9–16 kg shoot N fixed/t legume DM). Although pulses often fixed more N than pastures, legume-dominant pastures provided greater net inputs of fixed N, since a much larger fraction of the total plant N was removed when pulses were harvested for grain than was estimated to be removed or lost from grazed pastures. Conclusions about the relative size of the contributions of fixed N to the N-economies of the different farming systems depended upon the inclusion or omission of an estimate of fixed N associated with the nodulated roots. The net amounts of fixed N remaining after each year of either legume-based pasture or pulse crop were calculated to be sufficient to balance the N removed by at least one subsequent non-legume crop only when below-ground N components were included. This has important implications for the interpretation of the results of previous N2 fixation studies undertaken in Australia and elsewhere in the world, which have either ignored or underestimated the N present in the nodulated root when evaluating the contributions of fixed N to rotations.

The northern Australian grains industry relies on nitrogen (N) fertiliser to optimise yield and protein, but N fertiliser can increase soil fluxes of nitrous oxide (N2O) and methane (CH4). We measured soil N2O and CH4 fluxes associated with wheat (Triticum aestivum) and barley (Hordeum vulgare) using automated (Expts 1, 3) and manual chambers (Expts 2, 4, 5). Experiments were conducted on subtropical Vertosol soils fertilised with N rates of 0–160kgNha–1. In Expt 1 (2010), intense rainfall for a month before and after sowing elevated N2O emissions from N-fertilised (80kgNha–1) wheat, with 417gN2O-Nha–1 emitted compared with 80g N2O-Nha–1 for non-fertilised wheat. Once crop N uptake reduced soil mineral N, there was no further treatment difference in N2O. Expt 2 (2010) showed similar results, however, the reduced sampling frequency using manual chambers gave a lower cumulative N2O. By contrast, very low rainfall before and for several months after sowing Expt 3 (2011) resulted in no difference in N2O emissions between N-fertilised and non-fertilised barley. N2O emission factors were 0.42, 0.20 and –0.02 for Expts 1, 2 and 3, respectively. In Expts 4 and 5 (2011), N2O emissions increased with increasing rate of N fertiliser. Emissions were reduced by 45% when the N fertiliser was applied in a 50:50 split between sowing and mid-tillering, or by 70% when urea was applied with the nitrification inhibitor 3,4-dimethylpyrazole-phosphate. Methane fluxes were typically small and mostly negative in all experiments, especially in dry soils. Cumulative CH4 uptake ranged from 242 to 435g CH4-Cha–1year–1, with no effect of N fertiliser treatment. Considered in terms of CO2 equivalents, soil CH4 uptake offset 8–56% of soil N2O emissions, with larger offsets occurring in non-N-fertilised soils. The first few months from N fertiliser application to the period of rapid crop N uptake pose the main risk for N2O losses from rainfed cereal cropping on subtropical Vertosols, but the realisation of this risk is dependent on rainfall. Strategies that reduce the soil mineral N pool during this time can reduce the risk of N2O loss.

Summer crop production on slow-draining Vertosols in a sub-tropical climate has the potential for large emissions of soil nitrous oxide (N2O) from denitrification of applied nitrogen (N) fertiliser. While it is well established that applying N fertiliser will increase N2O emissions above background levels, previous research in temperate climates has shown that increasing N fertiliser rates can increase N2O emissions linearly, exponentially or not at all. Little such data exists for summer cropping in sub-tropical regions. In four field experiments at two locations across two summers, we assessed the impact of increasing N fertiliser rate on both soil N2O emissions and crop yield of grain sorghum (Sorghum bicolor L.) or sunflower (Helianthus annuus L.) in Vertosols of sub-tropical Australia. Rates of N fertiliser, applied as urea at sowing, included a nil application, an optimum N rate and a double-optimum rate. Daily N2O fluxes ranged from –3.8 to 2734g N2O-Nha–1day–1 and cumulative N2O emissions ranged from 96 to 6659g N2O-Nha–1 during crop growth. Emissions of N2O increased with increased N fertiliser rates at all experimental sites, but the rate of N loss was five times greater in wetter-than-average seasons than in drier conditions. For two of the four experiments, periods of intense rainfall resulted in N2O emission factors (EF, percent of applied N emitted) in the range of 1.2–3.2%. In contrast, the EFs for the two drier experiments were 0.41–0.56% with no effect of N fertiliser rate. Additional 15N mini-plots aimed to determine whether N fertiliser rate affected total N lost from the soil–plant system between sowing and harvest. Total 15N unaccounted was in the range of 28–45% of applied N and was presumed to be emitted as N2O+N2. At the drier site, the ratio of N2 (estimated by difference)to N2O (measured) lost was a constant 43%, whereas the ratio declined from 29% to 12% with increased N fertiliser rate for the wetter experiment. Choosing an N fertiliser rate aimed at optimum crop production mitigates potentially high environmental (N2O) and agronomic (N2+N2O) gaseous N losses from over-application, particularly in seasons with high intensity rainfall occurring soon after fertiliser application.

Nitrous oxide (N2O) emissions from Australian grain cropping systems are highly variable due to the large variations in soil and climate conditions and management practices under which crops are grown. Agricultural soils contribute 55% of national N2O emissions, and therefore mitigation of these emissions is important. In the present study, we explored N2O emissions, yield and emissions intensity in a range of management practices in grain crops across eastern Australia with the Agricultural Production Systems sIMulator (APSIM). The model was initially evaluated against experiments conducted at six field sites across major grain-growing regions in eastern Australia. Measured yields for all crops used in the experiments (wheat, barley, sorghum, maize, cotton, canola and chickpea) and seasonal N2O emissions were satisfactorily predicted with R2=0.93 and R2=0.91 respectively. As expected, N2O emissions and emissions intensity increased with increasing nitrogen (N) fertiliser input, whereas crop yields increased until a yield plateau was reached at a site- and crop-specific N rate. The mitigation potential of splitting N fertiliser application depended on the climate conditions and was found to be relevant only in the southern grain-growing region, where most rainfall occurs during the cropping season. Growing grain legumes in rotation with cereal crops has great potential to reduce mineral N fertiliser requirements and so N2O emissions. In general, N management strategies that maximise yields and increase N use efficiency showed the greatest promise for N2O mitigation.

Dryland cereal production systems of south-eastern Australia require viable options for reducing nitrous oxide (N2O) emissions without compromising productivity and profitability. A 4-year rotational experiment with wheat (Triticum aestivum L.)–canola (Brassica napus L.)–grain legumes–wheat in sequence was established at Wagga Wagga, NSW, Australia, in a semiarid Mediterranean-type environment where long-term average annual rainfall is 541mm and the incidence of summer rainfall is episodic and unreliable. The objectives of the experiment were to investigate whether (i) tillage increases N2O emissions and (ii) nitrogen (N) application can improve productivity without increasing N2O emissions. The base experimental design for each crop phase was a split-plot design with tillage treatment (tilled versus no-till) as the whole plot, and N fertiliser rate (0, 25, 50 and 100kgN/ha) as the subplot, replicated three times. This paper reports high resolution N2O emission data under a canola crop. The daily N2O emission rate averaged 0.55g N2O-N/ha.day, ranging between –0.81 and 6.71g N2O-N/ha.day. The annual cumulative N2O-N emitted was 175.6 and 224.3g N2O-N/ha under 0 and 100kgN/ha treatments respectively. There was no evidence to support the first hypothesis that tillage increases N2O emissions, a result which may give farmers more confidence to use tillage strategically to manage weeds and diseases where necessary. However, increasing N fertiliser rate tended to increase N2O emissions, but did not increase crop production at this site.

During the 2007 winter cropping season in Australia, severe leaf-spotting (necrosis) symptoms resembling chloride (Cl–) deficiency found in North America were reported in the newly released durum wheat variety Jandaroi. Testing for bacterial, fungal and viral pathogens all proved negative. Four Australian durum and four Australian bread wheat varieties were grown, along with a North American variety of each, in a glasshouse experiment using a sterile sand–vermiculite mix and nutrient solutions containing 0 (nil), 0.5, 1.0 or 2.0 mm Cl–. When grown in the nil Cl– solution, all durum and some bread wheat varieties produced leaf-spotting symptoms the same as observed in the field. Nil Cl– also delayed flowering, reduced biomass, decreased grain size, and depressed grain yield in most durum and bread wheat varieties. In field experiments, additions of Cl– fertiliser as KCl at sowing provided no biomass or yield response from a range of wheat varieties, probably because the plants accessed sufficient Cl– from below 0.9 m depth in the soil. Chloride concentrations in whole-plant tissue sampled at head emergence suggested that unfertilised plants were borderline deficient in Cl– according to critical values established in North America. An in-crop foliar Cl– application experiment showed linear uptake of applied Cl–, as MgCl2, until the end of tillering. However, because leaf-spotting symptoms typically appear only after tillering, it is not possible to correct Cl– deficiency by adding Cl– fertiliser to the affected crop after symptoms appear. Managing Cl– in susceptible crops therefore needs to be preventative rather than curative. Among commercial varieties, Jandaroi was highly sensitive to low Cl–, Caparoi was moderately sensitive, and EGA Bellaroi was tolerant. Several elite durum breeding lines grown in 2010 showed considerably reduced leaf spotting compared with Jandaroi under low Cl– conditions, indicating potential for conventional breeding to reduce the potential impact of low Cl– soils on durum production in northern Australia.

Farmers on Vertosols in the northern grains region of Australia are increasingly using pre-crop broadcasting and in-crop topdressing of nitrogen (N) fertilisers. Surface application risks gaseous loss via ammonia volatilisation, but the magnitude of N loss is unknown. Because both soil properties and environmental conditions influence ammonia volatilisation, measurements need to be field-based and non-intrusive, e.g. micrometeorological. We used an integrated horizontal flux technique to measure ammonia volatilised from neutral to alkaline Vertosols for a month after the application of several fertiliser products in 10 bare-fallow paddocks, seven mid-tillering wheat crops, and two perennial-grass-based pastures. Ammonia loss from urea averaged 11% (5.4–19%) when applied to fallow paddocks, 4.8% (3.1–7.6%) when applied to wheat, and 27% when applied to pasture. Volatilisation from urea applied to pastures was high, because there was little rain after spreading. Losses from ammonium sulfate applied to pastures were >60% less than from urea. Nitrogen losses from ammonium sulfate were high (18.6–33.8%) from soils with >10 g 100 g–1 of calcium carbonate (CaCO3), but were 52% less than from urea at five of eight fallow paddocks on non-calcareous soils, and 76% less than from urea at the two pasture paddocks. Coating urea with N-(n-butyl)thiophosphoric triamide reduced ammonia loss at just two of eight fallow paddocks and one of three in-crop paddocks. Ammonia volatilisation from aqueous solutions of urea, urea ammonium nitrate, and ammonium nitrate were either less than or no different from losses from granulated urea, but not consistent. With the exception of ammonium sulfate applied to soils with >10 g 100 g–1 of CaCO3, surface application of N fertiliser during autumn–winter on cropped Vertosols in the Australian northern grains region does not lead to major N loss via ammonia volatilisation.

Sown tropical perennial grass pastures may be a means to restore soil organic carbon (C) lost by cropping with conventional tillage to the levels originally present in native grass pastures. To assess this, total organic carbon and related soil properties were measured under sown tropical pastures, conventionally cultivated cropping, and native pastures on 75 Chromosols and 70 Vertosols to 0.3 m depth in the New South Wales North-West Slopes and Plains region of Australia. The impact of several perennial pasture species on soil organic carbon was also assessed in a 6-year-old, sown pasture experiment on a previously cropped Chromosol. Soil cores in 0.1-m segments to 0.3 m were analysed for total organic carbon, total nitrogen (N), pH, and phosphorus (Colwell-P). Mid-infrared scans were used to predict the particulate, humus, and resistant fractions of the total organic carbon. Bulk density was used to calculate stocks of C, N, and C fractions. In Chromosols, total organic carbon in the surface 0–0.1 m was greater under sown tropical pastures (23.1 Mg ha–1) than conventional tillage cropping (17.7 Mg ha–1), but still less than under native pastures (26.3 Mg ha–1). Similar land-use differences were seen for particulate and resistant organic C, and total N. The proportional differences between land uses were much greater for particulate organic C than other measures, and were also significant at 0.1–0.2 and 0.2–0.3 m. Subsurface bulk density (0.1–0.2 m) was lower under sown tropical pastures (1.42 Mg m–3) than conventionally tilled cropping (1.52 Mg m–3). For Vertosols, total organic carbon in the surface 0–0.1 m was greater under sown tropical pastures (19.0 Mg ha–1) and native pastures (20.5 Mg ha–1) than conventional tillage cropping (14.0 Mg ha–1). Similar land-use effects were seen for the particulate and humus organic C fractions, and total N. In the sown pasture species experiment, there was no significant difference in total N, total organic carbon, or any C fraction between soils under a native-grass species mixture, two improved tropical grass species, or a perennial pasture legume. Regular monitoring is required to better discern whether gradual changes are being masked by spatial and temporal variation. The survey results support previous research on Vertosols within the New South Wales North-West Slopes and Plains that show sown tropical grass pastures can improve total organic carbon. Improvements in total organic carbon on Chromosols have not previously been documented, so further targeted soil monitoring and experimentation is warranted for the region.

Reduced carbon stock levels in Australian soil due to cropping provide a significant opportunity for carbon sequestration, and the recent initiative to consider soil carbon in domestic emissions trading requires a scientific assessment of soil carbon levels under a range of cropping soil management practices. Some of the previous research in southern and western New South Wales (NSW) showed that the rate of carbon decline in cropping soils is slowed under minimum tillage when the stubble is also retained. However, such comparison is rare in the NSW North-West Slopes and Plains region, particularly on the red soils (Chromosols) which are one of the major soil types in the region. We surveyed 50 dryland Chromosols, 72 dryland Vertosols, and 25 irrigated Vertosols on commercial farms across this region to examine the effects of conventional tillage, minimum tillage, and irrigation on total soil organic carbon. Samples of 0.1 m segments to 0.3 m depth were analysed for total organic carbon and other soil properties. Mid-infrared scans were used to predict the particulate, humus, and resistant soil organic carbon fractions. Bulk density was used to calculate total organic carbon stock for each segment, and equivalent soil mass (ESM) for 0–0.3 m. In Vertosols, for 0–0.3 m ESM, total organic carbon and particulate organic carbon were not different between management practices, whereas humic organic carbon and resistant organic carbon were consistently lower under conventional tillage. However, in 0–0.1 m, total organic carbon was greater under minimum tillage (15.2 Mg ha–1) than conventional tillage (11.9 Mg ha–1) or irrigation (12.0 Mg ha–1), reflecting less soil surface disturbance under minimum tillage. In Chromosols, only total organic carbon was higher under minimum tillage than conventional tillage in the 0–0.3 m ESM (39.8 v. 33.5 Mg ha–1) and in 0–0.1 m (19.7 v. 16.9 Mg ha–1). The strong influences of rainfall, temperature, bulk density, texture, and management history on soil carbon stocks suggested that these environmental and management factors require further consideration when gauging soil carbon sequestration potential under current and novel tillage practices in key regional locations.

Life Cycle Assessment (LCA) has become an increasingly common approach across different industries, including agriculture, for environmental impact assessment. A single-issue LCA focusing on greenhouse gas emissions was conducted to determine the emissions profile and total carbon footprint of wheat produced in the Central Zone (East) of New South Wales. Greenhouse gas emissions (in carbon dioxide equivalents; CO2-e) from all stages of the production process, both pre-farm and on-farm, were included. Total emissions were found to be 200 kg CO2-e per t of wheat at the farm gate, based on a 3.5 t/ha grain yield. The relative contribution of greenhouse gas emissions from different components of the production system was determined, with most emissions (37%) coming from pre-farm production and transport of fertiliser (30%) and lime (7%) and from the nitrous oxide (N2O) emitted from the nitrogenous fertiliser applied to the crop (26%). Other important emissions included the CO2 emissions from the use of fertiliser and lime (15%) and the production, transport and use of diesel (16%). The relative importance of other minor emissions is also discussed. For a higher yielding crop (5.0 t/ha), total emissions were found to be 150 kg CO2-e per t of wheat. This paper considers the effect of different management scenarios on the emissions profile and the effect of adopting a N2O emissions factor, which is based on current New South Wales field research, rather than the current Australian National Greenhouse Accounts National Inventory Report default value. This LCA provides a template from which comparative farming systems LCA can be developed and provides data for the Australian Life Cycle Inventory.

Productivity of grain crops grown under dryland conditions in north-eastern Australia depends on efficient use of rainfall and available soil moisture accumulated in the period preceding sowing. However, adverse subsoil conditions including high salinity, sodicity, nutrient imbalances, acidity, alkalinity, and high concentrations of chloride (Cl) and sodium (Na) in many soils of the region restrict ability of crop roots to access this stored water and nutrients. Planning for sustainable cropping systems requires identification of the most limiting constraint and understanding its interaction with other biophysical factors. We found that the primary effect of complex and variable combinations of subsoil constraints was to increase the crop lower limit (CLL), thereby reducing plant available water. Among chemical subsoil constraints, subsoil Cl concentration was a more effective indicator of reduced water extraction and reduced grain yields than either salinity or sodicity (ESP). Yield penalty due to high subsoil Cl was seasonally variable, with more in-crop rainfall (ICR) resulting in less negative impact. A conceptual model to determine realistic yield potential in the presence of subsoil Cl was developed from a significant positive linear relationship between CLL and subsoil Cl: Since grid sampling of soil to identify distribution of subsoil Cl, both spatially across landscape and within soil profile, is time-consuming and expensive, we found that electromagnetic induction, coupled with yield mapping and remote sensing of vegetation offers potential to rapidly identify possible subsoil Cl at paddock or farm scale. Plant species and cultivars were evaluated for their adaptations to subsoil Cl. Among winter crops, barley and triticale, followed by bread wheat, were more tolerant of high subsoil Cl concentrations than durum wheat. Chickpea and field pea showed a large decrease in yield with increasing subsoil Cl concentrations and were most sensitive of the crops tested. Cultivars of different winter crops showed minor differences in sensitivity to increasing subsoil Cl concentrations. Water extraction potential of oilseed crops was less affected than cereals with increasing levels of subsoil Cl concentrations. Among summer crops, water extraction potential of millet, mungbean, and sesame appears to be more sensitive to subsoil Cl than that of sorghum and maize; however, the differences were significant only to 0.7 m. Among pasture legumes, lucerne was more tolerant to high subsoil Cl concentrations than the others studied. Surface applied gypsum significantly improved wheat grain yield on soils with ESP >6 in surface soil (0–0.10 m). Subsurface applied gypsum at 0.20–0.30 m depth did not affect grain yield in the first year of application; however, there was a significant increase in grain yield in following years. Better subsoil P and Zn partially alleviated negative impact of high subsoil Cl. Potential savings from improved N fertilisation decisions for paddocks with high subsoil Cl are estimated at ~$AU10 million per annum.

Salinity, sodicity, acidity, and phytotoxic levels of chloride (Cl) in subsoils are major constraints to crop production in many soils of north-eastern Australia because they reduce the ability of crop roots to extract water and nutrients from the soil. The complex interactions and correlations among soil properties result in multi-colinearity between soil properties and crop yield that makes it difficult to determine which constraint is the major limitation. We used ridge-regression analysis to overcome colinearity to evaluate the contribution of soil factors and water supply to the variation in the yields of 5 winter crops on soils with various levels and combinations of subsoil constraints in the region. Subsoil constraints measured were soil Cl, electrical conductivity of the saturation extract (ECse), and exchangeable sodium percentage (ESP). The ridge regression procedure selected several of the variables used in a descriptive model, which included in-crop rainfall, plant-available soil water at sowing in the 0.90–1.10 m soil layer, and soil Cl in the 0.90–1.10 m soil layer, and accounted for 77–85% of the variation in the grain yields of the 5 winter crops. Inclusion of ESP of the top soil (0.0–0.10 m soil layer) marginally increased the descriptive capability of the models for bread wheat, barley and durum wheat. Subsoil Cl concentration was found to be an effective substitute for subsoil water extraction. The estimates of the critical levels of subsoil Cl for a 10% reduction in the grain yield were 492 mg cl/kg for chickpea, 662 mg Cl/kg for durum wheat, 854 mg Cl/kg for bread wheat, 980 mg Cl/kg for canola, and 1012 mg Cl/kg for barley, thus suggesting that chickpea and durum wheat were more sensitive to subsoil Cl than bread wheat, barley, and canola.

Soils with high levels of chloride and/or sodium in their subsurface layers are often referred to as having subsoil constraints (SSCs). There is growing evidence that SSCs affect wheat yields by increasing the lower limit of a crop’s available soil water (CLL) and thus reducing the soil’s plant-available water capacity (PAWC). This proposal was tested by simulation of 33 farmers’ paddocks in south-western Queensland and north-western New South Wales. The simulated results accounted for 79% of observed variation in grain yield, with a root mean squared deviation (RMSD) of 0.50 t/ha. This result was as close as any achieved from sites without SSCs, thus providing strong support for the proposed mechanism that SSCs affect wheat yields by increasing the CLL and thus reducing the soil’s PAWC. In order to reduce the need to measure CLL of every paddock or management zone, two additional approaches to simulating the effects of SSCs were tested. In the first approach the CLL of soils was predicted from the 0.3–0.5 m soil layer, which was taken as the reference CLL of a soil regardless of its level of SSCs, while the CLL values of soil layers below 0.5 m depth were calculated as a function of these soils’ 0.3–0.5 m CLL values as well as of soil depth plus one of the SSC indices EC, Cl, ESP, or Na. The best estimates of subsoil CLL values were obtained when the effects of SSCs were described by an ESP-dependent function. In the second approach, depth-dependent CLL values were also derived from the CLL values of the 0.3–0.5 m soil layer. However, instead of using SSC indices to further modify CLL, the default values of the water-extraction coefficient (kl) of each depth layer were modified as a function of the SSC indices. The strength of this approach was evaluated on the basis of correlation of observed and simulated grain yields. In this approach the best estimates were obtained when the default kl values were multiplied by a Cl-determined function. The kl approach was also evaluated with respect to simulated soil moisture at anthesis and at grain maturity. Results using this approach were highly correlated with soil moisture results obtained from simulations based on the measured CLL values. This research provides strong evidence that the effects of SSCs on wheat yields are accounted for by the effects of these constraints on wheat CLL values. The study also produced two satisfactory methods for simulating the effects of SSCs on CLL and on grain yield. While Cl and ESP proved to be effective indices of SSCs, EC was not effective due to the confounding effect of the presence of gypsum in some of these soils. This study provides the tools necessary for investigating the effects of SSCs on wheat crop yields and natural resource management (NRM) issues such as runoff, recharge, and nutrient loss through simulation studies. It also facilitates investigation of suggested agronomic adaptations to SSCs.

External agricultural inputs such as mineral fertilisers, organic amendments, microbial inoculants, and pesticides are applied with the ultimate goal of maximising productivity and economic returns, while side effects on soil organisms are often neglected. We have summarised the current understanding of how agricultural inputs affect the amounts, activity, and diversity of soil organisms. Mineral fertilisers have limited direct effects, but their application can enhance soil biological activity via increases in system productivity, crop residue return, and soil organic matter. Another important indirect effect especially of N fertilisation is soil acidification, with considerable negative effects on soil organisms. Organic amendments such as manure, compost, biosolids, and humic substances provide a direct source of C for soil organisms as well as an indirect C source via increased plant growth and plant residue returns. Non-target effects of microbial inoculants appear to be small and transient. Among the pesticides, few significant effects of herbicides on soil organisms have been documented, whereas negative effects of insecticides and fungicides are more common. Copper fungicides are among the most toxic and most persistent fungicides, and their application warrants strict regulation. Quality control of organic waste products such as municipal composts and biosolids is likewise mandatory to avoid accumulation of elements that are toxic to soil organisms.

In dryland agricultural systems of the subtropical, semi-arid region of north-eastern Australia, water is the most limiting resource. Crop productivity depends on the efficient use of rainfall and available water stored in the soil during fallow. Agronomic management practices including a period of fallow, stubble retention, and reduced tillage enhance reserves of soil water. However, access to stored water in these soils may be restricted by the presence of growth-limiting conditions in the rooting zone of the crop. These have been termed as subsoil constraints. Subsoil constraints may include compacted or gravel layers (physical), sodicity, salinity, acidity, nutrient deficiencies, presence of toxic elements (chemical) and low microbial activity (biological). Several of these constraints may occur together in some soils. Farmers have often not been able to obtain the potential yield determined by their prevailing climatic conditions in the marginal rainfall areas of the northern grains region. In the past, the adoption of soil management practices had been largely restricted to the top 100 mm soil layer. Exploitation of the subsoil as a source of water and nutrients has largely been overlooked. The key towards realising potential yields would be to gain better understanding of subsoils and their limitations, then develop options to manage them practically and economically. Due to the complex nature of the causal factors of these constraints, efforts are required for a combination of management approaches rather than individual options, with the aim to combat these constraints for sustainable crop production, managing natural resources and avoiding environmental damage.

During the last decade, a range of indicators has been advocated for assessing soil, farm and catchment health. This paper assembles some recent experiences of the authors in developing and using indicators from paddock to national scales. Indicators are merely a subset of the attributes that are used to quantify aspects of catchment or farm health. Their selection and use in the past has led to criticism of indicators, but, given an explicit approach, most of the criticisms can be overcome. Reliable indicators provide positive and negative signals about the current status of natural resources and how these properties have changed over time. They are used both to identify potential risks and to confirm that current farming practices and systems of land use are effective in maintaining the resource base or economic status. They should be precursors for change and future on-ground investments when problems are observed or identified.A structured approach is needed to ensure indicators are selected and used efficiently. This approach involves: deciding local issues and selecting the most appropriate indicators to reflect those issues; interpreting both positive and negative signals from the monitoring process; taking appropriate action to resolve problems; and, using indicators to monitor the outcomes from the action taken.Finally, we have drawn on these and other experiences to compile a list of indicators that may be used to address sustainability issues associated with farm productivity, soil health and catchment health identified in recent strategic plans developed for the northern grains region of Australia, the focus of this special journal issue.

Two issues prompted this paper. The first was the measured soil organic carbon decline in fertile northern Australian soils under continual cropping using traditional management practices. We wanted to see whether it was theoretically possible to maintain or improve soil organic carbon concentrations with modern management recommendations. The second was the debate about use of sustainability indicators for on-farm management, so we looked at soil organic carbon as a potential indicator of soil health and investigated whether it was useful in making on-farm crop decisions. The analytical results indicated first that theoretically the observed decline in soil organic carbon concentrations in some northern cracking clay soils can be halted and reversed under continuous cropping sequences by using best practice management. Second, the results and associated discussion give some support to the use of soil organic carbon as a sustainability indicator for soil health. There was a consistent correlation between crop input decisions (fertilisation, stubble management, tillage), outputs (yield and profits) and outcomes (change in soil organic carbon content) in the short and longer term. And this relationship depended to some extent on whether the existing soil organic carbon status was low, medium or high. A stock dynamics relationship is one where the change in a stock (such as soil organic carbon) through time is related not only to the management decisions made and other random influences (such as climatic effects), but also to the concentration or level of the stock itself in a previous time period. Against such a requirement, soil organic carbon was found to be a reasonable measure. However, the inaccuracy in measuring soil organic carbon in the paddock mitigates the potential benefit shown in this analysis of using soil organic carbon as a sustainability indicator.These results are based on a simulation model (APSIM) calibrated for a cracking clay (Vertosol) soil typical of much of the intensively-cropped slopes and plains region of northern New South Wales and southern Queensland, and need to be interpreted in this light. There are large areas of such soils in north-western New South Wales; however, many of these experience lower rainfalls and plant-available soil water capacities than in this case, and the importance of these characteristics must also be considered.

The objectives of this study were to quantify below-ground nitrogen (BGN) of rainfed fababean (Vicia faba), chickpea (Cicer arietinum), and barley (Hordeum vulgare) and to use the values to determine N balances for the 3 crops. The BGN fraction of legumes in particular represents a potentially important pool of N that has often been grossly underestimated or ignored in calculating such balances. A field experiment was conducted at Breeza on the Liverpool Plains, New South Wales, in which BGN of fababean, chickpea, and barley was estimated using 15N methodologies. Plants were grown in 0.32-m2 microplots and labelled with 15N on 5 occasions during vegetative growth with a total of 1.0 mL of 0.5% 15N urea (98 atom% 15N) using leaf-flap (fababean), leaf-tip (barley), or cut petiole (chickpea) shoot-labelling procedures. At peak biomass (146–170 days after sowing), all plant material and soil to 45 cm depth was sampled from one microplot in each replicate plot and analysed for dry matter (DM), %N, and 15N. At plant maturity, the remaining 3 microplots in each replicate plot were harvested for shoot and grain DM and N. With fababean, 15N enrichments of intact roots and shoots were reasonably uniform at 537‰ and 674‰, respectively. Microplot soil at 0–25 cm depth had a 15N enrichment of 18‰ (natural abundance of 6.1‰). The 25–45 cm soil enrichment was 8.7‰ (natural abundance of 6.3‰). In contrast, 15N enrichment of chickpea shoots was about twice that of recovered roots (685‰ v. 331‰), and the soil enrichment was relatively high (30‰ and 8.8‰ for the 0–25 and 25–45 cm depths, respectively). The 15N enrichments of barley shoots and recovered roots were 2272‰ and 1632‰, respectively, with soil enrichments of 34‰ and 10.7‰ for the 0–25 and 25–45 cm depths, respectively. Estimates of BGN as a percentage of total plant N, after adjusting the 15N shoot-labelling values of fababean and chickpea for uneven distribution of 15N-depleted nodules, were 24% for fababean, 68% for chickpea, and 36% for barley. The BGN values were combined with N2 fixation (fababean and chickpea only) and shoot and grain yield data (all 3 species) to construct N budgets. The inclusion of BGN in the budgets increased N balances by 38 kg N/ha to +36 kg N/ha for fababean and by 93 kg N/ha to +94 kg N/ha for chickpea. As there was no external (N2 fixation) input of N to barley, the inclusion of BGN made no difference to the N balance of the crop of –74 kg N/ha. Such values confirm the importance of BGN of N2-fixing legumes in the N economies of cropping systems.

Concern over the long-term sustainability of post-mining ecosystems at Weipa (North Queensland, Australia) led to investigations of soil organic matter dynamics, a key process linking soil and vegetation development in maintenance-free systems. Paper I of this series examined the short-term effects of rehabilitation operations on soil organic matter. Here, we assess the medium-term development of post-rehabilitation soil organic matter quantity and quality using mine soil chronosequences of up to 22 years post-rehabilitation at Weipa. Soils had been respread either immediately after stripping or after stripped soil had been stockpiled for several years. Sites surveyed were revegetated with native tree and shrub species, forestry (Khaya senegalensis), or pasture (Brachiaria decumbens/Stylosanthes spp.). Three areas of undisturbed native forest were included for comparison. Compared with the undisturbed forest, rehabilitated soils were shallower and more compacted, contained more gravel, and, as a result of topsoil–subsoil mixing, stored less organic matter in the surface soil. Rehabilitated sites respread with stockpiled soil were more compacted and lower in all quantitative and qualitative measures of organic matter than freshly replaced soils. With time, organic matter accumulated in the surface soil under all vegetation types at rates of up to 1.25 t C/ha.year, but new equilibrium levels were yet to be reached. Accumulated organic matter was mostly associated with clay and silt-sized particles, indicating effective cycling of litter to humus. Nitrogen mineralisation capacity increased with time under all vegetation types. The incidence of fire led to increased total and light-fraction organic C, but this was probably as charcoal C. Sites where volunteer grass biomass was reduced pre-planting by late-season stripping or disc-ploughing accumulated less organic C. To optimise post-mining soil organic matter development, we recommend that soil stockpiling be avoided, that more volunteer grasses be retained to ensure continuity of organic inputs, and that attention be focussed on minimising soil compaction and gravel incorporation—both permanent limitations to plant growth.