Soil mineralizable nitrogen and soil nitrogen supply under two-year potato rotations

Two-year potato rotations were evaluated for their effects on soil mineralizable N and soil N supply. Pre-plant soil samples (0–15 cm) collected from the potato year after seven rotation cycles were used to estimate soil mineralizable N using a 24 week aerobic incubation. Potentially mineralizable N (N ₀ ) ranged from 102 to 149 kg N ha^?1, and was greater after pea/white clover and oats/Italian ryegrass than after oats by an average of 35 and 22%, respectively. Labile, intermediate and stable mineralizable N pools were increased after pea/white clover compared with oats, whereas only the stable mineralizable N pool was increased after oats/Italian ryegrass. Potato plant N uptake with no fertilizer applied was greater in potato-pea/white clover compared with the three other rotations (126 vs. average of 67 kg N ha^?1). Choice of rotation crop in potato production influences both the quantity and quality of soil mineralizable N.     

N2O and CH4 Emissions,and NO3 − Leaching on a Crop-Yield Basis from a Subtropical Rain-fed Wheat–Maize Rotation in Response to Different Types of Nitrogen Fertilizer

Guaranteeing high crop yields while reducing environmental impacts of nitrogen fertilizer use due to associated losses of N₂O emissions and nitrate (NO₃ ^?) leaching is a key challenge in the context of sustainable intensification of crop production. However, few field data sets are available that explore the effect of different forms of N management on yields as well as on N losses in the form of N₂O or NO₃ ^?. Here we report on a large-scale field lysimeter (8 × 4 m²) experiment, which was designed to determine soil CH₄ and N₂O emissions, NO₃ ^? leaching losses and crop yields from a subtropical rain-fed wheat–maize rotation in the Sichuan Basin, one of the most intensively used agricultural regions in China. One control and three different fertilizer treatments with the same total rate of N application (280 kg N ha^?1 y^?1) were included: NF: control (no fertilizer); NPK: synthetic N fertilizer; OMNPK: synthetic N fertilizer plus pig manure; RSDNPK: synthetic N fertilizer plus crop residues. As compared to the standard NPK treatment, annual NO₃ ^? leaching losses for OMNPK and RSDNPK treatments were decreased by 36 and 22%, respectively (P < 0.05). Similarly, crop yield-scaled NO₃ ^? leaching for NPK treatment was higher than those for either OMNPK or RSDNPK treatments (P < 0.05). Direct N₂O emissions for RSDNPK treatment were decreased as compared with NPK and OMNPK treatments (P < 0.05). Furthermore, the yield-scaled GWP (global warming potential) was lower for the treatments where either pig manure or crop residues were incorporated as compared to the standard NPK treatment (P < 0.05). Our study indicates that it is possible to reduce the negative environmental impact of NO₃ ^? leaching and N₂O emissions without compromising crop productivity. Yield-scaled NO₃ ^? leaching, similar to the yield-scaled GWP, represents another valuable-integrated metric to address the dual goals of reducing nitrogen pollution and maintaining crop grain yield for a given agricultural system.     

Nitrogen dynamics following field application of biochar in a temperate North American maize-based production system

Background and aims

Biochar additions to tropical soils have been shown to reduce N leaching and increase N use efficiency. No studies exist verifying reduced N leaching in field experiments on temperate agricultural soils or identifying the mechanism for N retention.

Methods

Biochar derived from maize stover was applied to a maize cropping system in central New York State at rates of 0, 1, 3, 12, and 30 t?ha^-1 in 2007. Secondary N fertilizer was added at 100, 90, 70, and 50 % of the recommended rate (108 kg N ha^-1). Nitrogen fertilizer enriched with ¹⁵?N was applied in 2009 to the 0 and 12 t?ha^-1 of biochar at 100 and 50 % secondary N application.

Results

Maize yield and plant N uptake did not change with biochar additions (p?>?0.05; n?=?3). Less N (by 82 %; p?<?0.05) was lost after biochar application through leaching only at 100 %?N fertilization. The reason for an observed 140 % greater retention of applied ¹⁵?N in the topsoil may have been the incorporation of added ¹⁵?N into microbial biomass which increased approximately three-fold which warrants further research. The low leaching of applied fertilizer ¹⁵?N (0.42 % of applied N; p?<?0.05) and comparatively high recovery of applied ¹⁵?N in the soil (39 %) after biochar additions after one cropping season may also indicate greater overall N retention through lower gaseous or erosion N losses with biochar.

Conclusions

Addition of biochar to fertile soil in a temperate climate did not improve crop growth or N use efficiency, but increased retention of fertilizer N in the topsoil.     

Nitrogen cycling in a15N-fertilized bean (Phaseolus vulgaris L.) crop

To increase our understanding of the fate of applied nitrogen inPhaseolus vulgaris crops grown under tropical conditions,¹⁵N-labelled urea was applied to bean crops and followed for three consecutive cropping periods. Each crop received 100 kg urea-N ha^?1 and 41 kg KCl?K ha^?1. At the end of each period we estimated each crop's recovery of the added nitrogen, the residual effects of nitrogen from the previous cropping period, the distribution of nitrogen in the soil profile, and leaching losses of nitrogen. In addition, to evaluate potential effects of added phosphorus on nitrogen cycling in this crop, beans were treated at planting with either 35 kg rock-phosphate-P, 35 kg superphosphate-P, or 0 kg P ha^?1. Results showed that 31.2% of the nitrogen in the first crop was derived from the applied urea, which represents a nitrogen utilization efficiency of 38.5%. 6.2% of the nitrogen in the second crop was derived from fertilizer applied to the first crop, and 1.4% of the nitrogen in the third crop. Nitrogen utilization efficiencies for these two crops, with respect to the nitrogen applied to the first crop, were 4.6 and 1.2%, respectively. In total, the three crops recovered 44.3% of the nitrogen applied to the first crop. The remainder of the nitrogen was either still in the soil profile or had been lost by leaching, volatilization or denitrification.¹⁵N enrichment of mineral-N(NO₃+NH₄) suggests that at the end of the second crop, the pulse of fertilizer applied to the first crop had probably passed the 120 cm depth.¹⁵N enrichment of organic-N suggests that root activity of beans and weeds transported nitrogen to 90–120 cm (or deeper). We could account for 109 kg fertilizer-N ha^?1 in harvested biomass, crop residue, and soil at the end of the first cropping period. This indicates an experimental error of about 10% if no nitrogen was lost by volatilization, denitrification, or leaching below 120 cm. At the end of the second and third crops, 76 and 80 kg N ha^?1, respectively, could be accounted for, suggesting that 20 to 25% of the applied-N was lost from the system over a 2-crop period. The two types of added phosphorus did not significantly differ in their effects on bean yields.     

Nitrogen fate and environmental consequence in paddy soil under rice-wheat rotation in the Taihu lake region, China

Field undisturbed tension-free monolith lysimeters and ¹⁵N-labeled urea were used to investigate the fate of fertilizer nitrogen in paddy soil in the Taihu Lake region under a summer rice-winter wheat rotation system. We determined nitrogen recovered by rice and wheat, N remained in soil, and the losses of reactive N (i.e., NH₃, N₂O, NO₃ ^?, organic N and NH₄ ⁺) to the environment. Quantitative allocation of nitrogen fate varied for the rice and wheat growing seasons. At the conventional application rate of 550 kg N ha^?1 y^?1 (250 kg N ha^?1 for wheat and 300 kg N ha^?1 for rice), nitrogen recovery of wheat and rice were 49% and 41%, respectively. The retention of fertilizer N in soil at harvest accounted for 29% in the wheat season and for 22% in the rice season. N losses through NH₃ volatilization from flooded rice paddy was 12%, far greater than that in the wheat season (less than 1%), while N leaching and runoff comprised only 0.3% in the rice season and 5% in the wheat season. Direct N₂O emission was 0.12% for the rice season and 0.14% for the wheat season. The results also showed that some dissolved organic N (DON) were leached in both crop seasons. For the wheat season, DON contributed 40–72% to the N- leaching, in the rice season leached DON was 64–77% of the total N leaching. With increasing fertilizer application rate, NH₃ volatilization in the rice season increased proportionally more than the fertilizer increase, N leaching in the wheat season was proportional to the increase of fertilizer rate, while N₂O emission increased less in proportion than fertilizer increase both in the rice season and wheat season.     

Nitrous oxide emissions and nitrate leaching from a rain-fed wheat-maize rotation in the Sichuan Basin, China

Aims

A 3-year field experiment (October 2004–October 2007) was conducted to quantify N₂O fluxes and determine the regulating factors from rain-fed, N fertilized wheat-maize rotation in the Sichuan Basin, China.

Methods

Static chamber-GC techniques were used to measure soil N₂O fluxes in three treatments (three replicates per treatment): CK (no fertilizer); N150 (300 kg N fertilizer ha^?1 yr^?1 or 150 kg N?ha^?1 per crop); N250 (500 kg N fertilizer ha^?1 yr^?1 kg or 250 kg N?ha^?1 per crop). Nitrate (NO ₃ ^? ) leaching losses were measured at nearby sites using free-drained lysimeters.

Results

The annual N₂O fluxes from the N fertilized treatments were in the range of 1.9 to 6.7 kg N?ha^?1 yr^?1 corresponding to an N₂O emission factor ranging from 0.12 % to 1.06 % (mean value: 0.61 %). The relationship between monthly soil N₂O fluxes and NO ₃ ^- leaching losses can be described by a significant exponential decaying function.

Conclusions

The N₂O emission factor obtained in our study was somewhat lower than the current IPCC default emission factor (1 %). Nitrate leaching, through removal of topsoil NO ₃ ^? , is an underrated regulating factor of soil N₂O fluxes from cropland, especially in the regions where high NO ₃ ^- leaching losses occur.     

Fate of ammonium 15N in a Norway spruce forest under long-term reduction in atmospheric N deposition

In the last decades, in particular forest ecosystems became increasingly N saturated due to elevated atmospheric N deposition, resulting from anthropogenic N emission. This led to serious consequences for the environment such as N leaching to the groundwater. Recent efforts to reduce N emissions raise the question if, and over what timescale, ecosystems recover to previous conditions. In order to study the effects on N distribution and N transformation processes under the lowered N deposition treatment, we investigated the fate of deposited NH₄ ⁺-¹⁵N in soil of a N-saturated Norway spruce forest (current N deposition: 34 kg ha^?1 year^?1; critical N load: 14 kg ha^?1 year^?1), where N deposition has been reduced to 11.5 kg ha^?1 year^?1 since 14.5 years. We traced the deposited ¹⁵N in needle litter, bulk soil, and amino acids, microbial biomass and inorganic N in soil. Under reduced N deposition, 123 ± 23% of the deposited N was retained in bulk soil, while this was only 72 ± 15% under ambient deposition. We presume that with reduced deposition the amount of deposited N was small enough to become completely immobilized in plant and soil and no leaching losses occurred. Trees receiving reduced N deposition showed a decline in N content as well as in ¹⁵N incorporation into needle litter, indicating reduced N plant uptake. In contrast, the distribution of ¹⁵N within the soil over active microbial biomass, microbial residues and inorganic N was not affected by the reduced N deposition. We conclude that the reduction in N deposition impacted only plant uptake and drainage losses, while microbial N transformation processes were not influenced. We assume changes in the biological N turnover to start with the onset of the decomposition of the new, N-depleted litter.     

Carbon losses and primary productivity decline in savannah soils under cotton-cereal rotations in semiarid Togo

Soil degradation in the savannah-derived agroecosystems of West Africa is often associated with rapid depletion of organic carbon stocks in soils of coarse texture. Field experiments were conducted over a period of more than 30 years at two sites in semiarid Togo to test the impact of agricultural management practices on soil C stocks and crop productivity. The resulting datasets were analysed using dynamic simulation models of varying complexity, to study the impact of crop rotation, fertiliser use and crop residue management on soil C dynamics. The models were then used to calculate the size of the annual C inputs necessary to restore C stocks to thresholds that would allow positive crop responses to fertilisers under continuous cultivation. Yields of all crops declined over the 30 years irrespective of crop rotation, fertiliser use or crop residue management. Both seed-cotton and cereal grain yields with fertiliser fluctuated around 1 t ha^?1 after 20 years. Rotations that included early maturing sorghum varieties provided larger C inputs to the soil through residue biomass; around 2.5 t C ha^?1?year^?1. Soil C stocks, originally of 15 t ha^?1 after woodland clearance, decreased by around 3 t ha^?1 at both sites and for virtually all treatments, reaching lower equilibrium levels after 5–10 years of cultivation. Soil C dynamics were well described with a two-pool SOM model running on an annual time step, with parameter values of 0.25 for the fraction of resistant plant material (K₁), 0.15–0.20 for the decomposition rate of labile soil C (K₂) and 8–10 t C ha^?1 for the fraction of stable C in the soil. Simulated addition of organic matter to the soil 30 years after woodland clearance indicated that additions of 3 t C ha^?1?year^?1 for 15–20 years would be necessary to build ‘threshold’ soil C stocks of around 13 t ha^?1, compatible with positive crop response to fertiliser. The simulated soil C increases of 0.5 to 1.6% per year are comparable with results from long-term experiments in the region. However, the amounts of organic matter necessary to build these soil C stocks are not readily available to resource-poor farmers. These experimental results question the assumption that crop residue removal and lack of fertiliser input are responsible for soil C decline in these soils. Even when residues were incorporated and fertilisers used at high rates, crop C inputs were insufficient to compensate for C losses from these sandy soils under continuous cultivation.     

Nitrogen recoveries from organic amendments in crop and soil assessed by isotope techniques under tropical field conditions

The integration of multipurpose legumes into low-input tropical agricultural systems is needed because they are a nitrogen (N) input through symbiotic fixation. The drought-tolerant cover legume canavalia (Canavalia brasiliensis) has been introduced for use either as forage or as a green manure into the crop-livestock system of the Nicaraguan hillsides. To evaluate its impact on the subsequent maize crop, an in-depth study on N dynamics in the soil-plant system was conducted. Microplots were installed in a 6-year old field experiment with maize-canavalia rotation. Direct and indirect ¹⁵N-labelling techniques were used to determine N uptake by maize from canavalia residues and canavalia-fed cows?? manure compared to mineral fertilizer. Litter bags were used to determine the N release from canavalia residues. The incorporation of N from the amendment into different soil N pools (total N, mineral N, microbial biomass) was followed during the maize cropping season. Maize took up an average of 13.3 g?N?m^?2, within which 1.0 g?N?m^?2 was from canavalia residues and 2.6 g?N?m^?2 was from mineral fertilizer, corresponding to an amendment N recovery of 12% and 32%, respectively. Recoveries in maize would probably be higher at a site with lower soil available N content. Most of the amendment N remained in the soil. Mineral N and microbial N were composed mainly of N derived from the soil. Combined total ¹⁵N recovery in maize and soil at harvest was highest for the canavalia residue treatment with 98% recovery, followed by the mineral fertilizer treatment with 83% recovery. Despite similar initial enrichment of soil microbial and mineral N pools, the indirect labelling technique failed to assess the N fertilizer value of mineral and organic amendments due to a high N mineralization from the soil organic matter.     

Crop residue incorporation negates the positive effect of elevated atmospheric carbon dioxide concentration on wheat productivity and fertilizer nitrogen recovery

Background and purpose

Rapid increases in atmospheric carbon dioxide concentration ([CO₂]) may increase crop residue production and carbon: nitrogen (C:N) ratio. Whether the incorporation of residues produced under elevated [CO₂] will limit soil N availability and fertilizer N recovery in the plant is unknown. This study investigated the interaction between crop residue incorporation and elevated [CO₂] on the growth, grain yield and the recovery of ¹⁵N-labeled fertilizer by wheat (Triticum aestivum L. cv. Yitpi) under controlled environmental conditions.

Methods

Residue for ambient and elevated [CO₂] treatments, obtained from wheat grown previously under ambient and elevated [CO₂], respectively, was incorporated into two soils (from a cereal-legume rotation and a cereal-fallow rotation) 1 month before the sowing of wheat. At the early vegetative stage ¹⁵N-labeled granular urea (10.22 atom%) was applied at 50 kg?N ha^?1 and the wheat grown to maturity.

Results

When residue was not incorporated into the soil, elevated [CO₂] increased wheat shoot (16 %) and root biomass (41 %), grain yield (19 %), total N uptake (4 %) and grain N removal (8 %). However, the positive [CO₂] fertilization effect on these parameters was absent in the soil amended with residue. In the absence of residue, elevated [CO₂] increased fertilizer N recovery in the plant (7 %), but when residue was incorporated elevated [CO₂] decreased fertilizer N recovery.

Conclusions

A higher fertilizer application rate will be required under future elevated [CO₂] atmospheres to replenish the extra N removed in grains from cropping systems if no residue is incorporated, or to facilitate the [CO₂] fertilization effect on grain yield by overcoming N immobilization resulting from residue amendment.     

Fertilization of Willow Coppice Over Three Consecutive 2-Year Rotations—Effects on Biomass Production,Soil Nutrients and Water

Short rotation coppice (SRC) willow is a promising bioenergy feedstock. Fertilization is an integrated part of the production system, but knowledge about the effects in consecutive rotations is scarce. The objective of this study was to identify an appropriate fertilization regime for achieving high yields, reducing risks of nutrient leaching and maintaining the soil nutrient stocks in SRC willow on a former arable land. Ten different fertilization treatments were applied, with different application frequencies, fertilizer types and doses over three consecutive 2-year rotations. The biomass production was determined at harvest, soil solution samples were collected monthly, water fluxes were modelled using CoupModel and nutrient budgets were calculated. The unfertilized control had a mean biomass production of 8.3, 8.3 and 9.5 odt ha^?1 year^?1, respectively, in the three rotations. This indicated that nutrients were adequately available to maintain production for at least 6 years without fertilization. When adding 60 kg N ha^?1 year^?1, biomass production tended to be higher than the control, by 33% (p = 0.055), and the treatment where 360 kg N ha^?1 rotation^?1 was added, by 31% (p = 0.08). Treatments with one-time addition of 240 and 360 kg N ha^?1 rotation^?1 had significantly higher nitrogen leaching than all other treatments. Organic fertilizers did not increase biomass production nor N leaching significantly compared to the control, but nutrient budgets indicated a nutrient build-up in the soil. We concluded that application of 60 kg N ha^?1 year^?1 is recommendable, for achieving high biomass yields, low nitrogen leaching and maintenance of the soil nutrient stock.     

Estimation of Nitrogen Pools in Irrigated Potato Production on Sandy Soil Using the Model SUBSTOR

Recent increases in nitrate concentrations in the Suwannee River and associated springs in northern Florida have raised concerns over the contributions of non-point sources. The Middle Suwannee River Basin (MSRB) is of special concern because of prevalent karst topography, unconfined aquifers and sandy soils which increase vulnerability of the ground water contamination from agricultural operations- a billion dollar industry in this region. Potato (Solanum tuberosum L.) production poses a challenge in the area due to the shallow root system of potato plants, and low water and nutrient holding capacity of the sandy soils. A four-year monitoring study for potato production on sandy soil was conducted on a commercial farm located in the MSRB to identify major nitrogen (N) loss pathways and determine their contribution to the total environmental N load, using a partial N budget approach and the potato model SUBSTOR. Model simulated environmental N loading rates were found to lie within one standard deviation of the observed values and identified leaching loss of N as the major sink representing 25 to 38% (or 85 to 138 kg ha^-1 N) of the total input N (310 to 349 kg ha^-1 N). The crop residues left in the field after tuber harvest represented a significant amount of N (64 to 110 kg ha^-1N) and posed potential for indirect leaching loss of N upon their mineralization and the absence of subsequent cover crops. Typically, two months of fallow period exits between harvest of tubers and planting of the fall row crop (silage corn). The fallow period is characterized by summer rains which pose a threat to N released from rapidly mineralizing potato vines. Strategies to reduce N loading into the groundwater from potato production must focus on development and adoption of best management practices aimed on reducing direct as well as indirect N leaching losses.     

Quality and decomposition in soil of rhizome, root and senescent leaf from Miscanthus x giganteus, as affected by harvest date and N fertilization

To predict the environmental benefits of energy crop production and use, the nature and fate of biomass residues in the soil need to be quantified. Our objective was to quantify Miscanthus x giganteus biomass recycling to soil and to assess how harvesting time and N fertilization affect their characteristics and subsequent biodegradability. The quantification of aerial and belowground biomass and their sampling were performed on 2- and 3-year-old Miscanthus stands, either fertilized with 120 kg N ha^?1 year^?1 or not fertilized, in autumn (maximal biomass production) and winter (maturity). Plant biomass was chemically characterized (total sugars, Klason lignin, C/N) and incubated in optimum decomposition conditions (15°C, ?80 kPa) for 263 days, for C and N mineralization. Accumulation of carbon in rhizomes and roots was 7.5 to 10 t C ha^?1 and represented about 50% of total plant biomass C. Senescent leaves represented about 1.5 t C ha^?1 year^?1. All residues, especially the roots, had high lignin contents, while the rhizomes also had a high soluble content due to their nutrient storage function. The C mineralization rates were closely related to the chemical characteristics of the residue, higher sugar and lower lignin contents leading to faster decomposition, as observed for rhizomes.     

Environmental Effects over the First 2½ Rotation Periods of a Fertilised Poplar Short Rotation Coppice

A short rotation coppice (SRC) with poplar was established in a randomised fertilisation experiment on sandy loam soil in Potsdam (Northeast Germany). The main objective of this study was to assess if negative environmental effects as nitrogen leaching and greenhouse gas emissions are enhanced by mineral nitrogen (N) fertiliser applied to poplar at rates of 0, 50 and 75 kg N ha^?1 year^?1 and how these effects are influenced by tree age with increasing number of rotation periods and cycles of organic matter decomposition and tree growth after each harvesting event. Between 2008 and 2012, the leaching of nitrate (NO₃ ^?) was monitored with self-integrating accumulators over 6-month periods and the emissions of the greenhouse gases (GHG) nitrous oxide (N₂O) and carbon dioxide (CO₂) were determined in closed gas chambers. During the first 4 years of the poplar SRC, most nitrogen was lost through NO₃ ^? leaching from the main root zone; however, there was no significant relationship to the rate of N fertilisation. On average, 5.8 kg N ha^?1 year^?1 (13.0 kg CO_2equ) was leached from the root zone. Nitrogen leaching rates decreased in the course of the 4-year study parallel to an increase of the fine root biomass and the degree of mycorrhization. In contrast to N leaching, the loss of nitrogen by N₂O emissions from the soil was very low with an average of 0.61 kg N ha^?1 year^?1 (182 kg CO_2equ) and were also not affected by N fertilisation over the whole study period. Real CO₂ emissions from the poplar soil were two orders of magnitude higher ranging between 15,122 and 19,091 kg CO₂ ha^?1 year^?1 and followed the rotation period with enhanced emission rates in the years of harvest. As key-factors for NO₃ ^? leaching and N₂O emissions, the time after planting and after harvest and the rotation period have been identified by a mixed effects model.     

Availability of residual fertilizer 15 N from forest floor and mineral soil to Douglas-fir seedlings ten years after fertilization

Background and aims

As low initial uptake and essentially zero later uptake limit efficacy of N fertilization for temperate conifers, we investigated factors limiting long-term tree uptake of residual ¹⁵?N-labeled fertilizer.

Methods

We used a pot bioassay to assess availability of ¹⁵?N from soil sampled 10 years after fertilization of a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stand with ¹⁵?N-urea (200 kg N ha^?1). Douglas-fir seedlings were grown for 2 years in organic (designated LFH) and mineral soil (0–10 cm) layers reconstructed from control and fertilized plots; residual fertilizer N amounted to 10 % of LHF and 5 % of MIN N.

Results

Percentage recovery of residual ¹⁵?N in seedlings was not affected by the original season of fertilization (spring vs. fall), but differed by the source of ¹⁵?N excess. LFH was a better source of residual ¹⁵?N; 12.4 % of residual LFH ¹⁵?N was taken up by seedlings and 7.6 % transferred to soil, whereas mineral soil yielded only 8.3 % of residual ¹⁵?N to seedling uptake and 2.4 % to LFH. Extractable inorganic N was 2–3 orders of magnitude higher in fallow pots.

Conclusions

Ten-year residual fertilizer ¹⁵?N was clearly cycling between LFH and mineral soil and available to seedlings, indicating that other factors such as denitrification, leaching, and asynchrony of soil N mineralization and tree uptake limit long-term residual N fertilizer uptake in the field.     

Yield gaps, nutrient use efficiencies and response to fertilisers by maize across heterogeneous smallholder farms of western Kenya

The need to promote fertiliser use by African smallholder farmers to counteract the current decline in per capita food production is widely recognised. But soil heterogeneity results in variable responses of crops to fertilisers within single farms. We used existing databases on maize production under farmer (F-M) and researcher management (R-M) to analyse the effect of soil heterogeneity on the different components of nutrient use efficiency by maize growing on smallholder farms in western Kenya: nutrient availability, capture and conversion efficiencies and crop biomass partitioning. Subsequently, we used the simple model QUEFTS to calculate nutrient recovery efficiencies from the R-M plots and to calculate attainable yields with and without fertilisers based on measured soil properties across heterogeneous farms. The yield gap of maize between F-M and R-M varied from 0.5 to 3 t grain ha^?1 season^?1 across field types and localities. Poor fields under R-M yielded better than F-M, even without fertilisers. Such differences, of up to 1.1 t ha^?1 greater yields under R-M conditions are attributable to improved agronomic management and germplasm. The relative response of maize to N–P–K fertilisers tended to decrease with increasing soil quality (soil C and extractable P), from a maximum of 4.4-fold to ?0.5-fold relative to the control. Soil heterogeneity affected resource use efficiencies mainly through effects on the efficiency of resource capture. Apparent recovery efficiencies varied between 0 and 70% for N, 0 and 15% for P, and 0 to 52% for K. Resource conversion efficiencies were less variable across fields and localities, with average values of 97 kg DM kg^?1 N, 558 kg DM kg^?1 P and 111 kg DM kg^?1 K taken up. Using measured soil chemical properties QUEFTS over-estimated observed yields under F-M, indicating that variable crop performance within and across farms cannot be ascribed solely to soil nutrient availability. For the R-M plots QUEFTS predicted positive crop responses to application of 30 kg P ha^?1 and 30 kg P ha^?1 + 90 kg N ha^?1 for a wide range of soil qualities, indicating that there is room to improve current crop productivity through fertiliser use. To ensure their efficient use in sub-Saharan Africa mineral fertilisers should be: (1) targeted to specific niches of soil fertility within heterogeneous farms; and (2) go hand-in-hand with the implementation of agronomic measures to improve their capture and utilisation.     

Soil Greenhouse Gas Emissions and Carbon Dynamics of a No-Till,Corn-Based Cellulosic Ethanol Production System

Crop residues like corn (Zea mays L.) stover perform important functions that promote soil health and provide ecosystem services that influence agricultural sustainability and global biogeochemical cycles. We evaluated the effect of corn stover removal from a no-till, corn-soybean (Glycine max (L.) Merr) rotation on soil greenhouse gas (GHG; CO₂, N₂O, CH₄) fluxes, crop yields, and soil organic carbon (SOC) dynamics. We conducted a 4-year study using replicated field plots managed with two levels of corn stover removal (none; 55 % stover removal) for four complete crop cycles prior to initiation of ground surface gas flux measurements. Corn and soybean yields were not affected by stover removal with yields averaging 7.28 Mg ha^?1 for corn and 2.64 Mg ha^?1 for soybean. Corn stover removal treatment did not affect soil GHG fluxes from the corn phase; however, the treatment did significantly increase (107 %, P?=?0.037) N₂O fluxes during the soybean phase. The plots were a net source of CH₄ (～0.5 kg CH₄-C ha^?1 year^?1 average of all treatments and crops) during the generally wet study duration. Soil organic carbon stocks increased in both treatments during the 4-year study (initiated following 8 years of stover removal), with significantly higher SOC accumulation in the control plots compared to plots with corn stover removal (0–15 cm, P?=?0.048). Non-CO₂ greenhouse gas emissions (945 kg CO₂-eq ha^?1 year^?1) were roughly half of SOC (0–30 cm) gains with corn stover removal (1.841 Mg CO₂-eq ha^?1 year^?1) indicating that no-till practices greatly improve the viability of biennial corn stover harvesting under local soil-climatic conditions. Our results also show that repeated corn stover harvesting may increase N loss (as N₂O) from fields and thereby contribute to GHG production and loss of potential plant nutrients.     

Assessment of nitrate leaching loss on a yield-scaled basis from maize and wheat cropping systems

Background and aims

It is so far a gap in knowledge to assess nitrate (NO₃ ^?) leaching loss linking with crop yield for a given cereal cropping system.

Methods

We conducted a meta-analysis on 32 published studies reporting both NO₃ ^? leaching losses and crop yields in the maize (N?=?20) and wheat (N?=?12) systems.

Results

On average, 22 % and 15 % of applied fertilizer N to wheat and maize systems worldwide are leached in the form of NO₃ ^?, respectively. The average area-scaled NO₃ ^- leaching loss for maize (57.4 kg N ha^?1) was approx. two times higher than for wheat (29.0 kg N ha^?1). While, if scaled to crop yields, the average yield-scaled NO₃ ^? losses were comparable between maize (5.40 kg N Mg^?1) and wheat (5.41 kg N Mg^?1) systems. Across all sites, the lowest yield-scaled NO₃ ^? leaching losses were observed at slightly suboptimal fertilization rates, corresponding to 90 % and 96 % of maximum maize or wheat yields, respectively.

Conclusions

Our findings suggest that small adjustments of agricultural N management practices can effectively reduce yield-scaled NO₃ ^? leaching losses. However, further targeted field experiments are still needed to identify at regional scale best agricultural management practices for reducing yield-scaled NO₃ ^? leaching losses in maize and wheat systems.     

Nitrogen cycling in leucaena (Leucaena leucocephala) alley cropping in semi-arid tropics

In an alley cropping system, prunings from the hedgerow legume are expected to supply nitrogen (N) to the associated cereal. However, this may not be sufficient to achieve maximum crop yield. Three field experiments with alley-cropped maize were conducted in a semi-arid environment in northern Australia to determine: (1) the effect of N fertilizer on maize growth in the presence of fresh leucaena prunings; (2) the effect of incorporation of leucaena and maize residues on maize yield and the fate of plant residue¹⁵N in the alley cropping system; and (3) the¹⁵N recovery by maize from¹⁵N-labelled leucaena, maize residues and ammonium sulphate fertilizer.Leucaena residues increased maize crop yield and N uptake although they did not entirely satisfy the N requirement of the alley crop. Additional N fertilizer further increased the maize yield and N uptake in the presence of leucaena residues. Placement of leucaena residues had little effect on the availability of N to maize plants over a 2 month period. The incorporation of leucaena residues in the soil did not increase the recovery of leucaena¹⁵N by maize compared with placement of the residues on the soil surface. After 2 months, similar proportions of the residue¹⁵N were recovered by maize from mulched leucaena (6.3%), incorporated leucaena (6.1%) and incorporated maize (7.6%). By the end of one cropping season (3 months after application) about 9% of the added¹⁵N was taken up by maize from either¹⁵N-labelled leucaena as mulch or¹⁵N-labelled maize residues applied together with unlabelled fresh leucaena prunings as mulch. The recovery of the added¹⁵N was much higher (42.7%) from the¹⁵N-labelled ammonium sulphate fertilizer at 40 kg N ha^-1 in the presence of unlabelled leucaena prunings. Most of the added¹⁵N recovered in the 200 cm soil profile was distributed in the top 25 cm soil with little leached below that. About 27–41% of the leucaena¹⁵N was apparently lost, largely through denitrification from the soil and plant system, in one cropping season. This compared with 35% of the fertilizer¹⁵N lost when the N fertilizer was applied in the presence of prunings. ei]H Lambers     

Transformation of15N-labelled urea in rice-wheat cropping system

Summary In a udic chromusterts the transformation of an initial application of¹⁵N-urea @ 80 kg N ha^–1 to rice (Oryza sativa L.) in rice-wheat (R-W) and to wheat (Triticum aestivum L.) in wheat-rice (W-R) rotations was followed in 6 successive crops in each rotation. All rice crops were grown in irrigated wetland and wheat in irrigated upland conditions.The first wheat crop in W-R rotation utilized 22 kg fertilizer N ha^–1 as compared to 19 kg by the corresponding rice crop in R-W rotation. But the latter absorbed more soil N than the former. About 69% of the total N uptake in rice was derived from mineralization of soil organic N as compared to 61% in wheat.The succeeding wheat crop in R-W rotation utilized 6.7% of the residual fertilizer N in the soil but the corresponding rice crop in W-R rotation only 2.2%. The higher utilization appeared to be related to a greater incorporation of labelled fertilizer N in mineral and hexosamine fractions of the soil N. After the second crop in each rotation, the average residual fertilizer N utilization in the next 4 crops ranged between 3 and 4%.The total recovery of¹⁵N-urea in all crops amounted to 21.7 and 24.3 kg N ha^–1 in R-W and W-R rotation, respectively. At the end of the experiment, about 9 to 10 kg ha^–1 of the applied labelled N was found in soil upto 60 cm depth. Most of the labelled soil N (69–76%) was located in the upper 0–20 cm soil layer indicating little movement to lower depths despite intensive cropping for 4 years.