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How much C can be stored in agricultural soils worldwide to mitigate rising carbon dioxide (CO2) concentrations, and at what cost? This question, because of its critical relevance to climate policy, has been a focus of soil science for decades. The amount of additional soil organic C (SOC) that could be stored has been estimated in various ways, most of which have taken the soil as the starting point: projecting how much of the SOC previously lost can be restored, for example, or calculating the cumulative effect of multiple soil management strategies. Here, we take a different approach, recognizing that photosynthesis, the source of C input to soil, represents the most fundamental constraint to C sequestration. We follow a simple “Fermi approach” to derive a rough but robust estimate by reducing our problem to a series of approximate relations that can be parameterized using data from the literature. We distinguish two forms of soil C: ‘ephemeral C’, denoting recently-applied plant-derived C that is quickly decayed to CO2, and ‘lingering C,’ which remains in the soil long enough to serve as a lasting repository for C derived from atmospheric CO2. First, we estimate global net C inputs into lingering SOC in croplands from net primary production, biomass removal by humans and short-term decomposition. Next, we estimate net additional C storage in cropland soils globally from the estimated C inputs, accounting also for decomposition of lingering SOC already present. Our results suggest a maximum C input rate into the lingering SOC pool of 0.44 Pg C yr−1, and a maximum net sequestration rate of 0.14 Pg C yr−1 – significantly less than most previous estimates, even allowing for acknowledged uncertainties. More importantly, we argue for a re-orientation in emphasis from soil processes towards a wider ecosystem perspective, starting with photosynthesis.

Extensive, progressive rock emergence causes localized variations in soil biogeochemical and microbial properties that may influence the capacity for the regeneration of degraded karst ecosystems. It is likely that karst ecosystem recovery relies on the persistence of soil functions at the microbial scale, and we aimed to explored the role of interactions between soil bacterial taxa and identify keystone species that deliver key biogeochemical functions, i.e. carbon (C) and nutrient (nitrogen, N and phosphorus, P) cycling. We applied high-throughput sequencing and phylogenetic molecular ecological network approaches to topsoils sampled at rock-soil interfaces and adjacent bulk soil along an established gradient of land-use intensity in the Chinese Karst Critical Zone Observatory. Bacterial α-diversity was greater under increased perturbation and at the rock-soil interface compared to bulk soils under intensive cultivation. However, bacterial ecological networks were less intricate and connected fewer keystone taxa as human disturbance increased and at the rock-soil interface. Co-occurrence within the bacterial community in natural primary forest soils was 13% larger than cultivated soils. The relative abundances of keystone taxa Acidobacteria, Bacteroidetes and Chloroflexi increased with land-use intensity, while Proteobacteria, Actinobacteria and Verrucomicrobia decreased by up to 6%. In general, Bacteroidetes, Verrucomicrobia and Chlorobi were related to C-cycling, Proteobacteria, Actinobacteria and Chloroflexi were related to N-cycling, and Actinobacteria and Nitrospirae were related to both N- and P-cycling. Proteobacteria and Chlorobi affected C-cycling and multiple functionality indexes in the abandoned land. We conclude that increasing land-use intensity changed the soil bacterial community structure and decreased bacterial interactions. However, increases in α-diversity at the rock-soil interface in cultivated soils indicated that major soil functions related to biogeochemical cycling were maintained within keystone taxa in this microenvironment. Our study provides foundations to test the success of different regeneration practices in restoring soil microbial diversity and the multifunctionality of karst ecosystems.

Conservation agriculture (CA), comprising minimum soil disturbance, retention of crop residues and crop diversification, is widely promoted for reducing soil degradation and improving agricultural sustainability. It is also claimed to mitigate climate change through soil carbon sequestration: we conducted a meta-analysis of soil organic carbon (SOC) stock changes under CA practices in two tropical regions, the Indo-Gangetic Plains (IGP) and Sub-Saharan Africa (SSA), to quantify this. In IGP annual increases in SOC stock compared to conventional practice were between 0.16 and 0.49 Mg C ha−1 yr−1. In SSA increases were between 0.28 and 0.96 Mg C ha−1 yr−1, but with much greater variation and a significant number of cases with no measurable increase. Most reported SOC stock increases under CA are overestimates because of errors introduced by inappropriate soil sampling methodology. SOC increases require careful interpretation to assess whether or not they represent genuine climate change mitigation as opposed to redistribution of organic C within the landscape or soil profile. In smallholder farming in tropical regions social and economic barriers can greatly limit adoption of CA, further decreasing realistic mitigation potential. Comparison with the decreases in greenhouse gas emissions possible through improved management of nitrogen (N) fertilizer in regions such as IGP where N use is already high, suggests that this is a more effective and sustainable means of mitigating climate change. However the mitigation potential, and other benefits, from crop diversification are frequently overlooked when considering CA and warrant greater attention. Increases in SOC concentration (as opposed to stock) in near-surface soil from CA cause improvements in soil physical conditions; these are expected to contribute to increased sustainability and climate change adaptation, though not necessarily leading to consistently increased crop yields. CA should be promoted on the basis of these factors and any climate change mitigation regarded as an additional benefit, not a major policy driver for its adoption.

Over-application and inefficient use of nitrogen (N) fertilizer is a serious issue throughout China, with adverse environmental and economic impacts. In this paper we present evidence of this in the wheat/maize double cropping system in the Guanzhong Plain in Shaanxi Province, northwest China. Results show the economic benefits of overcoming this problem are greatest for the lowest income farmers. We also outline new advisory approaches that could aid delivery of information to farmers. Evidence of excessive N fertilizer applications, and opportunities to maintain or even increase crop yields with lower rates of N, are presented from several sources. A survey of N applications to maize by 80–100 farmers showed that 77% were applying N at rates in excess of those recommended by the local advisory agencies. Experiments with maize and wheat at 120 sites, testing a range of N application rates, show remarkably small yield responses to applied N and high yields even when no N is applied. This is mainly because of large nitrate residues accumulated in the soil from past N fertilizer applications. Trials were conducted in 30 farmers’ fields comparing the farmer’s usual N rate with a lower rate based on a combination of local recommendations and measurements of nitrate in soil. On average, N rates to maize and wheat could be decreased by 70% and 20%, respectively, with no loss of yield and sometimes small increases. Economic assessments and household surveys showed the economic benefits for farmers of moving to more rational use of N fertilizer. Even a 30% reduction in N use would increase household income by 2–9%, and a 50% reduction by 4–15%. In all cases the poorest farmers benefit the most because fertilizer represents a larger percentage of their expenditure, so policies and practices leading to more rational N use are clearly pro-poor. Advisory approaches based on an N budget approach are outlined as an alternative to traditional approaches where farmers are simply given a recommended application rate. Simple in-field measurements of nitrate concentration in soil, using commercially available nitrate-sensitive strips giving a color reaction, may be a useful supplement for field-specific advisory work if the logistics at village level can be organised.

Results from the UK were reviewed to quantify the impact on climate change mitigation of soil organic carbon (SOC) stocks as a result of (1) a change from conventional to less intensive tillage and (2) addition of organic materials including farm manures, digested biosolids, cereal straw, green manure and paper crumble. The average annual increase in SOC deriving from reduced tillage was 310 kg C ± 180 kg C ha−1 yr−1. Even this accumulation of C is unlikely to be achieved in the UK and northwest Europe because farmers practice rotational tillage. N2O emissions may increase under reduced tillage, counteracting increases in SOC. Addition of biosolids increased SOC (in kg C ha−1 yr−1 t−1 dry solids added) by on average 60 ± 20 (farm manures), 180 ± 24 (digested biosolids), 50 ± 15 (cereal straw), 60 ± 10 (green compost) and an estimated 60 (paper crumble). SOC accumulation declines in long-term experiments (>50 yr) with farm manure applications as a new equilibrium is approached. Biosolids are typically already applied to soil, so increases in SOC cannot be regarded as mitigation. Large increases in SOC were deduced for paper crumble (>6 t C ha−1 yr−1) but outweighed by N2O emissions deriving from additional fertiliser. Compost offers genuine potential for mitigation because application replaces disposal to landfill; it also decreases N2O emission.&nbsp

We compared alternate uses of cereal straw (4.25t dry matter ha(-1) containing 1.7t carbon (C)) for their effectiveness in relation to climate change mitigation. The scenarios were (1) incorporation into soil to increase soil organic carbon (SOC) content ("carbon sequestration") and (2) combustion to generate electricity. The Rothamsted Carbon Model was used to estimate SOC accumulation in a silty clay loam soil under the climatic conditions of north-west Europe. Using straw for electricity generation saved seven times more CO2 than from SOC accumulation. This comparison assumed that electricity from straw combustion displaced that generated from coal and used the mean annual accumulation of SOC over 100yr. SOC increased most rapidly in the early years, but then more slowly as a new equilibrium value was approached. We suggest that increased SOC from straw incorporation does not represent genuine climate change mitigation through carbon sequestration. In Europe, most straw not already incorporated in the field where it is grown is subsequently returned elsewhere, e.g., after use for animal bedding and production of manure. Only additional retention of C in soil compared to the alternative use represents sequestration. Maintenance of SOC for soil functioning is a more appropriate rationale for returning straw to soil than climate change mitigation. This analysis shows that considerably greater climate change mitigation is achieved through saved CO2 emissions by burning straw for electricity generation, replacing some use of fossil fuel.

National and sub-national scale estimates of soil organic carbon (SOC) stocks and changes can provide information land degradation risk, C sequestration possibilities and the potential sustainability of proposed land management plans. Under a GEF co-financed project, ‘The GEFSOC Modelling System’ was used to determine SOC stocks and projected stock change rates for four case study areas; The Brazilian Amazon, The Indo-Gangetic Plains of India, Kenya and Jordan. Each case study represented soil and vegetation types, climates and land management systems that are under represented globally, in terms of an understanding of land use and land management systems and the effects these systems have on SOC stocks. The stocks and stock change rates produced were based on detailed geo-referenced datasets of soils, climate, land use and management information. These datasets are unique as they bring together national and regional scale data on the main variables determining SOC, for four contrasting non-temperate eco-regions. They are also unique, as they include information on land management practices used in subsistence agriculture in tropical and arid areas. Implications of a greater understanding of SOC stocks and stock change rates in non-temperate areas are considered. Relevance to national land use plans are explored for each of the four case studies, in terms of sustainability, land degradation and greenhouse gas mitigation potential. Ways in which such information will aid the case study countries in fulfilling obligations under the United Nations Conventions on Climate Change, Biodiversity and Land Degradation are also considered. The need for more detailed land management data to improve SOC stock estimates in non-temperate areas is discussed.

The GEFSOC Project developed a system for estimating soil carbon (C) stocks and changes at the national and sub-national scale. As part of the development of the system, the Century ecosystem model was evaluated for its ability to simulate soil organic C (SOC) changes in environmental conditions in the Indo-Gangetic Plains, India (IGP). Two long-term fertilizer trials (LTFT), with all necessary parameters needed to run Century, were used for this purpose: a jute (Corchorus capsularis L.), rice (Oryza sativa L.) and wheat (Triticum aestivum L.) trial at Barrackpore, West Bengal, and a rice-wheat trial at Ludhiana, Punjab. The trials represent two contrasting climates of the IGP, viz. semi-arid, dry with mean annual rainfall (MAR) of 1600 mm. Both trials involved several different treatments with different organic and inorganic fertilizer inputs. In general, the model tended to overestimate treatment effects by approximately 15%. At the semi-arid site, modelled data simulated actual data reasonably well for all treatments, with the control and chemical N + farm yard manure showing the best agreement (RMSE = 7). At the humid site, Century performed less well. This could have been due to a range of factors including site history. During the study, Century was calibrated to simulate crop yields for the two sites considered using data from across the Indian IGP. However, further adjustments may improve model performance at these sites and others in the IGP. The availability of more long-term experimental data sets (especially those involving flooded lowland rice and triple cropping systems from the IGP) for testing and validation is critical to the application of the model's predictive capabilities for this area of the Indian sub-continent.

A range of spectroscopic, wet chemical, gas chromatographic (GC) and mass spectrometric (MS) techniques was applied to the characterisation of three soil organic matter (SOM) fractions that have been proposed as the basis of a new SOM turnover model based on measurable, physically defined fractions. The fractions were: the free light fraction (obtained by density separation in NaI solution at a density of 1.80 g cm � 3 , without disruption of aggregates), the intra-aggregate light fraction (obtained using a second density separation after disrupting aggregates using ultrasonic dispersion) and the organomineral fraction corresponding to the residual heavy material. The techniques employed to investigate the composition of the organic constituents of each fraction were: 13 C nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and pyrolysis-gas chromatography/mass spectrometry (py-GC/MS) to study bulk composition. Lipid, lignin and carbohydrate fractions were assessed using GC and GC/MS with appropriate derivatisation, following oxidative and hydrolytic treatments, respectively, in the case of the latter two classes. Proteinaceous components were determined as amino acids using reversed-phase high performance liquid chromatography (HPLC) following 6 M HCl treatment and derivatisation. Each technique revealed marked differences in chemical composition between the organomineral and the two light fractions, with the results being consistent with the organomineral fraction having different biological sources or having undergone a greater degree of degradation or transformation. Several techniques detected differences between the composition of the free light fraction and the intra-aggregate light fraction. With the exception of carbohydrate composition, the results were consistent with the order of reactivity previously proposed from incubation studies with isotopically labelled substrates, namely: free > intra-aggregate > organomineral. The investigation illustrates the importance of using a range of different chemical characterisation techniques in studies of complex SOM fractions as each has limitations that could, if used alone, produce ambiguous findings or fail to detect differences between them. � 2005 Elsevier Ltd. All rights reserved.

A C and N mineralization model (DETRAN) was used to simulate C and N dynamics in an unfertilized soil (NIL plot) and a soil annually fertilized with organic (FYM plot) or inorganic fertilizer (NPK plot). The soils amended with or without rye, i.e. 500 mg C and 30 mg N kg ˇ1 dry soil (D.S.), were incubated for 180 d at 258C and microbial biomass C and N, CO2 production and inorganic N (NH4 ,N O2 ,N O3 ) were monitored. The production of CO2 was greater in the NIL plot than in the NPK plot but three times lower than in the FYM plot. The soil microbial biomass C and N decreased in the FYM soil but not in the NIL and NPK plots. The N mineralization was 7.5 times greater in the FYM plot than in the NPK and NIL plots but the application of rye had no significant eAect on it. In the unamended soil, an eAciency for C of 40%, i.e. the amount of C incorporated into the microbial biomass while the rest or 60% evolved as CO2, had to be used to link the C and N dynamics in the NPK and NIL plots but the eAciency was only 26% in the FYM plot. The eAciency for C added with the rye was 45% in the NIL plot and 60% in the NPK plot but it was diAcult to link C and N mineralization in the FYM plot. We conclude that the dynamics of C and N as governed by the microbial biomass C and N were diAerent for the FYM plot compared with the NPK and NIL plots. The diAerence, a result of long-term organic fertiliser application, could be due to a diAerence in eAciency for C, a diAerence in the C-to-N ratio of the microbial biomass; a diAerence not reflected in the measured

Nine soil organic models were evaluated using twelve datasets from seven long-term experiments. Datasets represented three different land-uses (grassland, arable cropping and woodland) and a range of climatic conditions within the temperate region. Different treatments (inorganic fertilizer, organic manures and different rotations) at the same site allowed the effects of differing land management to be explored. Model simulations were evaluated against the measured data and the performance of the models was compared both qualitatively and quantitatively. Not all models were able to simulate all datasets; only four attempted all. No one model performed better than all others across all datasets. The performance of each model in simulating each dataset is discussed. A comparison of the overall performance of models across all datasets reveals that the model errors of one group of models (RothC, CANDY, DNDC, CENTURY, DAISY and NCSOIL) did not differ significantly from each other. Another group (SOMM, ITE and Verberne) did not differ significantly from each other but showed significantly larger model errors than did models in the first group. Possible reasons for differences in model performance are discussed in detail.&nbsp

The amount and quality of inputs into soil organic matter will be altered by both climate and landuse change. The increase in growth of plants caused by increasing CO2 concentration implies not only potential increases in yields but also increases in plant residues. Simulation models using doubled CO2 levels predict global net primary productivity (NPP) to increase by 16.3%, over half of which will occur in the tropics. For tropical ecosystems increases in NPP will be dominated by the effects of elevated CO2, with water and nitrogen availability and temperature playing a less significant role. Phosphorus limitation may determine whether the potential for increased plant growth will be realized. The distribution of C3 and C4 species in the tropics could be affected by landuse change and estimates of yield increases will be dependent on their proportions. The allocation of photosynthate to the root will increase under elevated CO2, resulting in increased fine root dry weight and root length. Root sink strength and the turnover of roots and associated symbionts are critical knowledge gaps. Carbon: nitrogen ratios in tissues will increase resulting in decreased decomposition rates. The concentration of secondary compounds will be affected more by nitrogen limitations than a direct CO2 effect. Changes in lignin, tannin and polyphenol levels are more important in the decomposability of tropical litters than changes in the C : N ratios. Decomposition models will have to be altered to take into account changes in plant composition. The role of models in predicting the effects of management practice on long-term fertility is addressed.

Results are presented from long-term experimental sites showing that land use and agricultural management practices play an important role in mediating the sink strength of aerobic soils for methane. At sites located within 1 km2 at Rothamsted Experimental Station, U.K. the methane sink strength of soil follows the order woodland > grassland > arable. Comparison of grassland plots receiving nitrate-N fertilizer compared to ammonium-N fertilizer shows that the long-term (138 years) application of ammonium-N fertilizer caused a significant decrease in the soil sink strength for methane but that the application of nitrate-N for the same length of time did not.These results are discussed in relation to land use and microbial ecology.

Net uptake of CH4 was measured in intact soil cores (6.4 cm dia, 12 cm deep) collected from an arable wheat field, from three sites left uncultivated for more than 110 years following arable cropping and from a permanent grassland with different mineral N treatments subdivided into four pH levels. Soil cores were incubated in sealed 1 litre jars at 25°C for 48 h with a CH4 -amended atmosphere of 10 μl 1−1 at the start of incubation. The decrease in CH4 concentration followed first-order-kinetics and by log-transformation individual uptake rates could be calculated for each treatment. Soil from a calcareous site (pH 7.4) under deciduous woodland (Broadbalk Wilderness wooded section) oxidized CH4 6 times faster than the arable plot (pH 7.8) with the highest activity in the adjacent Broadbalk Wheat Experiment (with uptake rates of −80 and −13 nl CH4 1−1 h−1, respectively). The CH4 uptake rate was only 20% of that in the woodland in an adjacent area that had been uncultivated for the same period but kept as rough grassland by the annual removal of trees and shrubs and, since 1960, grazed during the summer by sheep. It is suggested that the continuous input of urea through animal excreta was mainly responsible for this difference. Another undisturbed woodland area with an acidic soil reaction (pH 4.1) did not oxidize any CH4. On a permanent grassland site (Park Grass Continuous Hay Experiment), the plot without N fertilization showed a distinct pH effect: CH4 consumption decreased from −67 to − 35nl CH4 1−1 h−1 with decreasing pH in the range 6.3–5.6 and declined to zero between pH 5.6 and 5.1. Mineral N applied annually as (NH4)2SO4, at either 96 or 144 kg N ha −1 for 130 years, completely inhibited CH4 oxidation, even where lime was applied to maintain a soil pH of about 6. By contrast, the long-term application of N as NaNO3 (96 kg N ha−1 a−1) caused no decline in CH4 oxidation compared to unfertilized grassland at the same pH and, in some cases, caused a small increase. Withholding NH4-N for 3 years caused no significant recovery of CH4 -oxidizing activity; withholding NO3-N caused a slight decline. Thus, land use (arable, cut grassland, grazed grassland or woodland), soil pH, N fertilizer inputs and form of N (NH4 or NO3) all have marked and interacting effects on the extent to which aerobic soil acts as a sink for CH4. The mechanisms through which the factors operate are not known but some possibilities are discussed. The results have important implications for the planning of land use and agricultural practices that will maximize the extent to which aerobic soils can act as a sink for CH4.

A new method for the determination of biomass in soil is described. Soil is fumigated with CHCl3 vapour, the CHCl3 removed and the soil then incubated. The biomass is calculated from the difference between the amounts of CO2 evolved during incubation by fumigated and unfumigated soil. The method was tested on a set of nine soils from long-term field experiments. The amounts of biomass C ha−1 in the top 23 cm of soil from plots on the Broadbalk continuous wheat experiment were 530 kg (unmanured plot), 590 (plot receiving inorganic fertilizers) and 1160 (plot receiving farmyard manure). Soils that had been fallowed for 1 year contained less biomass than soils carrying a crop. A calcareous woodland soil contained 1960 kg biomass C ha−1, and an unmanured soil under permanent grass 2020. The arable soils contained about 2% of their organic C in the biomass; uncultivated soils a little more—about 3%.

The straw and stubble of spring barley (Hordeum vulgare; 5t dry matter ha−1) were either burned or incorporated into soil annually for 18 yr in two field experiments in Denmark. Both experiments were on light soils situated at Studsgaard (loamy sand) and Rønhave (sandy loam). At both sites 18 yr of annual straw incorporation increased total soil organic C by only 5% and total N by about 10% but produced large increases in microbial biomass measured by the CHCl3-fumigation method. The increases in biomass C were 45 and 37% at Studsgaard and Rønhave, respectively: the corresponding increases in biomass N were 50 and 46%. Biomass measurements thus gave an early indication of slow changes in organic matter content long before these could be measured accurately against the background of organic matter already present in the soils. Increases in biomass P due to straw incorporation appeared to be even greater. However, the amounts of P released by CHC13 were small so the measurements of biomass P were less accurate than those of biomass C or N. During a 60-day laboratory incubation at 25°C, evolution of CO2-C was 55–79% greater in soil from straw incorporated plots than in soil from burned plots. Mineralization of N was 40–50% greater where straw had been incorporated, indicating thaf the long-term incorporation of straw had increased the quantity of mineralizable N in soil. RESP-09802

Respiration and mineralisation of N were measured in a set of contrasting soils that had either been autoclaved, air-dried, fumigated (with chloroform or methyl bromide) or exposed to gamma radiation. The soils used were a manured and an unmanured arable soil, an acid and a neutral woodland soil, an arable sandy soil and an organic soil under grass. With the exception of the acid woodland soil, the flushes of decomposition (i.e. the increases in O2 consumption, CO2 evolution and N mineralisation that occurred when the treated soil was inoculated and incubated for 10 days) were in the order: air-drying < CH3Br ⩽ CHCl3 < irradiation < autoclaving. All of the treatments, except air-drying, decreased the ratio (C mineralised after treatmcnt)/(N mineralised after treatment). All of the treatments increased the amount of 1N K2SO4 extractable organic C, autoclaving causing by far the greatest increase. Neither of the fumigants increased respiration in the acid soil over the whole 10 day period, although N mineralisation was slightly increased. Irradiation, air-drying and autoclaving did, however, produce a flush in the acid soil, the order being: irradiation < air-drying < autoclaving. A soluble substrate, extracted from yeast cells by ultrasonic disintegration, decomposed to about the same extent in neutral and in acid soil. When 14C labelled glucose was added to the acid soil and incubated for 52 days, the retention of labelled C was slightly greater (31·6%) than in a comparable near-neutral soil (28·8%). However, the flush that followed fumigation of the acid soil was only half that in the near-neutral soil, suggesting that less biomass is formed under acid conditions. Liming increased the size of the flush in an acid soil. For soils from the same field but under different management, the size of the flush caused by CHCl3 is in the order: grassland > cropped arable > bare fallow. The flush is much more sensitive to differences in soil management than is the total amount of soil organic matter; a fallowed soil lost half its organic C in 10 yr whereas the increase in respiration that followed fumigation fell to one-seventh its original value. Two Nigerian soils behaved similarly; a soil that had been 2 years under cultivation contained only 16% less total organic C than an adjacent soil still under secondary forest, yet the flush in the cultivated soil was half that in the forest soil. The amount of substrate metabolised during the flush is thus very sensitive to changes in soil management that alter the amount of fresh organic matter entering the soil each year.

Phosphorus in the soil microbial biomass (biomass P) and soil biomass carbon (biomass C) were linearly related in 15 soils (8 grassland, 6 arable, 1 deciduous woodland), with a mean P concentration of 3.3% in the soil biomass. The regression accounted for 82% of the variance in the data. The relationship was less close than that previously measured between soil biomass C and soil ATP content and indicates that biomass P measurements can only provide a rough estimate of biomass C content. Neither P concentration in the soil biomass, nor the amount of biomass P in soil, were correlated with soil NaHCO3-extractable inorganic, organic or total P. The calculated mean annual flux of P through the biomass (in a soil depth of 10 cm) in 8 grassland soils was large, 23 kg P ha−1 yr−1, and more than three times the mean annual P flux through 6 arable soils (7 kg P ha−1 yr−1), suggesting that biomass P could make a significant contribution to plant P nutrition in grassland. About 3% of the total soil organic P in the arable soils was in microbial biomass and from 5 to 24% in the grassland soils. The decline in biomass P when an old grassland soil was put into an arable rotation for about 20 yr was sufficient to account for about 50% of the decline in total soil organic P during this period. When an old arable soil reverted to woodland, soil organic P doubled in 100 yr; biomass P increased 11-fold during the same period.

The effect of temperature on denitrification was compared in a temperate zone soil, a Paleudalf cropped to wheat in south-east England, and a sub-tropical soil, a Vertisol (Typic Pellustert) cropped to sorghum in Queensland. Australia. The 2 soils were incubated anaerobically in the presence of KNO3 and glucose at 5, 10, 15, 20 or 25°C for either 2 d or 7 d. Immobilization of the added nitrate was measured in parallel incubations using K15NO3. After 2 d more nitrate was lost from the English soil than from the Australian at every temperature. After 7d at 10°C, all nitrate had been reduced in the English soil, 92% denitrified and 8% immobilized, whilst in the Australian soil 91% of the nitrate still remained. Only at 20°C or above was all of the nitrate reduced (89% denitrified, 11% immobilized) in the Australian soil. The results indicate that the denitrifying populations in each soil was adapted to its own environment. The denitrifiers in the temperate soil reduced nitrate at lower temperatures than did those in the sub-tropical soil. The occurrence of considerable denitrification at 10°C in the temperate soil, and a sharp increase between 5 and 10°C, shows that denitrification could be a major cause of N loss in temperate areas in spring, when much N fertilizer is applied. Even though soil temperatures can fall to similar values in the subtropics in winter, much less denitrification would be expected.

A method for measuring the amount of P held in soil micro-organisms (biomass P) is described and the assumptions on which it is based are discussed. Biomass P is calculated from the difference between the amount of inorganic P (Pi) extracted by 0.5 (Spm) NaHCO3 (pH 8.5) from fresh soil fumigated with CHCl3 and the amount extracted from unfumigated soil. Some CHCl3-released Pi is sorbed by soil during fumigation and extraction: an approximate allowance for this is made by incorporating a known quantity of Pi during extraction and correcting for recovery. Most of the P released is in inorganic form and the proportion increases with duration of fumigation. Non-microbial P is little, if at all, affected by fumigation. Microbial biomass P is calculated from CHCl3-released Pi by dividing by 0.4, i.e. by assuming that 40% of the P in the biomass is rendered extractable as Pi by CHCl3. Measurements of biomass P must be done in fresh soil, CHCl3 releases much less P in air-dry soil.&nbsp

Short- and medium-term changes in soil organic matter content following a change in soil management or land use are often difficult to measure because they occur slowly against a large background of soil organic matter which can have considerable spatial variability. Results from an experiment with grain sorghum (Sorghum bicolor L.) on a vertisol in sub-tropical Australia demonstrate the usefulness of two techniques for detecting trends in surface soil organic matter before they can be assessed by conventional methods. Firstly, using soil microbial biomass C as a sensitive indicator of changes in soil organic matter. Secondly, by using initial values of soil organic C or total N, measured before imposition of treatments, as a covariate in an analysis of variance. The combination of these techniques provided the most sensitive approach for detecting changes. The above-ground residues of sorghum (41 dry matter ha−1) were either retained or removed from plots that received conventional or zero tillage for 6 yr. Averaged over tillage treatments, soil organic C in the surface 0–10 cm layer was 8% greater in the residue-retained than in the residue-removed treatment, a difference equivalent to 16% of the C added as residues. The trend to increased soil total N was not significant. Residue retention caused larger percentage increases in microbial biomass C, measured by the chloroform fumigation-incubation method, than in total organic C and total N. The increase in biomass C was 12%, biomass N 23% and biomass P 45%, equivalent to 0.7% of the C, 7% of the N and 32% of the P added in residues. Residue retention decreased the biomass C-to-P ratio from 48 to 35, but these values were still much wider than those previously measured in U.K. soils. Residue retention increased respiration by about 45% (measured by CO2 evolution during a 30-day incubation) but had little effect on biomass C-to-N ratio or mineralization of N. Averaged over the two residue management treatments, soil organic C in the surface 10 cm layer was 7% greater under zero tillage than under conventional tillage. The corresponding increase in biomass C was 14–21%, but there were no differences in biomass N or biomass P. CO2 evolution and specific respiration by the biomass (μ g CO2-C evolved g−1 biomass C day−1) were less in zero-tilled than in conventionally tilled soils. The combined effects of residue retention and zero tillage caused increases of 15% in surface soil organic C, 18% in soil total N and 31% in biomass C.

A single pulse of 15 N-labelled fertilizer was applied in spring as NH 4 NO 3 to each of four plots on the Broadbalk Continuous Wheat Experiment. Wheat has been grown on this experiment for more than 140 yr. The labelled N was given at the customary rates for the four plots, nominally 48, 96, 144 and 192 kg N ha −1 yr −1 . In subsequent years the plots reverted to unlabelled N, again given at the customary rates for the main Broadbalk experiment. Soils receiving inorganic fertilizer contained more biomass than soil from the corresponding plot that has never received inorganic N, but there was little difference in the microbial biomass N content of soils that had had 48, 96, 144 or 192 kg N ha −1 for many years. There were no consistent changes during a 4-yr period in total microbial biomass N, which averaged 190 kg N ha −1 in the plot receiving 192kg fertilizer N ha −1 yr −1 . Of the labelled fertilizer applied, 3–8% was present in the soil microbial biomass at the first harvest after application of labelled fertilizer. Expressed as a percentage of the total labelled N remaining in the soil at the first harvest, 19–27% was present in the microbial biomass. In the plot receiving 192 kg N ha −1 yr −1 , labelled biomass N declined from 5.69 kg ha −1 at the first harvest, to 4.50 at the second, to 3.35 at the third and to 2.35 at the fourth. In a subsidiary experiment, more N was retained in the soil at harvest when the fertilizer was added in the ammonium form than as nitrate; 32.6 and 19.5 kg ha −1 , respectively, for additions of 147 kg fertilizer N ha −1 . However, of the labelled N retained in the soil, 34% was present in the microbial biomass, whether the labelled N had originally been added in the ammonium form or in the nitrate form.

[Short communication - no abstract]

[Short communication - no abstract]