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Nitrogen fertilizer management can impact soil organic C (SOC) stocks in cereal-based cropping systems by regulating crop residue inputs and decomposition rates. However, the impact of long-term N fertilizer management, and associated changes in SOC quantity and quality, on the fate of N fertilizer inputs is uncertain. Using two 15-year N fertilizer rate experiments on continuous maize (Zea mays L.) in Iowa, which have generated gradients of SOC, we evaluated the legacy effects of N fertilizer inputs on the fate of added N. Across the historical N fertilizer rates, which ranged from 0 to 269 kg N ha−1 yr−1, we applied isotopically-labeled N fertilizer at the empirically-determined site-specific agronomic optimum rate (202 kg N ha−1 at the central location and 269 kg N ha−1 at the southern location) and measured fertilizer recovery in crop and soil pools, and, by difference, environmental losses. Crop fertilizer N recovery efficiency (NREcrop) at physiological maturity averaged 44% and 14% of applied N in central Iowa and southern Iowa, respectively (88 kg N ha−1 and 37 kg N ha−1, respectively). Despite these large differences in NREcrop, the response to historical N rate was remarkably similar across both locations: NREcrop was greatest at low and high historical N rates, and least at the intermediate rates. Decreasing NREcrop from low to intermediate historical N rates corresponded to a decline in early-season fertilizer N recovery in the relatively slow turnover topsoil mineral-associated organic matter pool (0–15 cm), while increasing NREcrop from intermediate to high historical N rates corresponded to an increase in early-season fertilizer N recovery in the relatively fast turnover topsoil particulate organic matter pool and an increase in crop yield potential. Despite the variation in NREcrop along the historical N rate gradient, we did not detect an effect of historical N rate on environmental losses during the growing season, which averaged 34% and 69% of fertilizer N inputs at the central and southern locations, respectively (69 kg N ha−1 and 185 kg N ha−1, respectively). Our results suggest that, while beneficial for SOC storage over the long term, fertilizing at the agronomic optimum N rate can lead to significant environmental N losses.

Agroforestry is often proposed as a ‘climate smart’ strategy for allowing agriculture to both adapt to and mitigate climate change and sustainably increase agricultural production. This is because shade trees in agroforests may buffer growing conditions by creating favorable microclimates (climate-change adaptation), and because shade trees can sequester additional carbon from the atmosphere (climate-change mitigation). However, a major challenge for agroforestry is to maximize these potential benefits while minimizing costs to production as a consequence of resource competition between shade trees and the primary crop. While the effects of shade-tree density and canopy cover on the costs and benefits of agroforests are increasingly well understood, the effects of the traits of shade trees on the effectiveness of agroforests have received less attention. Here, we assess how shade trees with different crown architecture influence production, adaptation, and mitigation goals in a major cocoa growing region in Ghana, West Africa. We quantified the effects of shade trees from nine different species across two classes of height-to-crown-base (low vs. elevated canopies) on yield, microclimate, and carbon storage. We show that shade trees with elevated crowns had large positive effects on carbon storage and neutral effects on yield, while shade trees with low crowns had smaller effects on carbon storage and simultaneously caused larger reductions in incoming light, which was associated with lower yield. Trees of both crown classes were equally effective at buffering sub-canopy temperatures and vapor pressure deficit, although trees with low crowns maintained higher relative humidity. Taken together, our results suggest that shade-tree species with elevated crowns improve the effectiveness of cocoa agroforests by providing maximum benefits for climate-change adaptation and mitigation, while minimizing short-term costs to cocoa production.

Currently, the only effective treatment for cocoa (Theobroma cacao L.) infected with the cocoa swollen shoot virus disease (CSSVD) is to cut and replant infected trees. Hence, the development of preventive control measures and strategies to mitigate the severity of the disease are of utmost importance. While past research has mainly focused on resistance breeding, mild strain cross protection and vector control, diversification measures such as agroforestry have received relatively less attention, despite their potential to mitigate CSSVD severity. Therefore, we studied the effects of shade on CSSVD symptom severity, capsid damage and cocoa yield along a gradient of increasing shade tree abundance in smallholder cocoa farms in Ghana. Furthermore, we measured photosynthetic active radiation and assessed soil fertility in order to elaborate on potential causal factors for possible shade effects on CSSVD symptom severity. Both CSSVD symptom severity and cocoa yields followed quadratic curves, and were found to be lowest and highest in plots with 54% and 39% shade, respectively. The simulated optimal shade levels for CSSVD symptom severity and cocoa yield overlapped between 45%–53%, indicating that agroforestry systems with around 50% shade cover may be an optimal coping strategy to balance CSSVD symptom severity versus reduced cocoa yield until diseased cocoa is replaced with more resistant varieties. Furthermore, our results suggest that rather than soil fertility, high-light and possibly also soil moisture stress may have been responsible for the shade effects on CSSVD symptom severity.

Retaining trees in low-input agroecosystems could be key to maintain mycelia of arbuscular mycorrhizal fungi (AMF) and hence, improve soil fertility and crop performance. We assessed the impact of faidherbia (Faidherbia albida, Fabaceae) and mango (Mangifera indica, Anacardiaceae) trees on AMF and soil fertility in smallholder farmers’ maize fields. Along distance-from-tree gradients (1, 4, 10, 15 m), we collected soil to assess AMF hyphal density, soil aggregation, and aggregate-associated carbon (C), nitrogen (N), and phosphorus (P) at the end of the non-cropping season. Further, we determined maize biomass and yield. The impact of faidherbia on maize N nutrition was assessed using the 15N natural abundance methodology. Our results show that hyphal density was largest at 4 and 10 m from trees and greater around faidherbia than mango. Soil aggregation decreased with distance from mango and was greater around faidherbia than mango. Macroaggregate-associated C, N, and P decreased with distance-from-tree, due to differences in aggregate distribution. Maize biomass was smallest at 1 m from trees and did not differ when under faidherbia versus mango. On average 69 ± 14, 24 ± 9, 20 ± 6, and 12 ± 5% of total foliar N of maize grown at 1, 4, 10, and 15 m from faidherbia trees was tree-derived. Our results suggest that faidherbia and mango trees can maintain AMF mycelia and combat declining soil fertility. Faidherbia is particularly suited to enhance measured soil parameters commonly associated with soil fertility and alleviate soil mining for N via improved internal N cycling. As such, agroforestry trees can contribute to a more sustainable agriculture positively affecting the environment via mitigating soil degradation. ISSN:0167-8809 ISSN:1873-2305

Complex agroforests have been promoted as a potential solutions to address trade-offs between environmental conservation efforts and the need for increased agricultural productivity for smallholder farmers in the tropics. However, the effects of tree diversification on soil fertility in tropical agroforests remain unclear. In this study, we examine whether tree diversification in cocoa plantations is associated with soil fertility benefits and can contribute to soil restoration after deforestation. We tested for positive associations between increasing tree species diversity and increased soil aggregation, soil nutrients and microbial communities across a diversity gradient ranging from cocoa monocultures to complex cocoa agroforests. Secondary forests and primary forests were used as reference ecosystems. Increase in tree diversity within cocoa plantations did not increase soil fertility parameters in topsoil layers or cocoa yields. Mean soil C contents were 8% lower, mean weight diameter of aggregates 48% lower and total bacterial biomass 35% lower in cocoa plantations than in primary and secondary forest systems, whereas soil P content was 22% higher. Across all land-use systems, microbial biomass was greater in sites with higher soil carbon contents and soil aggregation. This suggests soil function restoration in terms of microbial communities, soil C and aggregate stabilization in secondary forests. However, in cocoa plantations tree diversification alone may not be an effective solution to mitigate soil degradation after deforestation. Rather, preserving remaining forests or promoting farming approaches that allow for secondary forest regeneration (e.g. implementing forest strips and regular fallow rotations) might have a more substantial impact on soil health.

Agroforestry is often promoted as a sustainable agricultural practice that can ameliorate causes of declining yields, such as soil degradation. However, despite the often-stated potential of agroforestry, quantitative data on the benefits of shade trees are limited to relatively few cropping systems, particularly maize and coffee. Furthermore, agroforests are not cost-free and the benefits of agroforests might not be sufficient to outweigh these costs in all cropping systems or environments. Here we quantify costs and benefits of agroforests for cocoa production in Ghana, West Africa. Specifically, we quantified the ability of shade trees to increase soil carbon stocks and soil fertility (i.e. total soil carbon, nitrogen and phosphorus, available phosphorus and potassium, cation exchange capacity, soil aggregation, pH, and foliar nitrogen and phosphorus concentrations), and investigate if these benefits are sufficient to outweigh the negative effects of shade trees on cocoa growth and yields. We measured cocoa yields, soil fertility and carbon-sequestration under individual shade trees, and in 30 × 30 m plots that were distributed along a gradient of shade-tree cover (plot-scale). We found localized positive effects of individual shade trees on soil carbon and nitrogen content, as well as soil aggregation. However, we found no evidence for positive effects of agroforests via improved soil fertility or carbon-sequestration with increasing shade-tree cover at the plot scale, a scale that more closely matches the scale at which agroforests are managed. Cocoa growth was lower under individual shade trees and decreased with increasing shade-tree cover in plots, and cocoa yields also decreased with increasing shade-tree cover. Our results indicate that the benefits of agroforestry for soil fertility and carbon sequestration in cocoa cultivation systems might not be as extensive as believed, and may not be sufficient to compensate for short-term costs to production.

Integrated Soil Fertility Management (ISFM) is a soil management approach that emphasizes combined application of organic and mineral fertilizer inputs with the goal of improving yields and fertilizer use efficiency. Combined applications have resulted in a positive interaction between organic inputs and mineral fertilizers on vegetable yields, where yields from combined treatments are greater than yields from sole fertilizer treatments. ISFM studies have been conducted with a diverse range of crops, including grains, legumes, tubers, and bananas, but not vegetable crops. Particularly lacking are ISFM studies conducted under participatory, smallholder farmer management. A researcher-designed, farmer-managed, on-farm study was conducted on highly weathered soils (Ferralsols) in the Lake Victoria Crescent of Uganda to determine the influence of combined organic and mineral fertilizer treatments on yields of a commonly grown indigenous leafy vegetable known as nakati ( Solanum aethiopicum ). Farmer-managed plots allowed for the effect of farmer participation and management to be analyzed in conjunction with fertilizer treatment effects. A gradient of 100% organic (sole manure) to 0% organic (sole mineral) fertilizer treatments were applied at both an upper (200 kg ha −1 ) and lower (100 kg ha −1 ) nitrogen (N) rate. N rates were derived from survey results on typical organic application rates used by smallholder farmers in their vegetable plots. Fertilizer treatments resulted in significantly different vegetable yields; however, combined treatments did not necessarily result in higher yields than sole treatments. Differences between organic-mineral ratios were only seen when fertilizers were applied at the higher N rate. The highest yields were obtained when fertilizer was applied at a ratio of 67% organic to 33% mineral fertilizer. Effects of soil properties on yield were also observed; after accounting for the effect of fertilizer treatment, yields significantly increased with increasing soil pH. Farmer participation level had a significant effect on yield. All treatment means were significantly increased by greater participation in the study, and the interactive effects of all treatments became less negative when participation was higher. On-farm studies are needed to demonstrate the applicability of a technology under real world conditions, but trials need to maintain farmers’ interest throughout the study period.

Subsoils have been identified as a potential carbon sink because they typically have low soil organic carbon (SOC) concentrations and high SOC stability. One proposed strategy to increase SOC stocks is to enhance C inputs to the subsoil by increasing crop rotation diversity with deep-rooted perennial crops. Using three long-term field trials in Iowa (study durations of 60, 35, and 12 years), we examined the effects of contrasting cropping systems [maize (Zea mays L.)-soybean (Glycine max (L.) Merr) (= two-year system) vs. maize-soybean-oat (Avena sativa L.)/alfalfa (Medicago sativa L.)-alfalfa or maize-maize-oat/alfalfa-alfalfa (= four-year system)] on above- and below-ground C inputs, as well as the content, biochemical composition, and distribution of SOC among physical fractions differing in stability to 90 cm depth. Average annual total C inputs were similar in the two-year and four-year systems, but the proportion of C delivered belowground was 20–35 % greater in the four-year system. Despite the long duration of these studies, the effect of cropping system on SOC content to 90 cm was inconsistent across trials, ranging from −7 % to +16 % in the four-year relative to the two-year system. At the one site where SOC was significantly greater in the four-year system, the effect of cropping system on SOC content was observed in surface and subsoil layers rather than limited to the subsoil (i.e., below 30 cm). Cropping system had minimal effects on biochemical indicators of plant-derived organic matter or on the proportions of SOC in labile particulate organic matter versus stable mineral-associated organic matter. We conclude that adoption of cropping systems with enhanced belowground C inputs may increase total profile SOC, but the effect is minimal and inconsistent; furthermore, it has minor impact on the vertical distribution, biochemical composition, and stability of SOC in Mollisols of the Midwest U.S.

The potential of switchgrass ( Panicum virgatum L.) to offset large-scale greenhouse gas (GHG) emissions depends on optimizing external inputs when the crop is primarily managed as a sustainable source for renewable energy production. Due to the heterogeneity of climate and soil conditions and the complexity of agriculture, an evaluation of the effect of adopting switchgrass as a new biofuel crop into agriculture needs to be done at the regional scale. The objective of the study was to predict long-term (100-yr) GHG emissions under different N fertilization (0, 112, and 224 kg N ha −1 ) and irrigation application (0, 25, 50, 75, and 99 cm H 2 O) levels across the Central Valley of California using the DAYCENT model. Six cultivars (Alamo, Kanlow, Cave-in-Rock, Blackwell, Sunburst, and Trailblazer) were selected. The model results suggest that switchgrass productivity is primarily constrained by N inputs when no or low water stress is expected in a Mediterranean climate. In the short-term (the first decade after establishment), soil organic carbon (SOC) stocks (0–20 cm) increased by 0.42–0.92 Mg C ha −1 yr −1 and N 2 O emissions were 1.37–2.48 kg N 2 O–N ha −1 yr −1 across the cultivars with baseline input rates of 224 kg N ha −1 yr −1 and 99 cm H 2 O. All cultivars were net CO 2 sinks in the near term and the potential decreased by 0.09–0.30 Mg C ha −1 yr −1 (15.5–52.8%) with reduced N input from baseline under varying irrigation rates. There was a reduction in N 2 O emissions by 47.2–61.6% by applying less N fertilizer when irrigated at rates ≥75 cm H 2 O per year over time. In general, higher-yielding cultivars (e.g., Alamo) tended to sequester more CO 2 but also led to higher N 2 O emissions. In the near term, the use of N fertilizer and irrigation is needed for switchgrass systems to be a soil GHG sink, but for longer-term GHG mitigation strategies reducing both N fertilization and irrigation inputs is required.

It is well known that agricultural production contributes to global warming through the release of greenhouse gases CO2, N2O, and CH4, the most potent of which is N2O. However, most N2O emission studies focus on intensively managed, annual cropping systems. Few have documented greenhouse gas production in perennial crops, which often have substantially different agricultural management practices and growth patterns compared to annual crops. Hence, the objectives of this study were to (1) quantify seasonal and annual direct N2O emissions from a Mediterranean vineyard and (2) determine how conventional management practices such as irrigation, fertigation, cover cropping and tillage affect the magnitude and duration of N2O emissions. This study took place in a wine grape vineyard (Vitis vinifera) in Arbuckle, CA over a two-year period using closed-flux chamber measurements. Annual emissions totaled 3.92 kg N2O–N ha−1 the first year, when a leguminous cover crop was planted in the alleys, while emissions in the second year when the alleys were fallow showed a 7-fold reduction, reaching only 0.56 kg N2O–N ha−1. During the growing season of both years, fertigation events in the crop rows produced slightly increased emissions, ranging from 11 to 23 g N2O–N ha−1 day−1 and lasting less than one week, compared to the low background values of 0.5 g N2O–N ha−1 day−1. The largest fluxes occurred during the dormant season in response to the first precipitation event of the year, especially in the alleys. Nitrous oxide emissions following precipitation events in the second year, when the alleys were fallow, did not follow the same pattern, indicating the significant influence of cover crop-derived N (and C), when coupled with precipitation events, on annual N2O emissions. The results of this study indicate that the effects of individual management practices such as cover cropping may not be seen immediately, but instead act in sequence with other events when conditions are favorable for N2O production. Hence, when determining specific management practices to employ, it is important to recognize the interaction between management, climatic events, and time as important drivers in total N2O production.

Manure generated by dairy cattle is a useful soil amendment but contributes to greenhouse gas (GHG) emissions and water pollution from nutrient leaching. In order to assess the impact of pine chip biochar produced at a peak temperature of 550 °C when added to a dairy grassland system, a one-year field study was conducted on a sandy loam soil under annual ryegrass (Lolium multiflorum Lam.) grown for silage in Petaluma, California. Manure was applied to all plots at a rate of ca. 150 m3 ha−1 (410 kg N ha−1). Control plots received no biochar, high application biochar plots (HB) received biochar (with a 17% ash content) at a rate of 18.8 t ha−1, and low application biochar plots (LB) received the same biochar at 5.7 t ha−1. Although the HB plots demonstrated the lowest cumulative nitrous oxide (N2O) and methane (CH4) emissions, there was no significant difference between treatments (p = 0.152 and p = 0.496, respectively). Soil pH results from samples collected throughout the year indicated a significant treatment effect (p = 0.046), though Tukey test results indicated that there was no difference between mean values. Soil total carbon was significantly higher in HB plots at the end of the experiment (p = 0.025) and nitrate (NO3−) intensity throughout the year (which expresses potential exposure of NO3− to the soil microbial community) was significantly lower in HB plots compared to the control (p = 0.001). Annual cumulative potassium (K+) loss from HB plots was significantly higher than from the other treatments (p = 0.018). HB plots also demonstrated a short-term increase in phosphorus (P) and ammonium (NH4+) in leachate during the first rainfall event following manure and biochar application (p

Biochar amendment to soil has been proposed as a mechanism to mitigate climate change through an array of mechanisms; one being the mitigation of soil nitrous oxide (N2O) emissions. Yet the extent and mechanisms through which this may be achieved in temperate agroecosystems is uncertain. We used a pine chip biochar produced at a moderate temperature (550 °C, PC biochar) and a walnut shell biochar produced at a higher temperature (900 °C, WS biochar). Biochar was applied at 10 Mg ha−1 to a working commercial wine grape system in North-Central California. The effects of biochar were assessed over two years at two distinct functional locations: the berm and row, which differed in N application and irrigation. N2O emissions and ancillary soil properties (NH4+, NO3, water filled pore space (WFPS), and pH) were closely monitored following management and precipitation events. Soil bulk density, cover crop yield and soil C and N were measured annually to address longer term changes in cropping system and soil properties. In the PC biochar treatment, annual cumulative N2O emissions were significantly higher than the control treatment each year (p

Fertilizer nitrogen (N) has been shown to impact both N 2 O and CH 4 emissions from flooded rice systems, yet there is limited research on the effects of N rate when assessing global warming potential (GWP = N 2 O + CH 4 ) per unit area and per unit grain yield (yield-scaled) on a seasonal and annual basis. A two-year on-farm experiment was conducted from 2010–2012 to test the hypothesis that optimal N rates result in maximum agronomic productivity and minimal yield-scaled GWP in water-seeded rice systems experiencing continuously flooded conditions during the growing season and fallow period. Five fertilizer N rates (0, 80, 140, 200 and 260 kg N ha −1 yr −1 ) were applied as aqua ammonia and annual N 2 O and CH 4 emissions were quantified using the vented, closed chamber method. Results indicate that low N 2 O emissions occurred regardless of N rate when a permanent flood was maintained, but that large N 2 O fluxes occurred during discrete field drainage periods prior to harvest, particularly at high N rates. Hence, cumulative N 2 O emissions increased with N rate in a nonlinear manner during the growing season. Over the entire cropping cycle, the highest CH 4 fluxes occurred during the middle of the growing season and following field drainage periods prior to harvest and at the conclusion of the fallow period. Mean seasonal and annual CH 4 emissions tended to increase with N addition compared to the control, but significant differences were not observed between N rates. While CH 4 and N 2 O emissions were generally not affected by N rate during the fallow period, the fallow period contributed significantly to annual emissions (e.g. 56% of annual N 2 O emissions across N rates). Across years, CH 4 represented 94% of total GWP and as a result, mean annual GWP increased with N rate up to 140 kg N ha −1 . Maximum yields occurred between 140 and 200 kg N ha −1 , thus by employing the yield-scaled metric to begin to integrate climate change and global food demand concerns, mean annual yield-scaled GWP significantly decreased by 49% at these N rates. These findings suggest that optimal yields can be achieved with simultaneous reductions in yield-scaled GWP through efficient fertilizer N management in water-seeded rice systems experiencing continuously flooded conditions during the growing season and fallow period.

Understanding the effect of various agricultural management practices on nitrous oxide (N2O) emissions is crucial to advise farmers and formulate policies for future greenhouse gas (GHG) reductions. In order to estimate present N2O emissions, annual N2O budgets must be thoroughly and precisely quantified from current farms under conventional and alternative management, but subject to practical and economic constraints. In this study, field sites were located on two on-farm processing tomato (Lycopersicon esculentum) fields, under contrasting irrigation managements and their associated fertilizer application strategy: (1) furrow irrigation and sidedress fertilizer injection (conventional system) and (2) drip irrigation, reduced tillage, and fertigation (integrated system). Nitrous oxide emissions were monitored for seven to ten days following major events of cultivation, irrigation, fertilization, harvest, and winter precipitations. Total weighted growing season emissions (15 March–1 November 2010) were 2.01 ± 0.19 kg N2O-N ha−1 and 0.58 ± 0.06 kg N2O-N ha−1 in the conventional and integrated systems, respectively. The highest conventional system N2O emission episodes resulted from fertilization plus irrigation events and the first fall precipitation. In the integrated system, the highest N2O fluxes occurred following harvest and the first fall precipitation. Soil chemical and physical properties of soil moisture, inorganic nitrogen (N), and dissolved organic carbon (DOC) were low and less spatially variable in the integrated system. Used as an index of substrate availability, soil ammonium (NH4+) and nitrate (NO3−) exposures were significantly lower in the integrated system. Of great importance is that the drip irrigation water and fertilizer management of the integrated system also increased crop yield (119 Mg ha−1 vs. 78 Mg ha−1), highlighting the potential for decreasing N2O emissions while simultaneously improving the use of water and fertilizer for plant production.

In order to accurately predict N 2 O emissions from agricultural soils and to develop effective management strategies, it is important to understand mechanisms underlying N 2 O emissions under field conditions. This involves identification of sources of N 2 O, which is currently methodologically challenging, especially under field conditions. We assessed the suitability of 15 N tracers and natural abundance 15 N to study N cycling and sources of N 2 O after a rainfall simulation in an annual cropping system in the Central Valley of California. Our natural abundance 15 N approach differed from other studies due to a combination of emphasizing a per-event (e.g. rainfall simulation in this study) assessment of N 2 O emissions, applying high temporal sampling frequency during this event, determination of 15 N of NH 4 + and NO 3 − in addition to N 2 O, and data analysis using isotope models. In our study, the suitability of 15 N tracers to assess N cycling and sources of N 2 O emissions was limited, likely due to a combination of a fine soil texture, the use of undisturbed soil cores, and a low 15 N application rate. Based on natural abundance 15 N, we were able to calculate gross NH 4 + mineralization, NH 4 + immobilization, nitrification and NO 3 − immobilization rates of 5.37 ± 1.72, 2.70 ± 1.72, 3.01 ± 1.13 and 0.15 ± 0.29 μg N g −1 soil d −1 , respectively. Natural abundance 15 N was, however, a rather poor predictor of the contribution of nitrification versus denitrification to N 2 O production. Nevertheless, important trends in N 2 O reduction rates could be observed, showing a sharp increase from 48% to 78% in reduction of produced N 2 O between 2 hours and 24 hours after rainfall simulation, followed by a gradual decrease to 46% of reduction by the fifth day after rainfall simulation. We conclude that the natural abundance 15 N approach is very promising to elucidate mechanisms driving N-cycling and N 2 O emissions during agricultural management or weather events, especially if isotope dynamics are incorporated in site-specific biogeochemical process models.

Agricultural management practices can have a significant effect on the emissions of nitrous oxide (N 2 O) from soils. The aim of this 2-year study was to investigate the effects of no-till (NT) and reduced tillage (RT) practices on annual fluxes of N 2 O from different soil types typical for the boreal region of northern Europe. We measured the fluxes of N 2 O in conventional tillage (CT) and NT at four sites of which two also had RT treatment. No-till and RT practices had been implemented 8–10 years before our study was initiated. Chamber measurements were carried out fortnightly in 2008–2010 on clayey (sites 1–3) and coarse (site 4) soils. Annual cumulative emissions of N 2 O varied from 2.4 to 8.3 in CT, 2.5–6.5 in RT and 4.9–10.2 kg N 2 O-N ha −1 in NT. High peaks in measured N 2 O fluxes occurred during and after thawing of the soil in April and after fertilization and high rain events. No-till or RT did not have any significant effects on soil C or N stocks or potential denitrification of the 0–20 cm soil layer. Dry bulk density and water-filled pore space (WFPS) were generally higher under NT compared to CT, most probably being the main reasons for the increased N 2 O emissions in the NT systems. Soil temperature varied less in NT by being higher during the colder periods of the year and slightly cooler during hot summer days. In conclusion, our results indicate that NT induces a risk of increased N 2 O emissions in clayey soils in small grain spring cereal agroecosystems in Northern European boreal climate.

The conversion from cereal fields to vegetable production in the last three decades represents a significant shift in land use in China. Here, we studied the effects of conversion form cereal fields to vegetable production in north China on soil organic carbon (SOC) and total nitrogen (TN) in both bulk soil and soil aggregates. We used two approaches: (1) measurements of paired soil samples from wheat ( Triticum aestivum L.) – maize ( Zea mays L.) fields and adjacent greenhouses vegetable fields in three vegetable production areas representing various management intensities in terms of C and N inputs and frequency of tillage; (2) fractionating soil to distinguish intra-aggregate particulate organic matter (iPOM) and organo-mineral complexes (silt + clay). Our results indicated that converting cereal fields to greenhouse vegetable production with intermediate and high management intensity led to increases in SOC and TN and decreases in C:N ratios in the top soil. The accumulation rates of C and N in the surface soil (0–30 cm) were estimated to be 1.37 Mg C ha −1 yr −1 and 0.21 Mg N ha −1 yr −1 over an average period of 8 years after cereal fields to greenhouse vegetable production conversion. At the soil aggregate level, only the coarse (>250 μm) and fine (53–250 μm) iPOM fraction contributed to the increases in soil C (e.g., 49% and 51% of total C increases, respectively), while the coarse and fine iPOM, and silt + clay fraction accounted for 22%, 30% and 48%, respectively, of total N increases. This illustrates how the addition of readily available C (manure) and N (manure and inorganic N) leads to a temporary stabilization of C in relatively labile SOM fractions, but to a preferential stabilization of N in organo-mineral SOM fractions. In conclusion, the conversion to highly intensive vegetable systems in China leads to marked differences in C and N stabilization dynamics.

We predicted changes in yields and direct net soil greenhouse gas (GHG) fluxes from converting conventional to alternative management practices across one of the world's most productive agricultural regions, the Central Valley of California, using the DAYCENT model. Alternative practices included conservation tillage, winter cover cropping, manure application, a 25% reduction in N fertilizer input and combinations of these. Alternative practices were evaluated for all unique combinations of crop rotation, climate, and soil types for the period 1997–2006. The crops included were alfalfa, corn, cotton, melon, safflower, sunflower, tomato, and wheat. Our predictions indicate that, adopting alternative management practices would decrease yields up to 5%. Changes in modeled SOC and net soil GHG fluxes corresponded to values reported in the literature. Average potential reductions of net soil GHG fluxes with alternative practices ranged from −0.7 to −3.3 Mg CO 2 -eq ha −1 yr −1 in the Sacramento Valley and −0.5 to −2.5 Mg CO 2 -eq ha −1 yr −1 for the San Joaquin Valley. While adopting a single alternative practice led to modest net soil GHG flux reductions (on average −1 Mg CO 2 -eq ha −1 yr −1 ), combining two or more of these practices led to greater decreases in net soil GHG fluxes of up to −3 Mg CO 2 -eq ha −1 yr −1 . At the regional scale, the combination of winter cover cropping with manure application was particularly efficient in reducing GHG emissions. However, GHG mitigation potentials were mostly non-permanent because 60–80% of the decreases in net soil GHG fluxes were attributed to increases in SOC, except for the reduced fertilizer input practice, where reductions were mainly attributed to decreased N 2 O emissions. In conclusion, there are long-term GHG mitigation potentials within agriculture, but spatial and temporal aggregation will be necessary to reduce uncertainties around GHG emission reductions and the delivery risk of the associated C credits.

Knowing underlying practices for current greenhouse gas (GHG) emissions is a necessary precursor for developing best management practices aimed at reducing N2O emissions. The effect of no-till management on nitrous oxide (N2O), a potent greenhouse gas, remains largely unclear, especially in perennial agroecosystems. The objective of this study was to compare direct N2O emissions associated with management events in a cover-cropped Mediterranean vineyard under conventional tillage (CT) versus no-till (NT) practices. This study took place in a wine grape vineyard over one full growing season, with a focus on the seven to ten days following vineyard floor management and precipitation events. Cumulative N2O emissions in the NT system were greater under both the vine and the tractor row compared to CT, with 0.13 ± 0.021 kg N2O–N ha−1 growing season−1 emitted from the CT vine compared to 0.19 ± 0.017 kg N2O–N ha−1 growing season−1 emitted from the NT vine and 0.07 ± 0.041 kg N2O–N ha−1 growing season−1 emitted from the CT row compared to 0.11 ± 0.018 kg N2O–N ha−1 growing season−1 from the NT row. Yet these variations were not significant, indicating no differences in seasonal N2O emissions following conversion from CT to NT compared to long-term CT management. Individual management events such as fertilization and cover cropping, however, had a major impact on seasonal emissions, indicating that management events play a critical role in N2O emission patterns.

The integrated soil fertility management paradigm, currently advocated in Sub-Saharan Africa for rehabilitating its soils, recognizes the possible interactive benefits of combining organic residues with mineral fertilizer inputs on agroecosystem functioning. Residue quality may be a controlling factor for any beneficial interactions. The objectives of this study were to determine the effect of different quality organic residues and mineral fertilizer on N cycling under field conditions in Embu, Kenya. We hypothesized that combining low quality residue with mineral N would reduce potential system losses of N by synchronizing N release with plant uptake. Residue treatments consisted of a control (no residue input), high quality tithonia (Tithonia diversifolia) residue (C to N ratio of 13:1) and low quality maize (Zea mays) stover residue (C to N ratio of 42:1) applied at a rate of 1.2 Mg C ha � 1 . Subplots of each residue

Whereas the contribution of agriculture to the emissions of greenhouse gases (GHGs) is well known, especially of NOx gases following the application of N-fertilizer additions, quantitative estimates across fields remain uncertain. Here, we quantified CO2 ,N 2O, and NO emissions from an irrigated field under standard tillage and in a field recently converted (� 5 years) to minimum tillage in Yolo County, California, under a Mediterranean climate. We focused on the spatiotemporal variation of GHG emissions among positions across a seed bed and at the field scale. Seasonal CO2 and N2O fluxes ranged from 4.6 to 52.4 kg C ha � 1 day � 1 and 0 to 23.7 g N ha � 1 day � 1 , respectively. There was a significant seasonal pattern of CO2 emissions as a function of crop growth, while the level of CO2 flux rates varied annually by crop type and the previous year’s soil C inputs. The seasonal N2O emissions coincided with N fertilization placement and irrigation events. With the exception of immediately after N fertilizing, NO emissions were on average 2–33 times lower than N2O emissions. Whereas gross effects of tillage and position in the seed bed on CO2 and N2O emissions were not significant, the emissions were significantly different in a specific seed bed position because of an interaction between tillage and position in the seed bed. For example, N2O fluxes in the side dress position were significantly greater than fluxes from other seed bed positions, and were further accentuated by a significant tillage effect. At the field scale, soil-water content and temperature were generally related to both optimum CO2 and N2O emissions, but the relationships were highly variable. The results suggest that position-specific variations and interaction with tillage should be accounted for to improve the estimates of GHG emissions from irrigated soils.

There is a general lack of information on the effects of irrigation on soil carbon (C) sequestration in (semi)arid regions. For that purpose we present results of the sediment and C budget of a 30 ha furrow-irrigated corn field in the Central Valley in California. This field was monitored to assess the effects of minimum tillage versus standard tillage on soil C sequestration and greenhouse gas emissions. Water samples of two irrigation events in July and August 2004, were collected and analyzed for suspended sediment, dissolved organic C (DOC) and N (DON), total C and N. Field and soil water budgets were estimated from meteorological data, flow measurements of applied irrigation and runoff water, and neutron-probe soil water measurements. Tail waters contained less sediment but more organic C than irrigation waters, due to particle settlement and enrichment in organic matter. Tillage treatment had no significant effect on composition of water or sediment. Furrow irrigation resulted in a net field input of 700 kg sediment ha−1, 21.4 kg C ha−1, and 7.7 kg N ha−1. The added C by the sedimentation accounted for about two-thirds of the total C increase. The corresponding soil C increase associated with these two irrigation events was about 20% of reported yearly C sequestration rates in long-term soil C sequestration experiments. Our experiments showed the importance of time scale in C budgeting for intensively irrigated agroecosystems, where fast dynamics and large variability of inputs are common.

The authors regret that there are corrections relating to the reported yield. Please find the corrections below: 1. Table 2: The tomato harvest/vines shredded row should have the tomato yield as 118 Mg ha . The total tomato fertilization row should have 53 Mg ha 1 deleted. 2. 3. Results: Section 3.1. N2O emissions and tomato yield a. Current: In the conventional system...The yield was 53 Mg ha . b. Correction: In the conventional system...The yield was 78 Mg ha . c. Current: In the integrated system...The yield was 53 Mg ha . d. Correction: In the integrated system...The yield was 118 Mg ha .