Two approaches for net ecosystem carbon budgets and soil carbon sequestration in a rice–wheat rotation system in China

Few studies have comprehensively evaluated the method of estimating the net ecosystem carbon budget (NECB). We compared two approaches for studying the NECB components on the crop seasonal scale as validated by the soil organic carbon (SOC) changes measured over the 5-year period of 2009–2014. The field trial was initiated with four integrated soil–crop system management (ISSM) practices at different nitrogen application rates relative to the local farmer’s practices (FP) rate, namely, N1 (25 % reduction), N2 (10 % reduction), N3 (FP rate) and N4 (25 % increase) with no nitrogen (NN) and FP as the controls. Compared with the FP, the four ISSM scenarios of N1, N2, N3 and N4 significantly increased rice yields by 9.5, 19, 33 and 41 %, while increasing the agronomic nitrogen use efficiency (NUE) by 71, 75, 99 and 79 %, respectively. The SOC sequestration potentials were estimated to be ?0.15 to 0.35 Mg C ha^?1 year^?1 from the net primary production minus heterotrophic respiration approach and ?0.32 to 0.67 Mg C ha^?1 year^?1 from the gross primary production minus ecosystem respiration approach for the 2010–2011 rice–wheat annual cycle. Similarly, the annual topsoil carbon sequestration rate over 2009–2014 was measured to be ?0.22 Mg C ha^?1 year^?1 for the NN plot and 0.13–0.42 Mg C ha^?1 year^?1 for the five fertilized treatments. Both NECB approaches provided a sound basis for accurate assessment of the SOC changes. Compared to the SOC sequestration rate from the FP, the proposed N3 and N4 scenarios increased the SOC sequestration rates while also improving rice yield and NUE.     

Crop yields and soil organic carbon fractions as influenced by straw incorporation in a rice–wheat cropping system in southeastern China

In agro-ecosystems, the relationship between soil fertility and crop yield is mediated by manure application. In this study, an 8-year field experiment was performed with four fertilizer treatments (NPK, NPKM₁, NPKM₂, and NPKM₃), where NPK refers to chemical fertilizer and M₁, M₂, and M₃ refer to manure application rates of 15, 30, and 45 Mg ha^?1 year^?1, respectively. The results showed that the NPKM (NPKM₁, NPKM₂, and NPKM₃) treatments produced greater and more stable yields (4.95–5.45 Mg ha^?1 and 0.59–0.75) than the NPK treatment (4.01 Mg ha^?1 and 0.50). Crop yields under the NPKM treatments showed two trends, with a rate of decrease of 0.48–0.83 Mg ha^?1 year^?1 during the first 4 years and a rate of increase of 0.10–0.25 Mg ha^?1 year^?1 during the last 4 years. The soil organic carbon (SOC) significantly increased under all treatments. The estimated annual SOC decomposition rate was 0.35 Mg ha^?1 year^?1 and the equilibrium SOC level was 6.22 Mg ha^?1. Soil total nitrogen (N), available N, total phosphorus (P) and available P under the NPKM treatments increased by 0.15–0.26, 15–33, 0.17–0.66 and 45–159 g kg^?1, respectively, compared with the NPK treatment. Manure application mainly influenced crop yield by affecting the soil TN, available N, and available P, which accounted for up to 64% of the crop yield variation. Taken together, applying manure can determine or at least improve the effects of soil fertility on crop yield in acidic soils in South China.     

Effects of pig and dairy slurry application on N and P leaching from crop rotations with spring cereals and forage leys

Two crop rotations dominated by spring cereals and grass/clover leys on a clay soil were studied over 2 years with respect to nitrogen (N) and phosphorus (P) leaching associated with pig or dairy slurry application in April, June and October. Leaching losses of total N (TN), total P (TP), nitrate-N and dissolved reactive P (DRP) were determined in separately tile-drained field plots (four replicates). Mean annual DRP leaching after October application of dairy slurry (17 kg P ha^?1) to growing grass/clover was 0.37 kg ha^?1. It was significantly higher than after October application of pig slurry (13 kg ha^?1) following spring cereals (0.16 kg ha^?1) and than in the unfertilised control (0.07 kg P ha^?1). The proportion of DRP in TP in drainage water from the grass/clover crop rotation (35 %) was higher than from the spring cereal rotation (25 %) and the control (14 %). The grass/clover rotation proved to be very robust with respect to N leaching, with mean TN leaching of 10.5 kg ha^?1 year^?1 compared with 19.2 kg ha^?1 year^?1 from the cereal crop rotation. Pig slurry application after cereals in October resulted in TN leaching of 25.7 kg ha^?1 compared with 7.0 kg ha^?1 year^?1 after application to grass/clover in October and 19.1 kg ha^?1 year^?1 after application to spring cereals in April. In conclusion, these results show that crop rotations dominated by forage leys need special attention with respect to DRP leaching and that slurry application should be avoided during wet conditions or combined with methods to increase adsorption of P to soil particles.     

Leaching losses from Kenyan maize cropland receiving different rates of nitrogen fertilizer

Meeting food security requirements in sub-Saharan Africa (SSA) will require increasing fertilizer use to improve crop yields, however excess fertilization can cause environmental and public health problems in surface and groundwater. Determining the threshold of reasonable fertilizer application in SSA requires an understanding of flow dynamics and nutrient transport in under-studied, tropical soils experiencing seasonal rainfall. We estimated leaching flux in Yala, Kenya on a maize field that received from 0 to 200 kg ha^?1 of nitrogen (N) fertilizer. Soil pore water concentration measurements during two growing seasons were coupled with results from a numerical fluid flow model to calculate the daily flux of nitrate-nitrogen (NO₃ ^?-N). Modeled NO₃ ^?-N losses to below 200 cm for 1 year ranged from 40 kg N ha^?1 year^?1 in the 75 kg N ha^?1 year^?1 treatment to 81 kg N ha^?1 year^?1 in the 200 kg N ha^?1 treatment. The highest soil pore water NO₃ ^?-N concentrations and NO₃ ^?-N leaching fluxes occurred on the highest N application plots, however there was a poor correlation between N application rate and NO₃ ^?-N leaching for the remaining N application rates. The drought in the second study year resulted in higher pore water NO₃ ^?-N concentrations, while NO₃ ^?-N leaching was disproportionately smaller than the decrease in precipitation. The lack of a strong correlation between NO₃ ^?-N leaching and N application rate, and a large decrease in flux between 120 and 200 cm suggest processes that influence NO₃ ^?-N retention in soils below 200 cm will ultimately control NO₃ ^?-N leaching at the watershed scale.     

Integrated management practices significantly affect N2O emissions and wheat–maize production at field scale in the North China Plain

In the North China Plain, a field experiment was conducted to measure nitrous oxide (N₂O) and methane (CH₄) fluxes from a typical winter wheat–summer maize rotation system under five integrated agricultural management practices: conventional regime [excessive nitrogen (N) fertilization, flood irrigation, and rotary tillage before wheat sowing; CON], recommended regime 1 (balanced N fertilization, decreased irrigation, and deep plowing before wheat sowing; REC-1), recommended regime 2 (balanced N fertilization, decreased irrigation, and no tillage; REC-2), recommended regime 3 (controlled release N fertilizer, decreased irrigation, and no tillage; REC-3), and no N fertilizer (CK). Field measurements indicated that pulse emissions after N fertilization and irrigation contributed 19–49 % of annual N₂O emissions. In contrast to CON (2.21 kg N₂O-N ha^?1 year^?1), the other treatments resulted in significant declines in cumulative N₂O emissions, which ranged from 0.96 to 1.76 kg N₂O-N ha^?1 year^?1, indicating that the recommended practices (e.g., balanced N fertilization, controlled release N fertilizer, and decreased irrigation) offered substantial benefits for both sustaining grain yield and reducing N₂O emissions. Emission factors of N fertilizer were 0.21, 0.22, 0.23, and 0.37 % under CON, REC-1, REC-3, and REC-2, respectively. Emissions of N₂O during the freeze–thaw cycle period and the winter freezing period accounted for 9.7 and 5.1 % of the annual N₂O budget, respectively. Thus, we recommend that the monitoring frequency should be increased during the freeze–thaw cycle period to obtain a proper estimate of total emissions. Annual CH₄ fluxes from the soil were low (?1.54 to ?1.12 kg CH₄-C ha^?1 year^?1), and N fertilizer application had no obvious effects on CH₄ uptake. Values of global warming potential were predominantly determined by N₂O emissions, which were 411 kg CO₂-eq ha^?1 year^?1 in the CK and 694–982 kg CO₂-eq ha^?1 year^?1 in the N fertilization regimes. When comprehensively considering grain yield, global warming potential intensity values in REC-1, REC-2, and REC-3 were significantly lower than in CON. Meanwhile, grain yield increased slightly under REC-1 and REC-3 compared to CON. Generally, REC-1 and REC-3 are recommended as promising management regimes to attain the dual objectives of sustaining grain yield and reducing greenhouse gas emissions in the North China Plain.     

Corn and soybean’s season-long in-situ nitrogen mineralization in drained and undrained soils

Many factors influence nitrogen (N) mineralization in agricultural soils. Our objective was to quantify cumulative (season-long) net N mineralization in corn (Zea mays L.) and soybean [Glycine max (L.) Merr] in a corn-soybean rotation under different N and soil drainage management (drained and undrained) in poorly-drained soils. In-situ incubations were conducted over two growing seasons using a sequential core-sampling technique to measure net N mineralization. Differential drainage was imposed three-years before this study, in which time, the soil lost 2.2 Mg C ha^?1 year^?1 and 0.14 Mg N ha^?1 year^?1 due to tile-drainage. Overall greater total soil organic carbon (TOC) and total soil nitrogen (TN) in the undrained soil resulted in 2.7 times greater net N mineralization compared to the drained soil in the unfertilized control (0N), but the effect of drainage was inconsistent across years with N fertilization. Across all variables, soils mineralized 2.89% of TN in soybean residue and 0.94% of TN in corn residue. Nitrogen fertilization increased mineralization rate, as high as 9.6 kg N ha^?1 day^?1, compared to <2.2 kg N ha^?1 day^?1 for 0N. Overall, net N mineralization was 3.4 times greater with N fertilizer than the 0N, but fertilization made mineralization more variable. The impact of fertilization on boosting mineralization under differential soil drainage needs further refinement if we are to improve decision-making tools for N application based on soil mineralization predictions.     

Nitrogen balance in dryland agroecosystem in response to tillage,crop rotation,and cultural practice

Accounting of N inputs and outputs and N retention in the soil provides N balance that measures agroecosystem performance and environmental sustainability. Because of the complexity of measurements of some N inputs and outputs, studies on N balance in long-term experiments are scanty. We examined the effect of 8 years of tillage, crop rotation, and cultural practice on N balance based on N inputs and outputs and soil N sequestration rate under dryland cropping systems in the northern Great Plains, USA. Tillage systems were no-tillage (NT) and conventional tillage (CT) and crop rotations were continuous spring wheat (Triticum aestivum L.) (CW), spring wheat–pea (Pisum sativum L.) (W–P), spring wheat–barley (Hordeum vulgaris L.) hay–pea (W–B–P), and spring wheat–barley hay–corn (Zea mays L.)–pea (W–B–C–P). Cultural practices were traditional (conventional seed rates and plant spacing, conventional planting date, broadcast N fertilization, and reduced stubble height) and improved (variable seed rates and plant spacing, delayed planting, banded N fertilization, and increased stubble height). Total N input due to N fertilization, pea N fixation, atmospheric N deposition, crop seed N, and nonsymbiotic N fixation was greater with W–B–C–P than CW, regardless of tillage and cultural practices. Total N output due to aboveground biomass N removal and N losses due to denitrification, volatilization, plant senescence, N leaching, gaseous N (NO_x) emissions, and surface runoff were not different among treatments. Nitrogen sequestration rate at 0–20 cm from 2004 to 2011 varied from 29 kg N ha^?1 year^?1 in CT with W–P to 89 kg N ha^?1 year^?1 in NT with W–P. Nitrogen balance varied from ? 39 kg N ha^?1 year^?1 in NT with CW and the improved practice to 41 kg N ha^?1 year^?1 in CT with W–P and the traditional practice. Because of legume N fixation and increased soil N sequestration rate, diversified crop rotations reduced external N inputs and increased aboveground biomass N removal, N flow, and N balance compared with monocropping, especially in the CT system. As a result, diversified legume–nonlegume crop rotation not only reduced the cost of N fertilization by reducing N fertilization rate, but also can be productive by increasing N uptake and N surplus and environmentally sustainable by reducing N losses compared with nonlegume monocropping, regardless of cultural practices in dryland agroecosystems.     

Seasonal N<Subscript>2</Subscript>O emissions respond differently to environmental and microbial factors after fertilization in wheat–maize agroecosystem

Biogeochemical processes regulating cropland soil nitrous oxide (N₂O) emissions are complex, and the controlling factors need to be better understood, especially for seasonal variation after fertilization. Seasonal patterns of N₂O emissions and abundances of archaeal ammonia monooxygenase (amoA), bacterial amoA, nitrate reductase (narG), nitrite reductase (nirS/nirK), and nitrous oxide reductase (nosZ) genes in long-term fertilized wheat–maize soils have been studied to understand the roles of microbes in N₂O emissions. The results showed that fertilization greatly stimulated N₂O emission with higher values in pig manure-treated soil (OM, 2.88 kg N ha^?1 year^?1) than in straw-returned (CRNPK, 0.79 kg N ha^?1 year^?1) and mineral fertilizer-treated (NPK, 0.90 kg N ha^?1 year^?1) soils. Most (52.2–88.9%) cumulative N₂O emissions occurred within 3 weeks after fertilization. Meanwhile, N₂O emissions within 3 weeks after fertilization showed a positive correlation with narG gene copy number and a negative correlation with soil NO₃^? contents. The abundances of narG and nosZ genes had larger direct effects (1.06) than ammonium oxidizers (0.42) on N₂O emissions according to partial least squares path modeling. Stepwise multiple regression also showed that log narG was a predictor variable for N₂O emissions. This study suggested that denitrification was the major process responsible for N₂O emissions within 3 weeks after fertilization. During the remaining period of crop growth, insufficient N substrate and low temperature became the primary limiting factors for N₂O emission according to the results of the regression models.     

Fertigation combined with catch crop maximize vegetable yield and minimize N and P surplus

Excessive fertilization is a common agricultural practice that often results in high risk of nitrogen (N) and phosphorus (P) losses in vegetable production in China. To reduce these losses, it is crucial to control residual nutrient levels in the rootzone and maintain crop growth. A 3-year field experiment was therefore conducted to investigate the effects of optimal fertigation (OF), OF combined with summer catch crop (OF-SCC; sweet corn with residue incorporation after harvest) or wheat straw application (OF-WSA; soil amended with wheat straw before cucumber seedling transplanting) on soil nutrients, soil residual N and P levels in the rootzone. The conventional management (flood irrigation with excessive fertilization and bare fallow during the summer period) served as control. The results showed that, although OF reduced irrigation amount, N input and P input by 49, 50 and 53%, respectively, it did not affect N and P uptake and fruit yields, and significantly reduced N and P surplus in the rootzone by 60 and 59%, respectively, when compared to the control. The SCC extracted 72–74 kg N ha^?1 year^?1 and 10–13 kg P ha^?1 year^?1 from soils. In addition, SCC and WSA increased soil soluble organic N in the rootzone but had little influence on N and P surplus. Generally, OF was efficient in reducing soil residual N and P, while SCC could temporarily retarded N leaching and improved nutrient recycling in the rootzone. Our results infer that OF combined with SCC is an efficient method for reducing soil N and P losses.     

Particulate and active soil nitrogen fractions are reduced by sheep grazing in dryland cropping systems

Sheep (Ovis aries L.) grazing, a cost-effective method of weed control compared to herbicide application and tillage, may influence N cycling by consuming crop residue and weeds and returning N through feces and urine to the soil. The objective of this experiment was to evaluate the effect of sheep grazing compared to tillage and herbicide application for weed control on soil particulate and active soil N fractions in dryland cropping systems. Our hypothesis was that sheep grazing used for weed control would increase particulate and active soil N fractions compared to tillage and herbicide application. Soil samples collected at the 0–30 cm depth from a Blackmore silt loam were analyzed for particulate organic N (PON), microbial biomass N (MBN), and potential N mineralization (PNM) under dryland cropping systems from 2009 to 2011 in southwestern Montana, USA. Treatments were three weed management practices [sheep grazing (grazing), herbicide application (chemical), and tillage (mechanical)] as the main plot and two cropping sequences [continuous spring wheat (Triticum aestivum L.; CSW) and spring wheat–pea (Pisum sativum L.)/barley (Hordeum vulgare L.) mixture hay–fallow; W–P/B–F] as the split-plot factor arranged in randomized complete block with three replications. The PON and MBN at 0–30 cm were greater in the chemical or mechanical than the grazing treatment with CSW. The PNM at 15–30 cm was greater in the chemical or mechanical than the grazing treatment in 2009 and 2011 and at 5–15 cm was greater with W–P/B–F than CSW in 2010. From 2009 to 2011, PON at 0–30 cm and PNM at 15–30 cm reduced from 2 to 580 kg N ha^?1 year^?1 in the grazing and chemical treatments, but the rate varied from ?400 to 2 kg N ha^?1 year^?1 in the mechanical treatment. Lower amount of labile than nonlabile organic matter returned to the soil through feces and urine probably reduced soil active and coarse organic matter N fractions with sheep grazing compared to herbicide application and tillage for weed control. Reduction in the rate of decline in N fractions from 2009 to 2011 compared to the herbicide application treatment, however, suggests that sheep grazing may stabilize N fractions in the long-term if the intensity of grazing is reduced. Animal grazing may reduce soil N fractions in annual cropping systems in contrast to known increased fractions in perennial cropping systems.     

Long-term tillage,straw and N rate effects on some chemical properties in two contrasting soil types in Western Canada

Long-term use of soil, crop residue and fertilizer management practices may affect some soil properties, but the magnitude of change depends on soil type and climatic conditions. Two field experiments with barley, wheat, or canola in a rotation on Gray Luvisol (Typic Cryoboralf) loam at Breton and Black Chernozem (Albic Argicryoll) loam at Ellerslie, Alberta, Canada, were conducted to determine the effects of 19 or 27 years (from 1980 to 1998 or 2006 growing seasons) of tillage (zero tillage [ZT] and conventional tillage [CT]), straw management (straw removed [S_Rem] and straw retained [S_Ret]) and N fertilizer rate (0, 50 and 100 kg N ha^?1 in S_Ret, and 0 kg N ha^?1 in S_Rem plots) on pH, extractable P, ammonium-N and nitrate–N in the 0–7.5, 7.5–15, 15–30 and 30–40 cm or 0–15, 15–30, 30–60, 60–90 and 90–120 cm soil layers. The effects of tillage, crop residue management and N fertilization on these chemical properties were usually similar for both contrasting soil types. There was no effect of tillage and residue management on soil pH, but application of N fertilizer reduced pH significantly (by up to 0.5 units) in the top 15 cm soil layers. Extractable P in the 0–15 cm soil layer was higher or tended to be higher under ZT than CT, or with S_Ret than S_Rem in many cases, but it decreased significantly with N application (by 18.5 kg P ha^?1 in Gray Luvisol soil and 20.5 kg P ha^?1 in Black Chernozem soil in 2007). Residual nitrate–N (though quite low in the Gray Luvisol soil in 1998) increased with application of N (by 17.8 kg N ha^?1 in the 0–120 cm layer in Gray Luvisol soil and 23.8 kg N ha^?1 in 0–90 cm layer in Black Chernozem soil in 2007) and also indicated some downward movement in the soil profile up to 90 cm depth. There was generally no effect of any treatment on ammonium-N in soil. In conclusion, elimination of tillage and retention of straw increased but N fertilization decreased extractable P in the surface soil. Application of N fertilizer reduced pH in the surface soil, and showed accumulation and downward leaching of nitrate–N in the soil profile.     

Assessment of nutrient returns in a tropical dry forest after clear-cut without burning

Tropical dry forests (TDFs) are being deforested at unprecedented rates. The slash/burn/agriculture/fallow-extensive livestock sequence causes significant nutrient losses and soil degradation. Our aim is to assess nutrient inputs and outputs in a TDF area under an alternative management system, for exclusive wood production. The study involved clear-cutting a preserved caatinga TDF site without burning, quantifying nutrients exported in firewood/timber and nutrients returned to the soil from the litter layer plus the slash debris, left to decompose unburned on the soil surface. Before clear-cut, the litter layer on the forest floor contained 6.1 t ha of dry matter (DM). After clear-cut, the aboveground biomass was 61.9 t DM ha^?1 (consisting of 21.5 t DM ha^?1 of commercial wood and 40.4 t DM ha^?1 of clear-cut debris that did not include the underlying litter layer). The litter layer was composed of fine and coarse litter, with turnovers of 0.86 and 0.31 year^?1, respectively, separately measured in uncut control plots during two rainy seasons (Dec-2007/June-2008 and Dec-2008/June-2009). In a single season, its decomposition returned to the soil 48.4, 1.16 and 12.3 kg ha^?1 of N, P and K. The clear-cut debris was mainly composed of branches, 33.4 t ha^?1, bromeliads, 5.63 t ha^?1 and green leaves, 1.32 t ha^?1. In-situ decomposition rates for branches and bromeliads were 0.24 and 1.47 year^?1, respectively. After two rainy seasons the clear-cut debris released 206, 6.5 and 106 kg ha^?1 of N, P and K respectively. This input plus that of the underlying litter layer exceeded exports in the commercial wood, and replenished a soil nutrient stock (0–30 cm) of approximately the same magnitude.     

Differential retention of carbon, nitrogen and phosphorus in grassland soil profiles with long-term manure application

Liquid hog manure (LHM) is a valuable source of nutrients for farm production. Long-term experimental plots that had received LHM applications of 0, 50, and 100 m³ ha^?1 annually for 20 years were analyzed for total soil C, N and P storage. Applications increased total soil N and P by 1,200 kg N ha^?1 and 850 kg P ha^?1 at 100 m^?3 LHM year^?1, compared to the control treatment. However, C storage did not increase with LHM rates and was lower in the 50 m³ ha^?1 LHM treatment (86 Mg C ha^?1) than in the 0 or 100 m³ ha^?1 treatments (100 Mg C ha^?1). In addition to the limited quantities and high decomposability of the C supplied by LHM, it is hypothesized that LHM stimulated the mineralization of both native soil C and fresh root-derived material. This priming effect was particularly apparent in deeper soil horizons where the decomposability of native C may be limited by the supply of fresh C. This study indicates that while LHM can be a significant source of crop nutrients, it has limited capacity for maintaining or increasing soil C.     

Mitigation potential of greenhouse gases under different scenarios of optimal synthetic nitrogen application rate for grain crops in China

Proper management of synthetic nitrogen (N) fertilizer can reduce direct N₂O emission from soil and indirect CO₂ emission from production and transportation of synthetic N. In the late 1990s, the average application rates of synthetic N were 212, 207 and 207 kg ha^?1, respectively, for rice, wheat, and maize in China’s croplands. But research suggests that the optimal synthetic N application rates for the main grain crops in China should be in the range of 110–150 kg ha^?1. Excessive application of synthetic N has undoubtedly resulted in massive emission of greenhouse gases. Therefore, optimizing N application rates for grain crops in China has a great potential for mitigating the emission of greenhouse gases. Nevertheless, this mitigation potential (MP) has not yet been well quantified. This study aimed at estimating the MP of N₂O and CO₂ emissions associated with synthetic N production and transportation in China based on the provincial level statistical data. Our research indicates that the total consumption of synthetic N on grain crops in China can be reduced by 5.0–8.4 Tg yr^?1 (28–47 % of the total consumption) if the synthetic N application rate is controlled at 110–150 kg ha^?1. The estimated total MP of greenhouse gases, including direct N₂O emission from croplands and indirect CO₂ emission from production and transportation of synthetic N, ranges from 41.7 to 70.1 Tg CO₂_eq. yr^?1. It was concluded that reducing synthetic N application rate for grain crops in China to a reasonable level of 110–150 kg ha^?1 can greatly reduce the emission of greenhouse gases, especially in the major grain-crop production provinces such as Shandong, Henan, Jiangsu, Hebei, Anhui and Liaoning.     

Nitrate leaching from organic and conventional arable crop farms in the Seine Basin (France)

In the Seine Basin, characterised by intensive arable crops, most of the surface and groundwater is contaminated by nitrate (NO₃ ^?). The goal of this study is to investigate nitrogen leaching on commercial arable crop farms in five organic and three conventional systems. In 2012–2013, a total of 37 fields are studied on eight arable crop rotations, for three different soil and climate conditions. Our results show a gradient of soil solution concentrations in function of crops, lower for alfalfa (mean 2.8 mg NO₃-N l^?1) and higher for crops fertilised after legumes (15 mg NO₃-N l^?1). Catch crops decrease nitrate soil solution concentrations, below 10 mg NO₃-N l^?1. For a full rotation, the estimated mean concentrations is lower for organic farming, 12 ± 5 mg NO₃-N l^?1 than for conventional farming 24 ± 11 mg NO₃-N l^?1, with however a large range of variability. Overall, organic farming shows lower leaching rates (14–50 kg NO₃-N ha^?1) than conventional farms (32–77 kg NO₃-N ha^?1). Taking into account the slightly lower productivity of organic systems, we show that yield-scaled leaching values are also lower for organic (0.2 ± 0.1 kg N kg^?1 N year^?1) than for conventional systems (0.3 ± 0.1 kg N kg^?1 N year^?1). Overall, we show that organic farming systems have lower impact than conventional farming on N leaching, although there is still room for progress in both systems in commercial farms.     

Can litter production and litter decomposition improve soil properties in the rubber plantations of different ages in Côte d’Ivoire?

Litter production and litter decomposition influence the availability of nutrients in the soil. The investigation aimed at characterizing the dynamics of leaf litter decomposition, and soil physico-chemical and biological parameters in rubber plantations of different ages. During a 12-months’ period, field studies were done in 7-, 12-, and 25-year-old rubber plantations. For measuring of litter decomposition and input from aboveground, 324 litter bags and 27 litter traps (1 m?×?1 m) were placed in 3 sampling areas per age class of rubber plantations. The soil parameters were also characterized. The results showed that the annual litter production and the amounts of organic carbon in leaves increased with the aging of the plantations. The annual decomposition constant (k) ranged from 0.0381?±?0.0040 year^?1 in the 25-year-old plantations to 0.0767?±?0.0111 year^?1 in the 7-year-old plantations. The annually decomposed litter mass varied between 2.7?±?0.3 t ha^?1 year^?1 in the 12-year-old plantations to 4.2?±?0.3 t ha^?1 year^?1 in the 25-year-old plantations. The soil of the 25-year-old plantations showed higher values of most physico-chemical and biological variables as compared to the 7-year-old plantations: annual litter production (+?32%), annual litter mass decomposed (+?11%), annual carbon (+?15%) and nitrogen (+?11%) inputs, soil organic carbon (+?52%), total nitrogen (+?32%), soil organic matter (+?52%), soil water content (+?74%), and the total density of soil invertebrates (+?121%). The results indicate an improvement of soil properties with the aging of the rubber plantations and the importance of this agricultural system for carbon sequestration.     

Nitrogen balance in a stockless organic cropping system with different strategies for internal N cycling via residual biomass

A major future challenge in agriculture is to reduce the use of new reactive nitrogen (N) while maintaining or increasing productivity without causing a negative N balance in cropping systems. We investigated if strategic management of internal biomass N resources (green manure ley, crop residues and cover crops) within an organic crop rotation of six main crops, could maintain the N balance. Two years of measurements in the field experiment in southern Sweden were used to compare three biomass management strategies: anaerobic digestion of ensiled biomass and application of the digestate to the non-legume crops (AD), biomass redistribution as silage to non-legume crops (BR), and leaving the biomass in situ (IS). Neither aboveground crop N content from soil, nor the proportion of N derived from N₂ fixation in legumes were influenced by biomass management treatment. On the other hand, the allocation of N-rich silage and digestate to non-legume crops resulted in higher N₂ fixation in AD and BR (57 and 58 kg ha^?1 year^?1), compared to IS (33 kg ha^?1 year^?1) in the second study year. The N balance ranged between ??9.9 and 24 kg N ha^?1, with more positive budgets in AD and BR than in IS. The storage of biomass for reallocation in spring led to an increasing accumulation of N in the BR and AD systems from one year to another. These strategies also provide an opportunity to supply the crop with the N when most needed, thereby potentially decreasing the risk of N losses during winter.     

Long-term soil accumulation of potentially toxic elements and selected organic pollutants through application of recycled phosphorus fertilizers for organic farming conditions

The pollutant input and the risk of contamination by long-term application of recycled P fertilizers (RPFs) in European agricultural soils were estimated by a mass balance approach. Calculations based on literature data were carried out for the potentially toxic elements (PTEs) Cd, Cr, Cu, Ni, Pb, Zn and for the persistent organic pollutants (POPs) PCBs, PAHs and polychlorinated dibenzo-dioxins and -furans. The PTE accumulation estimation during 200 years of fertilizer application, equivalent to 11 kg P ha^?1 year^?1, the mean P export via harvested products on European stockless organic farms, regarded soil background concentrations and proposed threshold concentrations. For PTEs, inputs were fertilizer application, atmospheric deposition and liming, output processes were leaching and crop harvest. The effect of varying site conditions was assessed by considering two precipitation excess (F) values and two soil pH values. For POPs, fertilizer application and the half-life time were considered. The PTE accumulation risk was low for most RPFs. For the analysed POPs no accumulation was found. The highest accumulation was found for all PTEs at pH 7 and F = 0.1 m year^?1, the lowest at pH 5 and F = 0.3 m year^?1. A high P concentration in fertilizers resulted in a low PTE flow per unit of P. Composts had the highest PTE accumulation risk due to lowest P contents. Struvite, meat and bone meal, digestate of catering waste, ash and biosolids would be better suited as P fertilizers. The use of fertilizers should be regulated based on their pollutant-to-nutrient ratio.     

Amending soil with used cooking oil to reduce nitrogen losses after cole crop harvest: a 15N study

After cole crop harvest, over 400 kg N ha^?1 may remain in the field as crop residues and soil mineral N. Thus, methods to reduce potential post-harvest N losses are needed. Urea with 5 % ¹⁵N excess was incorporated in mini-plots to produce ¹⁵N enriched broccoli (Brassica olecerea var italica L.). The fate of above-ground crop residue-derived N (¹⁵N_{residue-above}) and below-ground residual fertilizer or root biomass N (¹⁵N_{residue-below}) were studied from broccoli harvest (Aug and Sept 2011) to spring wheat (Triticum durum L.) harvest (July 2012), with and without an amendment of used cooking oil. The ¹⁵N_{residue-below} remained mostly as organic N, was not influenced by the amendment, and was resistant to post-harvest losses. With the oil amendment, soil mineral ¹⁵N_{residue-above} was reduced by 19 kg ha^?1 and microbial biomass ¹⁵N_{residue-above} was increased by 21 kg ha^?1 2 weeks after broccoli harvest, indicating immobilization of ¹⁵N_{residue-above} and reduced potential N losses. At spring wheat harvest, amended soil had greater total, organic, and mineral ¹⁵N_{residue-above} compared to the unamended control, by 44, 43, and 0.75 kg ha^?1, respectively. The amendment increased the recovery of ¹⁵N_{residue-above} in the soil total N pool by 209 % at spring wheat harvest, and it did not affect spring wheat yields or plant ¹⁵N_{residue-above} content. It is possible that the amendment facilitated the incorporation of ¹⁵N_{residue-above} into organic compounds, which were less susceptible to losses. Growers should consider applying used cooking oil at harvest to minimize potential N losses and to increase the soil organic N fraction.     

Reducing nitrous oxide emissions and optimizing nitrogen-use efficiency in dryland crop rotations with different nitrogen rates

Recent interests in improving agricultural production while minimizing environmental footprints emphasized the need for research on management strategies that reduce nitrous oxide (N₂O) emissions and increase nitrogen-use efficiency (NUE) of cropping systems. This study aimed to evaluate N₂O emissions, annualized crop grain yield, emission factor, and yield-scaled- and NUE-scaled N₂O emissions under continuous spring wheat (Triticum aestivum L.) (CW) and spring wheat–pea (Pisum sativum L.) (WP) rotations with four N fertilization rates (0, 50, 100, and 150 kg N ha^?1). The N₂O fluxes peaked immediately after N fertilization, intense precipitation, and snowmelt, which accounted for 75–85% of the total annual flux. Cumulative N₂O flux usually increased with increased N fertilization rate in all crop rotations and years. Annualized crop yield and NUE were greater in WP than CW for 0 kg N ha^?1 in all years, but the trend reversed with 100 kg N ha^?1 in 2013 and 2015. Crop yield maximized at 100 kg N ha^?1, but NUE declined linearly with increased N fertilization rate in all crop rotations and years. As N fertilization rate increased, N fertilizer-scaled N₂O flux decreased, but NUE-scaled N₂O flux increased non-linearly in all years, regardless of crop rotations. The yield-scaled N₂O flux decreased from 0 to 50 kg N ha^?1 and then increased with increased N fertilization rate. Because of non-significant difference of N₂O fluxes between 50 and 100 kg N ha^?1, but increased crop yield, N₂O emissions can be minimized while dryland crop yields and NUE can be optimized with 100 kg N ha^?1, regardless of crop rotations.