Nitrogen leaching and indirect nitrous oxide emissions from fertilized croplands in Zimbabwe

Agricultural efforts to end hunger in Africa are hampered by low fertilizer-use-efficiency exposing applied nutrients to losses. This constitutes economic losses and environmental concerns related to leaching and greenhouse gas emissions. The effects of NH₄NO₃ (0, 60 and 120?kg?N?ha^?1) on N uptake, N-leaching and indirect N₂O emissions were studied during three maize (Zea mays L.) cropping seasons on clay (Chromic luvisol) and sandy loam (Haplic lixisol) soils in Zimbabwe. Leaching was measured using lysimeters, while indirect N₂O emissions were calculated from leached N using the emission factor methodology. Results showed accelerated N-leaching (3?C26?kg?ha^?1?season^?1) and N-uptake (10?C92?kg?ha^?1) with N input. Leached N in groundwater had potential to produce emission increments of 0?C94?g N₂O-N?ha^?1?season^?1 on clay soil, and 5?C133?g N₂O-N?ha^?1?season^?1 on sandy loam soil following the application of NH₄NO₃. In view of this short-term response intensive cropping using relatively high N rate may be more appropriate for maize in areas whose soils and climatic conditions are similar to those investigated in this study, compared with using lower N rates or no N over relatively larger areas to attain a targeted food security level.     

Effect of climate change on greenhouse gas emissions from arable crop rotations

The DAISY soil–plant–atmosphere model was used to simulate crop production and soil carbon (C) and nitrogen (N) turnover for three arable crop rotations on a loamy sand in Denmark under varying temperature, rainfall, atmospheric CO₂ concentration and N fertilization. The crop rotations varied in proportion of spring sown crops and use of N catch crops (ryegrass). The effects on CO₂ emissions were estimated from simulated changes in soil C. The effects on N₂O emissions were estimated using the IPCC methodology from simulated amounts of N in crop residues and N leaching. Simulations were carried out using the original and a revised parameterization of the soil C turnover. The use of the revised model parameterization increased the soil C and N turnover in the topsoil under baseline conditions, resulting in an increase in crop N uptake of 11 kg N ha^–1 y^–1 in a crop rotation with winter cereals and a reduction of 16 kg N ha^–1 y^–1 in a crop rotation with spring cereals and catch crops. The effect of increased temperature, rainfall and CO₂ concentration on N flows was of the same magnitude for both model parameterizations. Higher temperature and rainfall increased N leaching in all crop rotations, whereas effects on N in crop residues depended on use of catch crops. The total greenhouse gas (GHG) emission increased with increasing temperature. The increase in total GHG emission was 66–234 kg CO₂-eq ha^–1 y^–1 for a temperature increase of 4°C. Higher rainfall increased total GHG emissions most in the winter cereal dominated rotation. An increase in rainfall of 20% increased total GHG emissions by 11–53 kg CO₂-eq ha^–1 y^–1, and a 50% increase in atmospheric CO₂ concentration decreased emissions by 180–269 kg CO₂-eq ha^–1 y^–1. The total GHG emissions increased considerably with increasing N fertilizer rate for a crop rotation with winter cereals, but remained unchanged for a crop rotation with spring cereals and catch crops. The simulated increase in GHG emissions with global warming can be effectively mitigated by including more spring cereals and catch crops in the rotation.     

Soil nutrient content and nutrient balances in newly-built solar greenhouses in northern China

Soil nutrient content and nutrient balances in newly-built solar greenhouses in the southern part of China??s Loess Plateau were investigated over two consecutive years. Farmers applied manure and inorganic fertilizers at average annual rates of 1,907?kg?N ha^?1, 1,601?kg?P₂O₅?ha^?1 and 1,742?kg?K₂O?ha^?1. Manure accounted for 65?% of the total N input, 57?% of the total P input and 55?% of the total K input. The average annual nutrient surpluses were 1,374?kg?N?ha^?1, 1,468?kg?P₂O₅?ha^?1 and 881?kg?K₂O?ha^?1. Soil organic matter, total N, available P, available K and electrical conductivity (EC) increased significantly across time in the topsoil (0?C20?cm depth), but not in the subsoil (20?C100?cm depth). The nitrate?CN concentrations (mg?N?kg^?1) of the 0?C100?cm depth increased by 163?C336?% over 2?years. The average accumulation of nitrate?CN (kg?N?ha^?1) of the 0?C100?cm depth increased by 241?% and leveled out at 511?kg?N?ha^?1; and it was 1,015?kg?N?ha^?1 in the 0?C200?cm depth. In conclusion, over-fertilization led to large nutrient surpluses in the soil of newly-built greenhouses.     

Nutrient uptake of iron,zinc, magnesium,and copper in transgenic maize (<Emphasis Type="Italic">Zea mays</Emphasis>) as affected by rotation systems and N application rates

Understanding the interaction of macro- and micronutrients is a prerequisite to targeting nutrient balance in crop production. A 3-year field study was conducted to determine mineral nutrient uptake of maize hybrids with N fertilizer application under different rotation systems. The experiment was arranged in a split-plot design with rotation [maize-alfalfa (MA), maize-soybean (MS), and continuous maize (MM)] by N rate (0, 50, 100 and 150 kg N ha^?1) as the mainplot and hybrid as the subplot. Two additional treatments (200 and 250 kg N ha^?1) were tested in MM. Maize plant total Mg, Zn, and Cu content were in the order: MA?>?MS?>?MM. Plant Fe uptake was the lowest in MA and not affected by N input. The increased Cu uptake with increasing N rates indicated the synergism of these two nutrients, whereas dilution effects of N application on stover Zn and Mg concentrations were recorded. Rotation systems and N rates interactively affected nutrient harvest index and internal efficiency of Zn, Mg, Fe, and Cu. Relationships of plant N with Cu and Mg concentrations, and N with Zn, Mg, and Cu content at the V6 stage were established, but they were not necessarily preserved at maturity due to the progressive synergistic and dilution effects. The findings of nutrient uptake of Cu, Zn, Mg and Fe and their relationships with N nutrition in maize with stacked transgenic traits are important for developing best management practices to achieve concurrent improvements in nutrient use efficiency and crop productivity.     

Fate of labeled urea-15N as basal and topdressing applications in an irrigated wheat-maize rotation system in North China plain: II summer maize

A field micro-plot experiment for summer maize was conducted in an irrigated winter wheat (Triticum aestivum)-summer maize (Zea mays L.) rotation system in Mazhuang, Xinji of Hebei province in the North China Plain, using the ¹⁵N isotope method to determine the effects of N application (rates and timing) on urea-¹⁵N fate, residual N effects and N recovery efficiency (NRE) by maize. The experiment included three N rates (90, 180, and 270 kg ha^?1), divided by two ¹⁵N-labeled groups of basal-¹⁵N (30, 60, and 90 kg ha^?1, respectively) and topdress-¹⁵N (60, 120, and 180 kg N ha^?1, respectively). All of the treatments were irrigated two times, once at seeding time and once at topdressing time. The absorbed N in the maize plant derived from basal-N (6.8?C13%) and topdress-N (17?C30%) was identified. The residual N in the 0?C150-cm soil depth ranged from 45 to 60% at the first maize harvest, mainly retained in the top 20-cm layers. Both NRE in grain and total N recovery in plant in the first maize crop were higher from topdress-¹⁵N (26?C31 or 41?C51%, respectively) than from basal-¹⁵N (18?C23 or 34?C43%, respectively). The residual N in the 0?C150-cm soil layer was lower from topdress-¹⁵N (45?C47%) than from basal-¹⁵N (55?C60%) after the first maize harvest. Residual N recovery was 6?C11% in the second and 1.5?C3.5% in the third crop. Cumulative N recovery in the maize-wheat-maize rotations was higher from the topdress-¹⁵N (49?C59%) than from basal-¹⁵N and (45?C55%). The unaccounted N loss was 14?C24% from the basal-¹⁵N and 20?C33% from the topdress-¹⁵N, with a double dose of basal-¹⁵N application. An N rate of approximately 180 kg ha^?1 appears to be an effective application rate to optimum maize yield and NRE on North China Plain, depending on the residual N and the crop yield potential.     

Effects of rainfall and fertilizer types on nitrogen and phosphorus concentrations in surface runoff from subtropical tea fields in Zhejiang, China

The increasing input of fertilizers in tea (Camellia sinensis L.) fields may contribute to the deterioration of surface water quality. A plot study was conducted over a 2-year period (2010?C2011) to evaluate the effects of rainfall and fertilizer types on nitrogen (N) and phosphorus (P) concentrations in surface runoff from tea fields. Studies were arranged on slope of 18?% of red clay at a subtropical tea fields in Tiaoxi watershed of Zhejiang province, southeast China. Organic (OF), slow-release (SRF), and conventional chemical fertilizers were applied to different plots at rates of 248?kg?N?ha^?1 and 125.2?kg?P?ha^?1 in 2010 and 300?kg?N?ha^?1 and 100?kg?P?ha^?1 in 2011. Rainfall amounts showed statistically significant correlations with concentrations of TN and TP in runoff water from all fertilized treatments. Although equivalent N and P were applied in each fertilized treatment, the OF treatment had the lowest annual arithmetic mean concentration of total N in runoff in 2010 (6.1?mg?L^?1) and was amongst the lowest in 2011 (9.2?mg?L^?1) with concentration statistically similar to SRF (9.0?mg?L^?1). The SRF treatment had the lowest annual arithmetic mean concentration of total P in runoff in 2010 (1.50?mg?L^?1), while few differences were observed in concentration of total P between fertilized treatments in 2011. The research results suggested that replacement of conventional chemical fertilizers with organic or slow-release fertilizers in tea fields could reduce N and P losses while maintaining tea yields.     

Fate of labeled urea-15N as basal and topdressing applications in an irrigated wheat�Cmaize rotation system in North China Plain: I winter wheat

A field micro-plot experiment for winter wheat was conducted in an irrigated winter wheat (Triticum aestivum)-summer maize (Zea mays L.) rotation system in Mazhuang, Xinji of Hebei province in the North China Plain, using the ¹⁵N isotope method to determine the effects of N application (rates and timing), and irrigation frequency on urea-¹⁵N fate, residual-N and N recovery efficiency (NRE) of wheat. The experiment was conducted under two irrigation treatments (I2 and I3, representing for two and three irrigations, respectively), at three N rates (150, 210, and 270, kg ha^?1), divided between two ¹⁵N-labeled applications of basal-¹⁵N (90 kg ha^?1) and topdress-¹⁵N (60, 120, and 180, kg N ha^?1, respectively). The total N uptake by wheat (ranging from 186 to 238 kg ha^?1) and the fertilizer-derived N (Ndff, about 34?C55%) were measured. The Ndff from labeled basal-¹⁵N and from labeled topdress-¹⁵N were about 15?C22% and 16?C40%, respectively. The NRE (measured either as recovery in grain or as the total N recovery in the plant) was higher with I3 (39?C41 or 47?C49%) than with I2 (35?C40 or 42?C47%), showing maximum NRE in grain of about 40% both at N210 with I2 and at N150 with I3 treatment. The NRE by the first wheat crop (in grain or the total N recovery in plant) was higher with labeled topdress-¹⁵N (39?C48 or 45?C56%) as compared to that with labeled basal-¹⁵N (30?C37 or 36?C45%), while the unaccounted N losses were lower with labeled basal-¹⁵N (14?C22%) relative to labeled topdress-¹⁵N (14?C35%). Higher residual N in soils was found with labeled basal-¹⁵N (41?C51%), as compared to labeled topdress-¹⁵N (18?C35%). Residual N in the 0- to 150-cm soil depth ranged from 26 to 44% while the unaccounted N losses ranged from 14 to 30%. Recovery of residual N by the 2nd and 3rd crops in the rotation was 5?C10% in the maize crop and a further 1.7?C3.5% in the subsequent wheat crop. The accumulated N recovery and the unaccounted N losses in continuous wheat?Cmaize?Cwheat rotations derived from labeled topdress-¹⁵N were 54?C64% and 16?C37%, respectively while they were 47?C53% and 16?C28%, respectively from labeled basal-¹⁵N. This study also suggested that an N rate of 210 kg ha^?1 (with a ratio of basal-N to topdress-N of 1:1.3) with two irrigation applications could optimize wheat grain yields and NRE, under the water limited conditions in North China Plain.     

How much nitrogen is fixed by biological symbiosis in tropical dry forests? 2. Herbs

Although biological nitrogen fixation (BNF) is considered the main input of N in mature and regenerating native tropical vegetation, it has seldom been quantified. Biomass and N accumulation and fixation were determined for spontaneously occurring herbaceous species in caatinga areas in four regeneration stages (2, 17, 39 and >50?years after abandonment from agricultural use). BNF was estimated using the ¹⁵N natural-abundance method. The 2-year regeneration area had the highest total herb (6,355?kg?ha^?1) and legume (262?kg?ha^?1) biomass production, N stocks (82?kg?ha^?1) and fixed N (5.0?kg?ha^?1). N₂-fixing legumes (nine species in the sampled area) contributed over 97?% of legume biomass in all areas. Macroptilium gracile added the largest amount of N (3.9?kg?ha^?1 in the 2-year regeneration area) because of its large biomass production (205?kg?ha^?1), although it was not the species with the highest proportion of fixed N (76?%). All of the N₂-fixing species obtained large proportions of their N from symbiosis, most of them more than 50?%.However, the amounts of fixed N per unit area were relatively low (0.22?C5.00?kg?ha^?1) because the biomass of N₂-fixing species was always less than 5?% of the total herb biomass.     

Grazing effects on the greenhouse gas balance of a temperate steppe ecosystem

Although a significant fraction of the global soil?Catmosphere exchange of greenhouse gases (GHGs) occurs in semi-arid zones little is known about the magnitude of fluxes in grazed steppe ecosystems and the interference with grazing intensity. In order to assess GHG burdens and to identify options of climate-optimized livestock farming, GHG emissions of sheep grazing in Inner Mongolia steppe were analyzed. Carbon sequestration and field-fluxes of methane (CH₄) and nitrous oxide (N₂O) were measured at a range of steppe sites differing in grazing intensity and management, i.e. ungrazed (UG), ungrazed with hay cutting (HC), lightly grazed (LG), moderately grazed (MG), and heavily grazed (HG). In addition, GHG emissions from enteric fermentation, manure management, and farming inputs (i.e. fossil fuels) were quantified for LG, MG, and HG. Monte Carlo simulation was used to estimate uncertainty. Sheep grazing changed the net GHG balance of the steppe from a significant sink at UG (?1476?±?2481?kg CO_2eq ha^?1?year^?1) to a significant source at MG (2350?±?1723?kg CO_2eq ha^?1?year^?1) and HG (3115?±?2327?kg CO_2eq ha^?1?year^?1). In a similar way, the GHG intensity increased from 8.6?±?79.2?kg CO_2eq?kg^?1 liveweight gain at LG up to 62.2?±?45.8 and 62.6?±?46.7?kg CO_2eq?kg^?1 liveweight gain at MG and HG, respectively. GHG balances were predominantly determined by CO₂ from changes in topsoil organic carbon. In grazing systems, CH₄ from enteric fermentation was the second most important component. The results suggest that sheep grazing under the current management changes this steppe ecosystem from a sink to a source of GHGs and that grazing exclusion holds large potential to restore soil organic carbon stocks and thus to sequester atmospheric CO₂. The balance between grazing intensity and grazing exclusion predominantly determines GHG balances of grass-based sheep farming in this region. Therefore, a high proportion of ungrazed land is most important for reducing GHG balances of sheep farms. This can be either achieved by high grazing intensity on the remaining grazed land or by confined hay feeding of sheep.     

Dryland soil nitrogen cycling influenced by tillage, crop rotation, and cultural practice

Management practices may influence dryland soil N cycling. We evaluated the effects of tillage, crop rotation, and cultural practice on dryland crop biomass (stems and leaves) N, surface residue N, and soil N fractions at the 0?C20?cm depth in a Williams loam from 2004 to 2008 in eastern Montana, USA. Treatments were two tillage practices (no-tillage [NT] and conventional tillage [CT]), two crop rotations (continuous spring wheat [Triticum aestivum L.] [CW] and spring wheat-barley [Hordeum vulgaris L.] hay-corn [Zea mays L.]-pea [Pisum sativum L.] [W-B-C-P]), and two cultural practices (regular [conventional seed rates and plant spacing, conventional planting date, broadcast N fertilization, and reduced stubble height] and ecological [variable seed rates and plant spacing, delayed planting, banded N fertilization, and increased stubble height]). Nitrogen fractions were soil total N (STN), particulate organic N (PON), microbial biomass N (MBN), potential N mineralization (PNM), NH₄?CN, and NO₃?CN. Crop biomass N was 30?% greater in W-B-C-P than in CW in 2005. Surface residue N was 30?C34?% greater in NT with the regular and ecological practices than in CT with the regular practice. The STN, PON, and MBN at 10?C20 and 0?C20?cm were 5?C41?% greater in NT or CW with the regular practice than in CT or CW with the ecological practice. The PNM at 5?C10?cm was 22?% greater in the regular than in the ecological practice. The NH₄?CN and NO₃?CN contents at 10?C20 and 0?C20?cm were greater in CT with W-B-C-P and the regular practice than with most other treatments in 2007. Surface residue and soil N fractions, except PNM and NO₃?CN, declined from autumn 2007 to spring 2008. In 2008, NT with W-B-C-P and the regular practice gained 400?kg?N?ha^?1 compared with a loss of 221?kg?N?ha^?1 to a gain of 219?kg?N?ha^?1 in other treatments. No-tillage with the regular cultural practice increased surface residue and soil N storage but conventional tillage with diversified crop rotation and the regular practice increased soil N availability. Because of continuous N mineralization, surface residue and soil N storage decreased without influencing N availability from autumn to the following spring.     

How much nitrogen is fixed by biological symbiosis in tropical dry forests? 1. Trees and shrubs

Despite the recognized importance of the process, estimates of the amount of nitrogen fixed by biological symbiosis in tropical dry forests are almost nonexistent. We estimated the nitrogen fixed annually by the leaves of trees and shrubs at sites regenerating for 16 and 38?years and in an old-growth dry forest using ¹⁵N abundance methodology. The total leaf biomass (1,824?C3,036?kg?ha^?1) and nitrogen contents (62?C90?kg?ha^?1) did not differ among the areas. In all of the areas, most of the leaf biomass belonged to legume plants, but the proportion of the N₂-fixing legumes decreased with increasing regeneration time. In the 16-year regenerating area, almost all of the N was in the leaves of the N-fixing Mimosa tenuiflora plants, but fixation was absent or very low as it was in the N-fixing species present in the 38-year regenerating area. In the old-growth Caatinga, all of the N-fixing species (M. tenuiflora, Piptadenia stipulacea and Anadenanthera colubrina) had large proportions (47?C62?%) of their N derived from atmospheric N₂, but the amount of fixed N (6?kg?ha^?1) was a small proportion of the total leaf N because these plant species were a small part of the vegetation. The total input of biologically fixed N to the old-growth forest was similar in magnitude to an estimate made for a humid tropical forest in Amazonia.     

Nitrous oxide emissions as influenced by legume cover crops and nitrogen fertilization

In this study, we measured nitrous oxide (N₂O) fluxes from plots of fall-planted hairy vetch (HV, Vicia villosa) and spring-planted broadleaf vetch (BLV, Vicia narbonensis) grown as nitrogen (N) sources for following summer forage crabgrass (Digitaria sanguinalis). Comparisons also included 60 kg ha^?1 inorganic N fertilizer for crabgrass at planting (60-N) and a control without N fertilizer. Each treatment had six replicated plots across the slope. Fluxes were measured with closed chamber systems during the period between spring growth of cover crops and first-cut of crabgrass in mid-July. HV had strong stand and aboveground biomass had 185?±?50 kg N ha^?1 (mean?±?standard error, n?=?6) at termination. However, BLV did not establish well and aboveground biomass had only 35?±?15 kg N ha^?1. Ratio vegetation index of crabgrass measured as proxy of biomass growth was highest in HV treatment. However, total aboveground biomass of crabgrass was statistically similar to 60-N plots. Fluxes of N₂O were low prior to termination of cover crops but were as high as 8.2 kg N₂O ha^?1 day^?1 from HV plots after termination. The fluxes were enhanced by large rainfall events recorded after biomass incorporation. Rainfall enhanced N₂O fluxes were also observed in other treatments, but their magnitudes were much smaller. The high N₂O fluxes from HV plots contributed to emissions of 30.3?±?12.4 kg N₂O ha^?1 within 30 days of biomass incorporation. Emissions were only 2.0?±?0.7, 3.4?±?1.3 and 1.0?±?0.4 kg N₂O ha^?1 from BLV, 60-N and control plots, respectively.     

Influence of cultivars and nitrogen rates on the residual effects of potassium applications to preceding cotton on maize

The long-term residual effects of K application rates and cultivars for preceding cotton (Gossypium hirsutum L.) on subsequent maize (Zea mays L.) and the influence of N rates applied to preceding cotton and to maize on the residual K effects were examined on maize under no-tillage in the United States. Two field experiments were conducted on a no-till Loring silt loam at Jackson, TN during 1995–2008 with N rates (90 and 179 kg ha^?1) × K rates (28, 56, and 84 kg ha^?1) and cotton cultivars (determinate and indeterminate) × K rates (56 and 112 kg ha^?1) as the treatments, respectively, in the preceding cotton seasons. Maize was planted under no-tillage on the preceding cotton experiments without any K application during 2009 through 2011. The residual effects of K rates applied to preceding cotton on soil K levels were significantly influenced by the N rates applied to preceding cotton and to maize when the data were combined from 2008 to 2011. Relative to the standard N management practices of 168 kg N ha^?1 for maize and 90 kg N ha^?1 for preceding cotton, the higher N application rate 269 kg N ha^?1 to maize and 179 kg N ha^?1 to preceding cotton reduced the residual effects of K rates on soil K. However, cultivar for preceding cotton did not affect the residual effects of K fertilizer on soil K fertility, leaf K nutrition, plant growth, or grain yield of subsequent maize on a high K field.     

Crop and soil nitrogen responses to phosphorus and potassium fertilization and drip irrigation under processing tomato

Shortage of water or nutrient supplies can restrict the high nitrogen (N) demand of processing tomato, leaving high residual soil N resulting in negative environmental impacts. A 4-year field experiment, 2006?C2009, was conducted to study the effects of water management consisting of drip irrigation (DI) and non-irrigation (NI), fertilizer phosphorus (P) rates (0, 30, 60, and 90?kg P?ha^?1), and fertilizer potassium (K) rates (0, 200, 400, and 600?kg?K?ha^?1) on soil and plant N when a recommended N rate of 270?kg?N?ha^?1 was applied. Compared with the NI treatment, DI increased fruit N removal by 101?%, plant total N uptake by 26?%, and N harvest index by 55?%. Consequently, DI decreased apparent field N balance (fertiliser N input minus plant total N uptake) by 28?% and cumulative post-harvest soil N in the 0?C100?cm depth by 33?%. Post-harvest soil N concentration was not affected by water management in the 0?C20?cm depth, but was significantly higher in the NI treatment in the 20?C100?cm depth. Fertilizer P input had no effects on all variables except for decreasing N concentration in the stems and leaves. Fertilizer K rates significantly affected plant N utilization, with highest fruit N removal and plant total N uptake at the 200?kg?K?ha^?1 treatment; therefore, supplementing K had the potential to decrease gross N losses during tomato growing seasons. Based on the measured apparent field N balance and spatial distribution of soil N, gross N losses during the growing season were more severe than expected in a region that is highly susceptible to post-harvest soil N losses.     

Nitrogen and sulphur fertilization effect on leaching losses, nutrient balance and plant quality in a wheat–rapeseed rotation under a humid Mediterranean climate

An unbalanced S and/or N fertilization may have low N and S use efficiency together with substantial negative implications for yield, nutrient losses and plant quality parameters. The effect of N and S fertilization and their interactions on N?CS balances, on N?CS losses and on some plant quality parameters were investigated in a field experiment with a wheat (Triticum aestivum L.)?Crapeseed (Brassica napus L.)?Cwheat rotation (2005?C2008). The study was conducted under humid Mediterranean climatic conditions on a potentially S deficient soil. The effects of N (0, 140, 180, 220?kg?N?ha^?1 in wheat; 0, 100, 140, 180, 220?N?ha^?1 in rapeseed) combined with S fertilizer rates (0, 16 and 32?kg S ha^?1 in wheat and 0, 30, 60?kg S ha^?1 in rapeseed) were studied. Nitrogen fertilization increased yield by 55?% in wheat and 60?% in rapeseed, N concentration in grain and straw and S concentration in the grain of wheat. However, it led to a reduction in the S concentration of straw and the oil content of the rapeseed seed. The S application did not increase yield but had a positive effect on S concentration in the wheat straw. Glucosinolate concentration, a potentially toxic secondary metabolite in rapeseed, was not influenced by N or S applications. Nitrate leaching tended to increase with N application while sulphate leaching decreased. A net N and S mineralization was observed in each growing season, except for the first year in which a net S immobilization was observed. To make N fertilizer recommendations, the N mineralization from the previous crop residues should be taken into account. For S fertilizer recommendations, N supply is the most important item both from a qualitative point of view (N/S ratio in wheat grain) and an environmental point of view (S leaching).     

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.     

Integrated soil management for the savanna zone of W. Africa: legume rotation and fertilizer N

Integrated soil management with leguminous cover crops was studied at two sites in the northern Guinea savanna zone of northern Nigeria, Kaduna (190 day growing season) and Bauchi (150 days). One-year planted fallows of mucuna, lablab, and crotalaria were compared with natural grass fallow and cowpea controls. All treatments were followed by a maize test crop in the second year with 0, 30, or 60 kg N ha^–1 as urea. Above ground legume residues were not incorporated into the soil and most residues were burned early in the dry season at the Kaduna site. Legume rotation increased soil total N, maize growth in greenhouse pots, and dry matter and N accumulation of maize. Response of maize grain yield to 30 kg N ha^–1 as urea was highly significant at both sites and much greater than the response to legume rotation. The mean N fertilizer replacement value from legume rotation was 14 kg N ha^–1 at Kaduna and 6 kg N ha^–1 at Bauchi. W ith no N applied to the maize test crop, maize grain yield following legume fallow was 365 kg ha^–1 higher than natural fallow at Bauchi and 235 kg ha^–1 higher at Kaduna. The benefit of specific legume fallows to subsequent maize was mostly related to above ground N of the previous legume at Bauchi, where residues were protected from fire and grazing. At Kaduna, where fallow vegetation was burned, maize yield was related to estimated below ground N. The results show that legume rotation alone results in small maize yield increases in the dry savanna zone.     

Nitrogen balances in farmers fields under alternative uses of a cover crop legume: a case study from Nicaragua

Canavalia brasiliensis (canavalia), a drought tolerant legume, was introduced into the smallholder traditional crop-livestock production system of the Nicaraguan hillsides as green manure to improve soil fertility or as forage during the dry season for improving milk production. Since nitrogen (N) is considered the most limiting nutrient for agricultural production in the target area, the objective of this study was to quantify the soil surface N budgets at plot level in farmers fields over two cropping years for the traditional maize/bean rotation and the alternative maize/canavalia rotation. Mineral fertilizer N, seed N and symbiotically fixed N were summed up as N input to the system. Symbiotic N₂ fixation was assessed using the ¹⁵N natural abundance method. Nitrogen output was quantified as N export via harvested products. Canavalia derived in average 69% of its N from the atmosphere. The amount of N fixed per hectare varied highly according to the biomass production, which ranged from 0 to 5,700 kg ha^?1. When used as green manure, canavalia increased the N balance of the maize/canavalia rotation but had no effect on the N uptake of the following maize crop. When used as forage, it bears the risk of a soil N depletion up to 41 kg N ha^?1 unless N would be recycled to the plot by animal manure. Without N mineral fertilizer application, the N budget remains negative even if canavalia was used as green manure. Therefore, the replenishment of soil N stocks by using canavalia may need a few years, during which the application of mineral N fertilizer needs to be maintained to sustain agricultural production.     

Effect of wheat straw application on ammonia volatilization from urea applied to a paddy field

A proper amount of nitrogen (N) fertilizer is critical for the ideal production in the wheat-rice rotation in the Yangtse Delta region of China and straw retention is important for sustaining soil quality and productivity. However, the effects of straw retention on paddy field ammonia volatilization from applied urea are unclear. The objectives of this study were to explore the effect of wheat straw retuned with urea and to evaluate how floodwater ammonium concentration and pH, soil Eh influence on flooded rice field ammonia volatilization. The study was conducted for 2?years using a lysimeter experiment included 5 treatments, urea applied at rates of 0, 180, 240?kg?N?ha^?1 with no retained straw, and at rates of 180 and 240?kg?N?ha^?1 with 6.5?t?ha^?1 of retained wheat straw. Urea was split into three applications: incorporated at transplanting, tillering, and topdressing at panicle emergence. Rice was flooded to a depth of 5?cm and grown in rotation with irrigated wheat as a source of straw. Averaged over the two levels of applied N, straw incorporation increased the floodwater ammonium concentration by 11.5?C22.5?%, pH by 0.13?C0.70 units but reduced topsoil Eh by 1.0?C47?mv. Ammonia volatilization increased with the increasing amounts of urea applied and with straw incorporated. With no retained straw, the average ammonia volatilization from the fertilized treatments was 40.4?kg?N?ha^?1, accounting for 15.8?% of the fertilizer-N. With retained wheat straw, the average ammonia volatilization from the fertilized treatments was 51.9?kg?N?ha^?1, accounting for 21.3?% of the fertilizer-N. The increase in ammonia volatilization caused by straw incorporation may be partly attributed to the presence of urease in the straw and to the increased pH in the floodwater. It is unclear whether the reduced redox potential also contributed to ammonia volatilization.     

Simulating impact of seasonal climatic variation on the response of maize (Zea mays L.) to inorganic fertilizer in sub-humid Ghana

Under low input subsistence farming systems, increased pressure on land use and decreased fallow periods have led to a decline in soil productivity. The soils in sub-humid region of Ghana are generally poor and require mineral fertilizer to increase crop productivity. This paper presents the use of Agricultural Production Systems sIMulator (APSIM) to simulate the long term influence of nitrogen (N) and phosphorus (P) on maize (Zea mays L.) yield in Sub-humid Ghana. The APSIM model was evaluated at two sites in Ejura, on a rainfed experiment carried out on maize in 2008 major and minor seasons, under various nitrogen and phosphorus rates. The model was able to reproduce the response of maize to water, N and P, and hence simulated maize grain yields with a coefficient of correlation (R²) of 0.90 and 0.88 for Obatanpa and Dorke cultivars, respectively. A 21-year long term simulation, with different rates of N and P mineral fertilizer application, revealed that moderate application of N (60?kg?N?ha^?1) and 30?kg P ha^?1 improves both the long term average and the minimum yearly guaranteed yield. Variability in grain yield increased with increasing application of N fertilizer in both seasons. Treatments with P fertilizer application shows a similar trend for the major season and reverse trend for the minor season, thereby suggesting an interactive effect with rainfall amounts and distribution. Application of 30?kg P ha^?1 significantly increased the response to N. The response to mineral fertilizer (N and P) applications varied between seasons, suggesting the need to have a range of fertilizer recommendations to be applied based on seasonal weather forecast.