How winter cover crops and tillage intensities affect nitrogen availability in eggplant

This study evaluates the fate of nitrogen (N) content in winter cover crops under different tillage intensities. Field trials were conducted over a 2-year period in a Mediterranean environment adopting a cover crop–eggplant sequence. The treatments were: three cover crops (hairy vetch, oat and oilseed rape); three tillage intensities (residue left on soil surface, shallow green manure and deep green manure). The measurements included: cover crop and eggplant characteristics, N mineralization from cover crops, soil inorganic N and soil CO₂ emission. At cover crop termination, N accumulated in the cover crops was 207, 77 and 77 kg N ha^?1 in hairy vetch, oat and oilseed rape, respectively. Tillage intensity affected biomass decomposition and N mineralization from cover crop residues which were slower when residues were left on soil surface (54 and 71%, respectively) than when incorporated into the soil (66 and 79%, respectively). Hairy vetch showed a greater ability to supply N to eggplant (151 kg N ha^?1), due to the fast decay of its residues, consequently, the N balance index was always high after hairy vetch throughout eggplant cultivation. N mineralized by cover crops was positively correlated with total soil CO₂ emission and soil inorganic N. Placing cover crop residues on soil surface enhances synchronization between N mineralized and eggplant N demand in hairy vetch, while in oat it appears to mitigate the shortage of soil inorganic N for the following vegetable. These findings may also be extended to other summer vegetables which have similar requirements to the eggplant.     

Land management between crops affects soil inorganic nitrogen balance in a tropical rice system

Sustainable production of lowland rice (Oryza sativa L.) requires minimising undesirable soil nitrogen (N) losses via nitrate (NO₃ ^?) leaching and denitrification. However, information is limited on the N transformations that occur between rice crops (fallow and land preparation), which control indigenous N availability for the subsequent crop. In order to redress this knowledge gap, changes in NO₃ ^? isotopic composition (δ¹⁵N and δ¹⁸O) in soil and water were measured from harvest through fallow, land preparation, and crop establishment in a 7 year old field trial in the Philippines. During the period between rice crops, plots were maintained either, continuously flooded, dry, or alternately wet and dry from rainfall. Plots were split with addition or removal of residue from the previous rice crop. No N fertilizer was applied during the experimental period. Nitrogen accumulated during the fallow (20 kg NH₄ ⁺–N ha^?1 in flooded treatments and 10 kg NO₃ ^?–N ha^?1 in treatments with drying), but did not influence N availability for the subsequent crop. Nitrate isotope fractionation patterns indicated that denitrification drove this homogenisation: during land preparation ~50 % of inorganic N in the soil (top 10 cm) was denitrified, and by 2 weeks after transplanting this increased to >80 % of inorganic N, regardless of fallow management. The 17 days between fallow and crop establishment controlled not only N attenuation (3–7 kg NO₃ ^?–N ha^?1 denitrified), but also N inputs (3–14 kg NO₃ ^?–N ha^?1 from nitrification), meaning denitrification was dependent on soil nitrification rates. While crop residue incorporation delayed the timing of N attenuation, it ultimately did not impact indigenous N supply. By measuring NO₃ ^? isotopic composition over depth and time, this study provides unique in situ measurements of the pivotal role of land preparation in determining paddy soil indigenous N supply.     

Nitrous oxide and methane emissions from cultivated seasonal wetland (dambo) soils with inorganic,organic and integrated nutrient management

In many smallholder farming areas southern Africa, the cultivation of seasonal wetlands (dambos) represent an important adaptation to climate change. Frequent droughts and poor performance of rain-fed crops in upland fields have resulted in mounting pressure to cultivate dambos where both organic and inorganic amendments are used to sustain crop yields. Dambo cultivation potentially increases greenhouse gas (GHG) emissions. The objective of the study was to quantify the effects of applying different rates of inorganic nitrogen (N) fertilisers (60, 120, 240 kg N ha^?1) as NH₄NO₃, organic manures (5,000, 10,000 and 15,000 kg ha^?1) and a combination of both sources (integrated management) on GHG emissions in cultivated dambos planted to rape (Brassica napus). Nitrous oxide (N₂O) emissions in plots with organic manures ranged from 218 to 894 µg m^?2 h^?1, while for inorganic N and integrated nutrient management, emissions ranged from 555 to 5,186 µg m^?2 h^?1 and 356–2,702 µg m^?2 h^?1 respectively. Cropped and fertilised dambos were weak sources of methane (CH₄), with emissions ranging from ?0.02 to 0.9 mg m^?2 h^?1, while manures and integrated management increased carbon dioxide (CO₂) emissions. However, crop yields were better under integrated nutrient management. The use of inorganic fertilisers resulted in higher N₂O emission per kg yield obtained (6–14 g N₂O kg^?1 yield), compared to 0.7–4.5 g N₂O kg^?1 yield and 1.6–4.6 g N₂O kg^?1 yield for organic manures and integrated nutrient management respectively. This suggests that the use of organic and integrated nutrient management has the potential to increase yield and reduce yield scaled N₂O emissions.     

Nitrogen and phosphorus benefits from faba bean (Vicia faba L.) residues to subsequent wheat crop in the humid highlands of Ethiopia

Faba bean–wheat rotation is one of the traditional cropping systems in most parts of the temperate, Mediterranean and tropical highland areas. However, the net contribution of legumes to soil nutrient balance is determined by the extent to which crop residue is removed from the field. Therefore, we assessed two possible faba bean residue management scenarios and their role in the faba bean–wheat rotation system in a two-phase field experiment. We further tested to what extent high N₂-fixing and P efficient faba bean varieties could benefit subsequently grown wheat. In the first phase, three improved faba bean varieties (Degaga, Moti, Obse) were grown at four levels of P fertilization (0, 10, 20 and 30 kg P ha^?1) along with local faba bean and reference wheat but without any fertilization. N₂-fixation, soil N balance and P uptake were determined for the faba beans. The N balance was determined via two possible residue management scenarios: scenario-I assumed that all the aboveground biomass is exported from the fields; scenario-II assumed that all the above ground biomass except grains and empty pods is incorporated to the soil. In the second phase, the N and P benefits of faba beans to rotational wheat were assessed. Scenario-I gave a negative net N balance (kg N ha^?1) in the range of ?86.5 ± 5.8 (Degaga) to ?9.4 ± 8.7 (Moti) with significant differences between varieties. Scenario-II showed that all balances were significantly (P < 0.01) improved and the varieties were positively contributing N to the system in the range of 50.6 ± 13.4 (Degaga) to 168.3 ± 13.7 (Moti) kg N ha^?1, which is equivalent to 110–365 kg N ha^?1 in the form of urea (46 % N). In the second crop phase, biomass and grain yield of wheat grown after the faba beans improved significantly (P < 0.05) by 112 and 82 %, respectively compared to the yield of wheat after wheat. Phosphorus application to the preceding faba bean varieties significantly improved total biomass and grain yield of the succeeding wheat (R² = 0.97). The incorporated legume root, nodule and straw clearly played a role in improving wheat yield through N addition via BNF and straw P. The study demonstrates the prospects and importance of improved faba bean germplasm and management as a key component for sustainable wheat based cropping systems in the humid tropical highlands.     

Nitrogen dynamics and yields of fresh bean and sweet corn with different cover crops and planting dates

Cover crops are recommended to mitigate N losses but effects on crop productivity have been variable and often negative. A better understanding of management-specific cover crop systems may lead to yield improvements. In 2011–2014, a split–split-plot factorial field experiment, with four replicates assessed the impact of cover crop (five species and no cover crop controls) and planting date (August vs. September) on crop yield and N dynamics. Fresh bean (Phaseolus vulgaris L.) and sweet corn (Zea mays L. var. rugosa) were grown with 45 and 101 kg N ha^?1 fertilizer, respectively, except for the no cover crop without fertilizer control (Nocc-NoN). Although there was a cover crop by planting date interaction (P < 0.05) for cover crop biomass and N accumulation in the fall and spring, this interaction was not observed in main crop yield nor N concentration and accumulation, suggesting that simply growing a cover crop and below-ground effects may be more influential than the quantity and quality of above-ground biomass. Based on greater yields (6.9%) than the no cover crop with fertilizer (Nocc+N) control, all cover crops tested were recommended before corn but only the oat cover crop increased fresh bean yield (by 10.5%), which suggests crop-specific recommendations are needed. Yield improvements could not be attributed to N fertility alone, suggesting another mechanism was involved. The increased productivity with cover crops while minimizing potential for N losses in the non-growing season has important implications for sustainable agroecosystems and food security.     

Soil N mineralization in a dairy production system with grass and forage crops

This paper describes the dynamics of soil N mineralization in the experimental intensive dairy farming system ‘De Marke’ on a dry sandy soil in the Netherlands. We hypothesized that knowledge of the effects of crop rotation on soil N mineralization and of the spatial and temporal variability of soil N mineralization in a farming system can be used to better synchronize N application with crop N requirements, and hence to increase the recovery of applied N and to reduce N losses. Soil N mineralization was recorded continuously in the soil layer 0–0.30 m, from 1992 to 2005, using a sequential in situ coring technique on six observation plots, of which two were located in permanent grassland and four in crop rotations with a 3 year grassland phase and an arable phase of 3 or 5 years, dominated by maize. Average annual soil N mineralization was highest under permanent grassland: 381 kg ha^?1 and lowest under ≥3rd years arable crops: 184 kg ha^?1. In temporary grassland, soil N mineralization increased in the order: 1st year, 2nd year, 3rd year grassland and in arable crops after grassland mineralization decreased in the order: 1st year, 2nd year, ≥3rd years. Total mineral N input, i.e. the sum of N mineralization and mineral N supply to soil, exceeded crop N requirements in 1st year maize and was lower than the requirements in 1st year temporary grassland. N mineralization in winter, outside the growing season, was 77 kg ha^?1 in maize and 60 kg ha^?1 in grassland. This points at the importance of a suitable catch crop to reduce the susceptibility to N leaching. Temporal and spatial variability of soil N mineralization was high and could not be related to known field conditions. This limits the extent to which N fertilization can be adjusted to soil N mineralization. Variability increased with the magnitude of soil N mineralization. Hence, situations with high soil N mineralization may be associated with high risks for N losses and to reduce these risks a strong build-up of soil organic N should be avoided. This might be achieved, for instance, by fermenting slurry before application on farmland to enhance the fraction mineral N in slurry at the expense of organic N.     

Pea–barley intercropping and short-term subsequent crop effects across European organic cropping conditions

Grain legumes are known to increase the soil mineral nitrogen (N) content, reduce the infection pressure of soil borne pathogens, and hence enhance subsequent cereals yields. Replicated field experiments were performed throughout W. Europe (Denmark, United Kingdom, France, Germany and Italy) to asses the effect of intercropping pea and barley on the N supply to subsequent wheat in organic cropping systems. Pea and barley were grown either as sole crops at the recommended plant density (P100 and B100, respectively) or in replacement (P50B50) or additive (P100B50) intercropping designs. In the replacement design the total relative plant density is kept constant, while the additive design uses the optimal sole crop density for pea supplementing with ‘extra’ barley plants. The pea and barley crops were followed by winter wheat with and without N application. Additional experiments in Denmark and the United Kingdom included subsequent spring wheat with grass-clover as catch crops. The experiment was repeated over the three cropping seasons of 2003, 2004 and 2005. Irrespective of sites and intercrop design pea–barley intercropping improved the plant resource utilization (water, light, nutrients) to grain N yield with 25–30% using the Land Equivalent ratio. In terms of absolute quantities, sole cropped pea accumulated more N in the grains as compared to the additive design followed by the replacement design and then sole cropped barley. The post harvest soil mineral N content was unaffected by the preceding crops. Under the following winter wheat, the lowest mineral N content was generally found in early spring. Variation in soil mineral N content under the winter wheat between sites and seasons indicated a greater influence of regional climatic conditions and long-term cropping history than annual preceding crop and residue quality. Just as with the soil mineral N, the subsequent crop response to preceding crop was negligible. Soil N balances showed general negative values in the 2-year period, indicating depletion of N independent of preceding crop and cropping strategy. It is recommended to develop more rotational approaches to determine subsequent crop effects in organic cropping systems, since preceding crop effects, especially when including legumes, can occur over several years of cropping.     

Gross nitrogen mineralization in pulse-crop rotations on the Northern Great Plains

Pulse crops represent an ever-increasing proportion of cropping systems in the Northern Great Plains. Previous studies have noted apparent benefits associated with pulse crop production that extend beyond the reduced need for N fertilizer in the year of production; these benefits have been attributed to the quality of pulse residues and their effects on N dynamics in subsequent years. This study used isotope dilution techniques to quantify the N-cycling effects of pulse crops in the rotation. Gross N mineralization was measured over three growing seasons at two Agriculture and Agri-Food Canada research sites in Saskatchewan, Canada: Scott (four rotations; one with pulse crop) and Swift Current (three rotations; one with pulse crop). Gross nitrification and the relative contribution of nitrification vs. denitrification to N₂O emissions were also measured. Across all dates and rotations, the average gross mineralization rate at Scott was 2.0 ± 4.0 mg NH₄ ⁺-N kg^?1 soil d^?1 and at Swift Current was 1.4 ± 3.9 mg NH₄ ⁺-N kg^?1 soil d^?1. At both sites, rates were highly variable across the growing season, but tended to be higher at anthesis than either pre-seeding or post-harvest. The only significant difference among rotations was at Swift Current, where the fertilized continuous wheat rotation had the highest gross mineralization rates (rotation average: 2.3 mg NH₄ ⁺-N kg^?1 soil d^?1). The lack of difference among most rotations was particularly notable given the differences in residue quantity among the crops. Ultimately, the lower quantity of residues produced by pulse crops appears to be offset by their higher quality.     

Effects of plant residues on crop performance, N mineralisation and microbial activity including field CO2 and N2O fluxes in unfertilised crop rotations

The impacts of crop rotation and input of organic matter in the form of green manure crops, straw residues and incorporation of catch crops on crop yield, nitrogen uptake, microbial biomass and activity were studied in unfertilised crop rotations differing in input of plant residues, i.e., high-input rotations with a grass-clover crop and catch crops included and low-input cereal rotations without catch crops. The parameters studied included substrate induced respiration (SIR), hydrolysis of fluorescein diacetate (FDA), arylsulfatase activity (ASA), N mineralisation, N₂O emission, and soil respiration. These parameters were measured in bare soil plots, to estimate the effects of previous years' crops and input of plant residues. In neighbouring plots crop performances were registered by measuring yields, above-ground biomass and nitrogen uptake during the growing season. Generally, all measured parameters were significantly higher in the high-input than in low-input rotations. Estimates of metabolic quotients indicated that the microbial communities in the low-input rotations were less efficient in utilising the C sources than those in the high-input rotations. Calculations of N₂O emission factors indicate that the current IPCC methodology for estimating N₂O emission from plant residues needs to be improved.     

Contribution of relay intercropping with legume cover crops on nitrogen dynamics in organic grain systems

Nitrogen (N) management is a key issue in livestock-free organic grain systems. Relay intercropping with a legume cover crop can be a useful technique for improving N availability when two cash crops are grown successively. We evaluated the benefits of four relay intercropped legumes (Medicago lupulina, Medicago sativa, Trifolium pratense and Trifolium repens) on N dynamics and their contribution to the associated and subsequent cash crops in six fields of organic farms located in South-East France. None of the relay intercropped legumes affected the N uptake of the associated winter wheat but all significantly increased the N uptake of the succeeding spring crop, either maize or spring wheat. The improvement of the N nutrition of the subsequent maize crop induced a 30 % increase in grain yield. All relay intercropped legumes enriched the soil–plant system in N through symbiotic fixation. From 71 to 96 % of the N contained in the shoots of the legumes in late autumn was derived from the atmosphere (Ndfa) and varied between 38 and 67 kg Ndfa ha^?1. Even if the cover crop is expected to limit N leaching during wintertime, the presence of relay intercropped legumes had no significant effect on N leaching during winter compared to the control.     

Soil organic matter,greenhouse gases and net global warming potential of irrigated conventional,reduced-tillage and organic cropping systems

Reducing tillage intensity and diversifying crop rotations may improve the sustainability of irrigated cropping systems in semi-arid regions. The objective of this study was to compare the greenhouse gas (GHG) emissions, soil organic matter, and net global warming potential (net GWP) of a sugar beet (Beta vulgaris L.)-corn (Zea mays L,) rotation under conventional (CT) and reduced-tillage (RT) and a corn-dry bean (Phaseolus vulgaris L.) rotation under organic (OR) management during the third and fourth years of 4-year crop rotations. The gas and soil samples were collected during April 2011–March 2013, and were analyzed for carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) emissions, water-filled pore space (WFPS), soil nitrate (NO₃ ^?–N) and ammonium (NH₄ ⁺–N) concentrations, soil organic carbon (SOC) and total nitrogen (TN), and net global warming potential (net GWP). Soils under RT had 26% lower CO₂ emissions compared to 10.2 kg C ha^?1 day^?1 and 43% lower N₂O emissions compared to 17.5 g N ha^?1 day^?1 in CT during cropping season 2011, and no difference in CO₂ and N₂O emissions during cropping season 2012. The OR emitted 31% less N₂O, but 74% more CO₂ than CT during crop season 2011. The RT had 34% higher SOC content than CT (17.9 Mg ha^?1) while OR was comparable with CT. Net GWP was negative for RT and OR and positive for CT. The RT and OR can increase SOC sequestration, mitigate GWP and thereby support in the development of sustainable cropping systems in semiarid agroecosystems.     

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.     

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.     

Net ecosystem CO2 exchange and carbon cycling in tropical lowland flooded rice ecosystem

The seasonal fluxes of CO₂ and its characteristics with relation to environmental variables were investigated under tropical lowland flooded rice paddies employing the open path eddy covariance technique. The seasonal net ecosystem carbon budget was quantified by empirical modelling approach. The integrated net ecosystem exchange (NEE), gross primary production (GPP) and ecosystem respiration (RE) in the flooded rice field was ?448, 811 and 363 g C m^?2 in wet season. Diurnal variations of mean NEE values during the season varied from +3.99 to ?18.50 μmol CO₂ m^?2 s^?1. The daily average NEE over the cropping season varied from +2.73 to ?7.74 g C m^?2 day^?1. The net ecosystem CO₂ exchange reached its maximum in heading to flowering stage of rice with an average value of ?5.67 g C m^?2 day^?1. On daily basis the flooded rice field acted as a net sink for CO₂ during most of the times in growing season except few days at maturity when it became a net CO₂ source. The rate of CO₂ uptake by rice as observed from negative NEE values increased proportionally with air temperature up to 34 °C. The carbon distribution in different component of soil-plant system namely, soil organic carbon, dissolved organic carbon, methane emission, rhizodeposition, carbon in algal biomass, crop harvest and residues were quantified and carbon balance sheet was prepared for the wet season in tropical rice. Carbon balance sheet for tropical rice revealed 7.12 Mg C ha^?1 was cycled in the system in wet season.     

Influence of agronomic management of legume crops on soil accumulation with nitrate

Nitrate is known to accumulate under legume crops. The effects of legume crop, inoculation, row width, sowing rate, sowing date, and intra-cropping with wheat, on the amount and soil distribution of mineral N, residual soil water, crop biomass and crop N were studied at Wagga Wagga in south-east Australia. After removal of most of the above-ground plant material, the treatment effects on the biomass, N content, grain yield and grain N of wheat, established in the following season, were also measured. In a later experiment at Wagga, the recovery of 15N applied to the mid-row of lupin crops established at three row widths was estimated at crop maturity. At Condobolin, row width effects on the soil distribution of mineral N, biomass, N accumulation and N fixation of crop legumes and cereals, were determined. At physiological maturity, at Wagga Wagga, very little nitrate was left beneath cereals. Significantly more was left under legume crops, mostly below 30 cm of soil depth, and it was distributed differently depending on crop, inoculation, and sampling location. More nitrate was left under pea and faba than under lupin, and in response to inoculation. Mixing wheat with narrow-leaf lupin did not prevent nitrate accumulation in soil. For most of the legumes more nitrate was left in the mid-row than in the in-row; and more nitrate was left at the mid-row of lupin crops sown with wider rows. The additional nitrate left with wider rows increased the growth, N content, grain yield and protein of wheat established in the following season. 15N labelled nitrate applied mid-row was used less effectively by lupin as row width increased, in a dry season. At Condobolin, lupin established with wide rows used less soil nitrate than with narrower rows but maintained crop N by increased N fixation. In contrast, field pea maintained N demand by increasing nitrate uptake at intermediate row spacing. The study shows that the amount of nitrate accumulated in soil during legume cropping is susceptible to agronomic management, particularly crop selection, row width and inoculation; and that variation in the amount of this nitrate may carry forward to impact wheat production in the follow-on season.     

Legume cover crops and mulches: effects on nitrate leaching and nitrogen input in a pepper crop (<Emphasis Type="Italic">Capsicum annuum</Emphasis> L.)

There is not sufficient knowledge concerning the risks involved in NO₃–N leaching in relation to the use of cover crops and mulches. A 2 year field experiment was carried out in a pepper (Capsicum annuum L.) crop transplanted into different soil management treatments which involved the addition of mulch of three different types of winter cover crops (CC) [hairy vetch (Vicia villosa Roth.), subclover (Trifolium subterraneum L.), and a mixture of hairy vetch/oat (Avena sativa L.)], and an un-mulched plot. At the time of CC conversion into mulch, the hairy vetch/oat mixture accumulated the highest aboveground biomass (5.30 t ha⁻¹ of DM), while hairy vetch in pure stand accumulated the highest quantity of N (177 kg ha⁻¹) and showed the lowest C/N ratio (12). The marketable pepper yield was higher in mulched than in conventional (on average 33.5, 28.9, 27.7 and 22.2 t ha⁻¹ of FM for hairy vetch, subclover, hairy vetch/oat mixture, and conventional, respectively). Generally, the NO₃–N content of the soil was minimum at CC sowing, slightly higher at pepper transplanting and maximum at pepper harvesting (on average 15.2, 16.8, and 23.3 mg NO₃-N kg⁻¹ of dry soil, respectively). The cumulative leachate was higher during the CC period (from October to April) than the pepper crop period (from April to September), on average 102.1 vs 66.1 mm over the years, respectively. The cumulative NO₃–N leached greatly depended on the type of mulch and it was 102.3, 95.3, 94.7, and 48.2 kg ha⁻¹ in hairy vetch, subclover, hairy vetch/oat mixture, and conventional, respectively. A positive linear correlation was found between the N accumulated in the CC aboveground biomass and the NO₃–N leached during pepper cultivation (R ² = 0.87). This research shows that winter legume cover crops, especially hairy vetch in pure stand, converted into dead mulch in spring could be used successfully for adding N to the soil and increasing the yield of the following pepper crop although the risks of N losses via leaching could be increased compared to an un-mulched soil. Therefore when leguminous mulches are used in the cultivation of a summer crop, appropriate management practices of the system, such as a better control of the amount of irrigation water and the cultivation of a graminaceous or a cruciferous catch crop after the harvesting of the summer crop, should be adopted in order to avoid an increase in NO₃–N leaching.     

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.     

Symbiotic nitrogen fixation by legumes in two Chinese grasslands estimated with the 15N dilution technique

Symbiotic nitrogen (N) fixation by legumes was investigated using the ¹⁵N dilution technique in two Chinese grasslands: one in the north-eastern Tibetan Plateau and the other in Inner Mongolia in China. A small amount (0.03 g N m^?2) of ¹⁵N labelled (NH₄)₂SO₄ fertilizer was evenly distributed in two soils. One month after the ¹⁵N addition, four legumes (Astragalus sp., Gueldenstaedtia diversifolia, Oxytropis ochrocephala and Trigonella ruthenica) in the alpine meadow and two legumes (Thermopsis lanceolata and Melissitus ruthenica) in the temperate steppe were collected. Several non-legume plant species were harvested as the reference. Above-ground biomass of legumes ranged from 8 to 24 g m^?2 in the alpine meadow and from 11 to 35 g m^?2 in the temperate steppe. The reference plants showed distinctly higher ¹⁵N atom% excess than legumes (0.08% vs. 0.02% in the alpine meadow, 0.10% vs. 0.02% in the temperate steppe). The N derived from atmosphere (%Ndfa) ranged from 50 to 90% N in the alpine meadow, while it ranged from 85 to 92% in the temperate steppe. Based on the legume above-ground biomass, total symbiotic N₂-fixation rate was estimated to be 1.00 g N m^?2 year^?1 in the alpine meadow and 1.15 g N m^?2 year^?1 in the temperate steppe. These N inputs by legumes can account for 9% of the gap between the N demand and the seasonal N release by mineralization in the alpine Kobresia grassland and 20% in the temperate Leymus grassland, respectively. Considering additional contribution of the root biomass, we suggest that biological N₂-fixation by legumes plays an important role in the cycling of N in both Kobresia and Leymus grasslands on an annual scale.     

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.     

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.