Phosphorus Mass Balances for Successive Crops of Fertilised Rainfed Rice on a Sandy Lowland Soil

Raising and sustaining rice yields in the rainfed lowlands requires an understanding of nutrient inputs and outputs. On sandy lowland rice soils, managing phosphorus (P) supply is a key factor in achieving increased yields and sustainable production. Phosphorus inputs, rice yields, and crop P uptake were used to quantify P requirements of rice: together with results on soil P fractions, P balance sheets were constructed over five consecutive cropping seasons on a sandy Plinthustalf near Phnom Penh, Cambodia. Grain yields ranged from 665 to 1557 kg ha⁻¹ with no added P. Average yields increased significantly with P fertiliser application over five consecutive crops by 117, 139 and 140% when the phosphate fertiliser was applied at 8.25, 16.5 and 33 kg P ha⁻¹, respectively. Without added P fertiliser, a net loss of 1.2 kg P ha⁻¹ per crop was estimated with straw return and 2.0 kg P ha⁻¹ per crop with straw removed from the field, whereas, with added P fertiliser, there was a net P gain in the soil of 5.6 or 9.5 kg ha⁻¹ per crop when straw was removed and returned to the soil, respectively. After one crop, the addition of P fertiliser significantly (P < 0.01) increased recovery in all soil P fractions. Across five successive crops, repeated application of 16.5 and 33 kg P ha⁻¹ rates resulted in progressive P accumulation in the soil, especially a labile NaOH–Po pool, but had no effect on yields and P uptake of rice. By contrast, 8.25 kg P ha⁻¹ per rice crop was generally adequate for grain yields of 2.5–3.0 t ha⁻¹ and to maintain soil P pools.     

Towards a Revised Coefficient for Estimating N2O Emissions from Legumes

The Intergovernmental Panel on Climate Change (IPCC) standard methodology to conduct national inventories of soil N₂O emissions is based on default (or Tier I) emission factors for various sources. The objective of our study was to summarize recent N₂O flux data from agricultural legume crops to assess the emission factor associated with rhizobial nitrogen fixation. Average N₂O emissions from legumes are 1.0 kg N ha⁻¹ for annual crops, 1.8 kg N ha⁻¹ for pure forage crops and 0.4 kg N ha⁻¹ for grass legume mixes. These values are only slightly greater than background emissions from agricultural crops and are much lower that those predicted using 1996 IPCC methodology. These field flux measurements and other process-level studies offer little support for the use of an emission factor for biological N fixation (BNF) by legume crops equal to that for fertiliser N. We conclude that much of the increase in soil N₂O emissions in legume crops may be attributable to the N release from root exudates during the growing season and from decomposition of crop residues after harvest, rather than from BNF per se. Consequently, we propose that the biological fixation process itself be removed from the IPCC N₂O inventory methodology, and that N₂O emissions induced by the growth of legume crops be estimated solely as a function of crop residue decomposition using an estimate of above- and below-ground residue inputs, modified as necessary to reflect recent findings on N allocation.     

Farmers’ agronomic and social evaluation of productivity,yield and N<Subscript>2</Subscript>-fixation in different cowpea varieties and their subsequent residual N effects on a succeeding maize crop

Cowpea–maize rotations form an important component of the farming systems of smallholder farmers in the forest/savannah transitional agro-ecological zone of Ghana. We evaluated five cowpea varieties for grain yield, N₂-fixation, biomass production, and contribution to productivity of subsequent maize grown in rotation. We further analyzed the interrelationship between these technical dimensions and the social acceptability of these cowpea varieties for farmers. Cowpea grain yield ranged between 1.1 and 1.4 t ha⁻¹ with no significant yield differences among the different varieties. Using the ¹⁵N natural abundance technique, the average proportion of N₂ fixed ranged between 61% for Ayiyi and 77% for Legon prolific. This resulted in average amounts of N₂ fixed in above-ground biomass ranging between 32 and 67 kg N ha⁻¹, respectively. Variation in estimates due to differences in δ¹⁵N among reference plants were larger than differences between cowpea varieties. The amount of soil-derived N ranged from 15 to 20 kg N ha⁻¹. The above-ground net N contribution of the cowpea varieties to the soil (after adjusting for N export in grains) was highest for Legon Prolific (31 kg N ha⁻¹) due to high N₂-fixation and high leaf biomass production. Maize grain yield after cowpea without application of mineral N fertilizer ranged between 0.4 t ha⁻¹ with maize after maize to 1.5 t ha⁻¹ with Legon Prolific. The N fertilizer equivalence values for the cowpea varieties ranged between 18 and 60 kg N ha⁻¹. IT810D-1010 was ranked by the farmers as the most preferred cowpea variety due to its white seed type, short-duration, ease of harvesting and good market value. Despite the high leaf biomass production and high amount of N₂ fixed by Legon Prolific, it was generally the least preferred variety due to lower market price, late maturity, least potential cash income (due to the red mottled seed type) and difficulty in harvesting. Although farmers recognized the contribution of cowpea to soil fertility and yields of subsequent maize, they did not consider this as an important criterion for varietal selection. Soil fertility improvement must be considered as an additional benefit rather than a direct selection criterion when designing more sustainable smallholder farming systems.     

On-farm estimation of indigenous nitrogen supply for site-specific nitrogen management in the North China plain

Estimating indigenous nitrogen supply (INS) by measurement of crop N uptake in N omission plots for site-specific N management is not feasible on a routine basis because it involves destructive plant sampling and plant tissue analysis, which is time-consuming and expensive. The objective of this study was to determine the amount of INS and develop a method to estimate it using soil testing in the North China plain (NCP). On-farm experiments at 229 sites were conducted from 2003 to 2005 in seven key winter wheat (Triticum aestivum L.)/summer maize (Zea mays L.) production regions of the NCP. The mean INS during the wheat-growing season was129 kg N ha⁻¹ with a range from 62 to 212 kg N ha⁻¹, and it varied from 69 to 202 kg N ha⁻¹ with a mean of 142 kg N ha⁻¹ during the maize-growing season. Considering all sites, the variability of INS was not simulated by initial soil N _min or apparent N mineralization (N _organic) alone, while together they could explain about 38% and 60% of INS during the wheat and maize-growing seasons, respectively. During the wheat-growing season, mean N _organic was 63 kg N ha⁻¹, and 59% and 33% of its variation could be explained by SOM in high-yielding regions (mean yield, 7.6 t ha⁻¹) and low-yielding regions (mean yield, 5.3 t ha⁻¹), respectively. Mean N _organic during the maize-growing season was 109 kg N ha⁻¹, 22% of which could be explained by SOM across all sites. An average of 40% and 42% of INS variation could be explained by both SOM and initial soil N _min content during the wheat and maize-growing seasons, respectively. We conclude that the accuracy of estimating crop N requirement for site-specific N management will be increased by using initial soil N _min and SOM.     

Nutrient Uptake and Balance of Cotton + Pigeonpea Strip Intercropping on Rainfed Vertisols of Central India

Nutrient uptake and balance of the cotton (Gossypium hirsutum L.) + pigeonpea (Cajanus cajan(L.) Millsp.), a traditional strip intercropping system practiced on the rainfed Vertisols of central India is not known to us. On-farm participatory trials were conducted on 10 farmer fields, five each on medium deep (MDS) and deep soils (DS) of Nagpur, central India to determine the effect of technological interventions on N, P and K uptake of cotton and pigeonpea. The nutrient balance was also quantified as a difference of nutrient inputs and removal. Nutrients accumulated by the crops (grain, stalk and leaves) and weeds removed off the field by hand weeding were considered as nutrient removal, while fertilizer was considered as nutrient input. The interventions included application of recommended dose of fertilizer (RDF), RDF + conservation tillage with in situ green manure (CT₁) and CT₁ + application of ZnSO₄ (CT₂) and compared with farmers’ practice (FP). Nutrient uptake, in general, was higher on DS than on MDS. Among the interventions, N, P and K uptake of cotton and pigeonpea followed the order: CT₂ > CT₁ > RDF > FP. Mean N and P balance was positive in all the treatments. The balance may become negative if nutrient losses are accounted. A negative K balance was observed in all the treatments and the magnitude was the greatest for the FP plots (−39.4 kg ha⁻¹ y⁻¹). In spite of fertilizer-K application in the intervention plots, K balance was negative (−14.4 to −19.5 kg ha⁻¹ y⁻¹). By way of leaf and fruit drop, cotton and pigeonpea litter recycled 12.2 kg N, 1.7 kg P and 6.7 kg K ha⁻¹ y⁻¹     

Plant recovery of 15N-labelled nitrogen applied to reed canary grass grown for biomass

Reed canary grass (Phalaris arundinacea L.) is apotential crop for production of bioenergy and biomass in northern Europe. In this study labelled ¹⁵N was used to follow the fate of applied N in roots and shoots of reed canary grass during a year. Two rates of¹⁵N fertiliser were applied in spring 1995 and 1996 to a clay (50 kg ha⁻¹ and 100 kg ha⁻¹) and an organic soil (30 kg ha⁻¹ and 60 kg ha⁻¹). The data did not indicate significant differences between recoveries of nitrogen following application of fertiliser at recommended and half of the recommended rates. The recovery of added N in shoots was highest at midsummer. The median values were 68% and 58% inorganic soil and 42% and 65% in clay soil, in 1995 and 1996respectively. Some of the N utilised by shoots was remobilised to the roots during autumn. The highest median recovery of applied N in roots was 19%in clay soil in October 1996, corresponding to a 13 percentage unit increase in recovery during autumn. In contrast, the lowest remobilisation was recorded after a rainy spring in clay soil, being only 3 percentage units. During winter the loss of N and fertiliser N from the shoots continued, and consequently the total N content in shoots was about half of that for autumn. In spring, one year after N application, the shoots contained 9–20% of applied N. The data suggest both intensive uptake and remobilisation of fertiliser N during over a year, following delayed harvest, and indicate the importance of the rhizome system in N turnover. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of cropping history and phosphorus source on yield and nitrogen fixation in sole and intercropped cowpea–maize systems

Symbiotic N₂-fixation, N uptake efficiency, biomass- and crop production of cowpea and maize as affected by P source, sole- and intercropped, and introduction of break crops were studied on a farmer’s fields in semi-arid Tanzania. Cowpea fixed around 60% of its N from the atmosphere amounting to 70 kg N ha⁻¹ under sole and 36 kg N ha⁻¹ under intercropping as estimated by the ¹⁵N isotope dilution method around peak biomass production. The amount of N₂-fixed was 30–40% higher when P was applied as either TSP or MRP whereas cowpea yield were unaffected. Intercropped maize with 19,000 plant ha⁻¹ accumulated the same amount of N as 38,000 sole cropped maize plants although intercropping reduced the dry matter accumulation by 25%. The N uptake efficiency of the applied ¹⁵N labelled fertiliser was 26%, which equal a total pool of early available plant N of 158 kg N ha⁻¹. Under the N deficient conditions, P application did not increase the grain yield of maize. The LER indicate that sole cropping required 18% more area than intercropping in order to produce the same grain yield, and 35% more land when LER was based on N uptakes. Introduction of break crops in the maize systems, more than doubled accumulation of dry matter and N in the grain compared to continuous maize cropping. During maturation sole crop cowpea shedded leaves containing 41 kg N ha⁻¹. The current findings underline the importance of crop diversity in Sub Saharan Africa agriculture and emphasise the need for including all residues, including shedded leaves, in nutrient balance studies.     

Effect of crop-specific field management and N fertilization on N2O emissions from a fine-loamy soil

Agricultural soils are a major source of atmospheric N₂O. This study was conducted to determine the effect of different crop-specific field management and N fertilization rates on N₂O emissions from a fine-loamy Dystric Eutrochrept. Fluxes of N₂O were measured for two years at least once a week on plots cropped with potatoes (Solanum tuberosum) fertilized with 50 or 150 kg N ha⁻¹ a⁻¹, winterwheat (Triticum aestivum) fertilized with 90 or 180 kg N ha⁻¹ a⁻¹, corn (Zea mays) fertilized with 65 or 130 kg N ha⁻¹ a⁻¹, and on an unfertilized, set-aside soil planted with grass (mainly Lolium perenne and Festuca rubra). The mean N₂O emission rate from the differently managed plots was closely correlated to the mean soil nitrate content in the Ap horizon for the cropping period (April to October, r ² = 0.74), the winter period (November to March, r ² = 0.93, one outlier excluded), and the whole year (r ² = 0.81). N₂O emissions outside the cropping period accounted for up to 58% of the annual emissions and were strongly affected by frost-thaw cycles. There was only a slight relationship between the amount of fertilizer N applied and the annual N₂O emission (r ² = 0.20). The mean annual N₂O-N emission from the unfertilized set-aside soil was 0.29 kg ha⁻¹. The annual N₂O-N emission from the fertilized crops for the low and the recommended rates of N fertilization were 1.34 and 2.41 kg ha⁻¹ for corn, 2.70 and 3.64 kg ha⁻¹ for wheat, and 5.74 and 6.93 kg ha⁻¹ for potatoes. The high N₂O emissions from potato plots were due to (i) high N₂O losses from the interrow area during the cropping season and (ii) high soil nitrate contents after the potato harvest. The reduction of N fertilization (fertilizer was applied in spring and early summer) resulted in decreased N₂O emissions during the cropping period. However, the emissions during the winter were not affected by the rate of N fertilization. The results show that the crop-specific field management had a great influence on the annual N₂O emissions. It also affected the emissions per unit N fertilizer applied. The main reasons for this crop effect were crop-specific differences in soil nitrate and soil moisture content. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Effect of Future Climate Perturbations on N<Subscript>2</Subscript>O Emissions from a Fertilized Humid Grassland

N₂O emissions from a fertilized humid grassland near Cork, Ireland were continuously measured during 2003 using an eddy covariance system. For most of the year emissions were close to zero and 60% of the emissions occurred in eight major events of 2–20 days’ duration. Two hundred and seven kg ha⁻¹ of synthetic N and 130 kg ha⁻¹ organic N were applied over the year and the total measured annual N₂O emission was 11.6 kg N ha⁻¹. The flux data were used to test the prediction of N₂O emissions by the DNDC (DeNitrification – DeComposition) model. The model predicted total emissions of 15.4 kg N ha⁻¹, 32 % more than the observed emissions. On this basis the model was further used to simulate (a) background (non-anthropogenic) N₂O emissions and (b) the effect on N₂O emissions of future climate perturbations based on the Hadley Center model output of the IS92a scenario for Ireland. DNDC predicts 1.7 kg N ha⁻¹ year⁻¹ of background N₂O emissions, accounting for 15% of the observed emissions. Climate shifts will increase total annual modeled N₂O emissions from 15.4 kg N ha⁻¹ to 22.4 kg N ha⁻¹ if current levels of N applications are maintained, or to 21.2 kg N ha⁻¹ if synthetic N applications are reduced to 170 kg N ha⁻¹ to comply with recent EU water quality legislation. Thus the projected increase in N₂O emissions due to climate change is far larger than the decrease expected from reduced fertilizer applications.     

Meat and bone meal as nitrogen and phosphorus fertilizer to cereals and rye grass

Meat and bone meal (MBM) contains appreciable amounts of total nitrogen (~8%), phosphorus (~5%) and calcium (~10%). It may therefore be a useful fertilizer for various crops. This paper shows results from both pot and field experiments on the N and P effects of MBM. In two field experiments with spring wheat, increasing amounts of MBM (500, 1000, 2000 kg MBM ha⁻¹) showed a linear yield increase related to the N-supply. A similar experiment with barley gave positive yield increase for 500 kg MBM ha⁻¹ and no further yield increase for larger amounts of MBM. Supply of extra mineral P gave no yield increase when 500 kg MBM ha⁻¹ or more was applied. Meat and bone meal as P fertilizer was studied in greenhouse experiments using spring barley and rye grass as test crops. N applications were 100 N kg ha⁻¹ to barley and 200 kg N ha⁻¹ to rye grass, either from mineral fertilizer or assuming that 80% of total N in MBM was effective. Four different P deficient soils were given increasing doses of MBM and compared with compound NPK fertilizer 11-5-18, mineral N fertilizer (0 kg P ha⁻¹) and a control (0 kg N ha⁻¹, 0 kg P ha⁻¹). In barley there was no significant yield difference between the NPK treatment and MBM treatment with equal N supply, and both had significant higher yield than the treatment receiving the same amount of mineral N without P-supply. The positive yield response of MBM was even larger in rye grass. Both in barley and rye grass a significant residual effect of P from MBM applied the year before was found when the treatments received the same amount of mineral N fertilizer (0 kg P ha⁻¹). The pot experiments confirmed the assumed N effect of MBM. When MBM is used according to the N demand of the crops, the P supply will be more than sufficient and residual P will be left in the soil. Since a part of this residual P was utilized by the crops of the following year, it is not recommended to apply P-fertilizer the year after MBM application.     

Nitrogen Leaching from <Superscript>15</Superscript>N Labelled Cow Urine and Dung Applied to Grassland on a Sandy Soil

Nitrogen (N) leaching under grazed pastures can be very high directly under urine spots. The amount of N which is returned by one excretion of urine or dung can locally exceed 1000 kg ha⁻¹ a⁻¹ which is far more than the uptake by surrounding plants during one grazing period. We therefore quantified the contribution of N deriving from urine and dung to the total N leaching under urine and dung patches. Dung N and urine N was separately sampled from a cow feed with ¹⁵N labelled grassilage, and were amended on lysimeters in October 2000 and October 2001. Lysimeters (350 mm diameter and 800 mm length) were filled with sand, and an intact grass sod from a pasture, 4 lysimeter each were amended with the ¹⁵N labelled dung and urine; 4 lysimeters without an application of dung or urine served as control. During 11 months after dung and urine amendment the amount of leachate was monitored and leachate was analysed for nitrate, ammonium and total N. ¹⁵N in these fractions was measured. Dung and urine applications of 1052 and 1030 kg N ha⁻¹ in autumn increased N leaching. Leaching loss of nitrate and dissolved organic N deriving from dung was only 37 kg N ha⁻¹ in both years, whereas under urine patches 447 kg nitrate-N ha⁻¹, 108 kg N ha⁻¹ ammonia-N and 53 kg ha⁻¹ dissolved organic N leached on average of both experimental years. N not deriving from dung and urine exceeded the leached N under the control by about 36 and 136 kg ha⁻¹ on average of both years, suggesting the contribution of different priming processes.     

Recovery of mineral fertiliser N and slurry N in continuous silage maize using the <Superscript>15</Superscript>N and difference methods

The stable isotope technique and the difference method are common approaches for estimating fertiliser N uptake efficiency. Both methods, however, have limitations and their suitability may depend on N management and environmental conditions. A field experiment was conducted on a humus sandy soil in northern Germany to estimate fertiliser N uptake efficiency of silage maize in the year of application (Zea mays L.) by the stable isotope and the difference method as influenced by the type of N fertiliser (mineral vs. cattle slurry), the application mode (separate or combined application), and N rate. Seven N treatments were included (0, 50, 100 and 150 kg mineral N ha⁻¹; 20, 40 m3 cattle slurry ha⁻¹; 50 kg mineral N ha⁻¹ plus 40 m3 slurry ha⁻¹), where either mineral N or slurry N was labelled, and mineral N was split into two dressings. In addition, 4.1 kg ha⁻¹ labelled mineral N was incorporated into otherwise unlabelled treatments (0, 20, 40 m3 ha⁻¹, and 50 kg mineral N ha⁻¹ plus 40 m3 ha⁻¹) to estimate N uptake from the upper soil layer. Uptake of ¹⁵N was followed in leaves, stalk, ear, and the whole crop. Fertiliser N uptake efficiency (FNUE_15N) of mineral fertiliser N obtained by the isotope technique ranged between 51 and 61%. Recovered fertiliser N was mainly found in the ear, while less labelled N remained in leaves and the stalk. The nitrogen rate tended to increase the amount of recovered N, but the effect was not consistent among plant parts and the whole crop. Plant N uptake from non-fertiliser N was found to increase N input up to 100 kg N ha⁻¹. Nitrogen recoveries of the two mineral N dressings were similar for the different plant parts as well as for the whole crop. Fertiliser N uptake efficiency (FNUE_diff) of mineral N estimated by the difference method resulted in substantially higher values compared to FNUE_15N, varying between 56 and 98%. More N was taken up from the upper soil layer with increasing N supply, which is regarded as a major error source of the difference method. Slurry N was taken up less efficient in the year of application than mineral fertiliser N as indicated by recovery rates of 21–22% (FNUE_15N) and 39–62% (FNUE_diff), respectively. When mineral N and slurry were applied together, the difference method estimated significantly lower N uptake efficiencies for both mineral and slurry N compared to a single application, while values obtained by the isotope method were not affected.     

Nitrogen Cycling in an Irrigated Wheat System in Sonora, Mexico: Measurements and Modeling

An improved version of an ecosystem nitrogen cycling model (NLOSS) is described, tested, and used to analyze nitrogen cycling in the Yaqui Valley, Sonora, Mexico. In addition to previously described modules in NLOSS that simulate soil water and solute fluxes, soil evaporation, soil energy balance, and denitrification, modules were added to estimate crop growth, soil carbon cycling, urea hydrolysis, and nitrification. We first tested the model against season-long measurements of soil NO₃⁻, NO₂⁻, and NH₄⁺ aqueous concentrations; NO and N₂O soil effluxes; and crop biomass accumulation in three fertilizer treatments. We used NLOSS to test the sensitivity of wheat production, NO₃⁻ losses, and trace-gas emissions to fertilizer application rate. With the␣model, we compared the typical farmer’s fertilization of 250 kg N ha⁻¹ with five other fertilization scenarios, ranging from 110 to 220 kg N ha⁻¹. The typical farmer’s practice produced higher wheat yield than the lower fertilization treatments. However, the increase in yield per increase in kg N applied decreased with increasing fertilizer addition as a result of higher leaching losses, higher residual N, and higher trace-gas emissions. In addition, with respect to the lowest fertilization treatment, the highest fertilization treatment resulted in an 11% decrease, a 10% increase, and a 157% increase in N₂, N₂O, and NO emissions, respectively, and a 41% increase in leached NO₃⁻ + NO₂⁻. These results demonstrate that a small decrease in fertilizer application rate can increase N-use efficiency for wheat growth, while reducing leaching losses and emissions of harmful trace gas fluxes.     

Intercropping of Wheat and Pea as Influenced by Nitrogen Fertilization

The effect of sole and intercropping of field pea (Pisum sativum L.) and spring wheat (Triticum aestivum L.) on crop yield, fertilizer and soil nitrogen (N) use was tested on a sandy loam soil at three levels of urea fertilizer N (0, 4 and 8 g N m⁻²) applied at sowing. The ¹⁵ N enrichment and natural abundance techniques were used to determine N accumulation in the crops from the soil, fertilizer and symbiotic N₂ fixation. Intercrops of pea and wheat showed maximum productivity without the supply of N fertilizer. Intercropping increased total dry matter (DM) and N yield, grain DM and N yield, grain N concentration, the proportion of N derived from symbiotic N₂ fixation, and soil N accumulation. With increasing fertilizer N supply, intercropped and sole cropped wheat responded with increased yield, grain N yield and soil N accumulation, whereas the opposite was the case for pea. Fertilizer N enhanced the competitive ability of intercropped wheat recovering up to 90% of the total intercrop fertilizer N acquisition and decreased the proportion of pea in the intercrop, but without influencing the total intercrop grain yield. As a consequence, Land Equivalent Ratios calculated on basis of total DM production decreased from a maximum of 1.34 to as low as 0.85 with increased fertilizer N supply. The study suggests that pea–wheat intercropping is a cropping strategy that use N sources efficiently due to its spatial self-regulating dynamics where pea improve its interspecific competitive ability in areas with lower soil N levels, and vice versa for wheat, paving way for future option to reduce N inputs and negative environmental impacts of agricultural crop production.     

Two-year field study on the emission of N2O from coarse and middle-textured Belgian soils with different land use

In the following study N₂O emissions from 3 different grasslands and from 3 different arable lands, representing major agriculture areas with different soil textures and normal agricultural practices in Belgium, have been monitored for 1 to 2 years. One undisturbed soil under deciduous forest was also included in the study. Nitrous oxide emission was measured directly in the field from vented closed chambers through photo-acoustic infrared detection. Annual N₂O emissions from the arable lands ranged from 0.3 to 1.5 kg N ha⁻¹ y⁻¹ and represent 0.3 to 1.0% of the fertilizer N applied. Annual N₂O emissions from the intensively managed grasslands and an arable land sown with grass were significantly larger than those from the cropped arable lands. Emissions ranged from 14 to 32 kg N ha⁻¹ y⁻¹, representing fertilizer N losses between 3 and 11%. At the forest soil a net N₂O uptake of 1.3 kg N₂O-N ha⁻¹ was recorded over a 2-year period. It seems that the N₂O-N loss per unit of fertilizer N applied is larger for intensively managed and heavily fertilized (up to 500 kg N ha⁻¹) grasslands than for arable lands and is substantially larger than the 1.25% figure used for the global emission inventory. Comparison of the annual emission fluxes from the different soils also indicated that land use rather than soil properties influenced the N₂O emission. Our results also show once again the importance of year-round measurements for a correct estimate of N₂O losses from agricultural soils: 7 to 76% of the total annual N₂O was emitted during the winter period (October–February). Disregarding the emission during the off-season period can lead to serious underestimation of the actual annual N₂O flux. This revised version was published online in August 2006 with corrections to the Cover Date.     

Integrating legumes to improve N cycling on smallholder farms in sub-humid Zimbabwe: resource quality, biophysical and environmental limitations

The release of mineral-N in soil from plant residues is regulated by their ‘quality’ or chemical composition. Legume materials used by farmers in southern Africa are often in the form of litter with N concentration <2%. We investigated the decomposition of Sesbania sesban and Acacia angustissima litter in the field using litterbags, and N mineralization of a range of legume materials using a leaching tube incubation method in the laboratory. The mass loss of the litter could be described using a modified exponential decay model: Y = (Y ₀−Q)e^−kt + Q. The relative decomposition constants for Sesbania and Acacia litter were 0.053 and 0.039 d⁻¹, respectively. The % N mineralized from fresh Sesbania prunings was 55% compared with only 27% for the Sesbania litter after 120 days of incubation under leaching conditions. During the same period, fresh prunings of Acacia released only 12% of the added N while Acacia litter released 9%. Despite the large differences in N concentration between Acacia prunings and its litter, the total mineralized N was similar, as mineralization from prunings was depressed by the highly active polyphenols. While N supply may be poor, these slow decomposing litter materials are potentially useful for maintaining soil organic matter in smallholder farms. In two field experiments with contrasting soil texture, Sesbania, Acacia and Cajanus produced large amounts of biomass (>5 Mg ha⁻¹) and improved N cycling significantly (>150 kg N ha⁻¹) on the clay loam soil, but adapted poorly on the sandier soil. There was a rapid N accumulation in the topsoil at the beginning of the rains in plots where large amounts of Sesbania or Acacia biomass had been incorporated. Despite the wide differences in resource quality between these two, there was virtually no difference in N availability in the field as this was, among other factors, confounded by the quantity of N added. A substantial amount of the nitrate was leached to greater than 0.4 m depth within a three-week period. Also, the incidence of pests in the first season, and drought in the second season resulted in poor nitrogen use efficiency. Our measurements of gaseous N losses in the field confirmed that N₂O emissions were <0.5 kg N ha⁻¹. As we had measurements of all major N flows, we were able to construct overall N budgets for the improved fallow – maize rotation systems. These budgets indicated that, in a normal rainfall season with no major pest problems, reducing nitrate leaching would be the single largest challenge to increased N recovery of added organic N in the light textured soils.     

Quantification of seasonal soil nitrogen mineralization for corn production in eastern Canada

Precise estimation of soil nitrogen (N) supply to corn (Zea mays L.) through N mineralization plays a key role in implementing N best management practices for economic consideration and environmental sustainability. To quantify soil N availability to corn during growing seasons, a series of in situ incubation experiments using the method of polyvinyl chloride tube attached with resin bag at the bottom were conducted on two typical agricultural soils in a cool and humid region of eastern Canada. Soil filled tubes were retrieved at 10-d intervals within 2 months after planting, and at 3- to 4-week intervals thereafter until corn harvest. Ammonium and nitrate in the soil and resin part of the incubation tubes were analyzed. In general, there was minimal NH₄⁺-N with ranges from 1.5 to 7.3 kg N ha⁻¹, which was declined in the first 30 d and fluctuated thereafter. Nitrate, the main form of mineral N, ranged from 20 to 157 kg N ha⁻¹. In the first 20–50 d, main portion of the NO₃⁻-N was in the soil and thereafter in the resin, reflecting the movement of NO₃⁻ in the soil, which was affected by rainfall events and amount. Total mineralized N was affected by soil total N and weather conditions: There was more total mineralized N in the soil with higher total N, and rainy weather stimulated N mineralization. The relationship between the accumulated mineral N and accumulated growing degree-days (GDD) fitted well into first order kinetic models. The accumulated mineralized soil N during corn growing season ranged from 96 to 120 kg N ha⁻¹, which accounted for 2–3% of soil total N. Corn plants took up 110–137 kg N ha⁻¹. While the mineralized N and crop uptake were in the same magnitude, a quantitative relationship between them could not be established in this study.     

Nitrogen-15 balance as affected by rice straw management in a rice-wheat rotation in northwest India

The sustainability of the productive rice-wheat systems of Northwest India is being questioned due to the complete removal of straw for animal consumption and fuel, or the burning of straw which has reduced the soil organic matter contents. However, straw incorporation at planting can temporarily reduce the availability of fertilizer-N and reduce crop yields. In a field study on a loamy sand soil, the effect of 6 mg ha⁻¹ rice straw incorporated into the soil 20 or 40 days before sowing (DBS) the wheat was compared with removal or burning of rice straw on the fate and balance of 120 kg ha⁻¹ of 5 atom% ¹⁵N-urea applied to wheat and to a following crop of rice. Wheat grain yield and agronomic efficiency (AE) of applied N (kg grain/kg N applied) were not influenced by rice straw management. However, N uptake (NU), and recovery efficiency (RE) of N by the difference method were lower with rice straw incorporation than with burning. Nitrogen-15 recovery by wheat was highest (41%) when the rice straw was removed or burned and lowest (30.4%) when 30 of the 120 kg N ha⁻¹ was applied at the time of straw incorporation at 20 DBS of wheat. However, this strategy of adding 25% of the urea-N dose at the time of straw incorporation resulted in the highest ¹⁵N losses (45.2%). Inorganic N remaining at harvest in the 0 to 60 cm soil profile, mostly NO₃ ⁻, was 5.5% after wheat and 4.2% after rice. Rice grain yields, NU, and RE were not influenced by rice straw management. Nitrogen-15 losses were similar in rice and wheat (31% with straw removed) despite total irrigation and rainfall inputs of 340 and 32 cm to rice and wheat, respectively. These results suggest to the farmers of northwest India that straw incorporation does not necessarily hurt grain yields, and indicates to researchers that work is still needed to improve N use efficiency in rice and wheat. This revised version was published online in June 2006 with corrections to the Cover Date.     

Crop productivity and nutrient use efficiency as affected by long-term fertilisation in North China Plain

Nutrient inputs into crop production systems through fertilisation have come under increased scrutiny in recent years because of reduced nutrient use efficiency and increased environmental impact. Fifteen years of experimental data on dynamics of N, P and K in soil, crop yield and nutrient uptake from nine fertilisation treatments at Zhengzhou, North China Plain, were used to analyse the contribution of different fertilisation treatments to crop yield, nutrient use efficiency and accumulation of nutrients in soil. The results showed that both N and P were limiting factors for crop growth. Without additional N and P fertilisation, only a very low yield level (ca 2 t ha⁻¹ for wheat and 3 t ha⁻¹ for maize) could be maintained. To achieve the potential productivity (i.e. yield level free of water and nutrient stresses) of wheat (6.9 t ha⁻¹) and maize (8.3 t ha⁻¹), wheat would need, on average, 170 kg N ha⁻¹, 32 kg P ha⁻¹ and 130 kg K ha⁻¹, while maize would need 189 kg N ha⁻¹, 34 kg P ha⁻¹ and 212 kg K ha⁻¹. The N and P demands correspond well to the N and P levels supplied in one of the fertilisation treatments (NPK), while K deficiency could occur in the future if no crop residues were returned or no extra K was applied. On average under this NPK treatment, 80% of N and 71% of P could be recovered by the wheat–maize system. Treatments with nutrient inputs higher than the NPK treatment and treatments without combination of N and P have led to accumulation of N and P in the soil profile. The input levels of N and P in the NPK treatment are recommended in fertiliser management, with additional K to avoid future soil K deficiency.     

Nitrogen rhizodeposition from soybean (<Emphasis Type="Italic">Glycine max</Emphasis>) and its impact on nutrient budgets in two contrasting environments of the Guinean savannah zone of Nigeria

Nitrogen (N) rhizodeposition by grain legumes such as soybean is potentially a large but neglected source of N in cropping systems of Sub-Saharan Africa. Field studies were conducted to measure soybean N rhizodeposition in two environments of the Guinean savannah of Nigeria using ¹⁵N leaf labelling techniques. The first site was located in Ibadan in the humid derived savannah. The second site was in Zaria in the drier Northern Guinean savannah. Soybean N rhizodeposition in the top 0.30 m of soil varied from 7.5 kg ha⁻¹ on a diseased crop in Ibadan to 33 kg ha⁻¹ in Zaria. More than two-thirds of soybean belowground N was contained in the rhizodeposits at crop physiological maturity, while the rest was found in the recoverable roots. Belowground plant-derived N was found to constitute 16–23% of the total soybean N. Taking rhizodeposited pools into account led to N budgets close to zero when all residues were removed. If residues were left in the field or recycled as manure after being fed to steers, soybean cultivation led to positive N budgets of up to +95 kg N ha⁻¹. The role and potential of grain legumes as N purveyors have been underestimated in the past by neglecting the N contained in their rhizodeposits.