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

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

Nitrogen management in organic farming: comparison of crop rotation residual effects on yields,N leaching and soil conditions

After 3 years of different crop rotations in an organic farming experiment on a sandy soil in northwest Germany, spring triticale was cultivated on all plots in the fourth year to investigate residual effects on yield, nitrogen (N) leaching and nutrient status in the soil. Previous crop rotations differed in the way N was supplied, either by farmyard manure (FYM, 100 and 200 kg N ha⁻¹ year⁻¹) or by arable legumes like grass-red clover and field beans, or as a control with no N. Other crops in the rotations were maize, winter triticale and spring barley. Additional plots had a 3-year grass-clover ley, that was ploughed-in for spring triticale in the fourth year. Yields of spring triticale were moderate and largest for ploughed-in grassland leys and grass-red clover and plots that had previously received farmyard manure. The former crop rotation, including grassland break-up, had a significant effect on most yield and environmental parameters like residual soil mineral nitrogen (SMN) and N leaching and on the level of available K in the soil. The single crop harvested in the year before spring triticale had a significant effect on yield parameters of spring triticale, less so on SMN and N leaching in the fourth year and no effect on available nutrients (P, K, Mg) and pH in the soil. We conclude that the effects of arable legumes were rather short lived while ploughing of 3-year grassland leys had a profound influence on mineralization processes and subsequently on yield and N losses.     

The performance of the Danish simulation model DAISY in prediction of Nmin at spring

In Denmark the Danish Agricultural Advisory Centre has for some years used the soil content of mineral nitrogen in spring in fertilizer recommendations. Since 1986 these recommendations have been based on soil samples carried out at all intersections of a nationwide 7 km square grid in Denmark. It was hoped that it may be possible to replace soil measurements with values of soil mineral-N calculated with a model. The Danish simulation model DAISY, which among other things simulates changes in the inorganic N content of the soil, was evaluated with respect to the N_min content in the early spring under bare soil and under winter cereal. For both situations the precrop was cereals. The performance of the model was evaluated in farming systems receiving mineral fertilizer and in some instances organic manures. The results were analysed according to type of subsoil: sandy or loamy. Predictions were 11 kg N ha^–1 less than the measured values as a mean and the differences between simulated and measured values were high for fields receiving organic manures. Predictions were less than ± 10 kg N ha^–1 of measured values in 25–58% of cases for the different types of crop cover at the time of soil sampling, type of subsoils and fertilizer strategies, respectively. Predictions were less than ± 20 kg N ha^–1 of measured values in 48–89% of cases for the different situations. The best predictions were obtained for sandy subsoils covered by winter cereal and supplied with mineral fertilizer only. It is concluded that the quality of the data used as input in the model has to be increased and that further developments of parts of the DAISY model are needed before modelling can be a useful tool in N fertilizer recommendations.     

Simulation of nitrogen in soil and winter wheat crops: A management model that makes the best use of limited information

A model that simulates changes in mineral N in the soil and N uptake by crops has been adapted to require as little detailed information as possible so that it is useful as an aid to management. The adapted model, which was developed in the UK, was tested against data from six experiments on winter wheat in the Netherlands. It proved reasonably successful in simulating the amounts of mineral N found in the soil in early spring and the changes that resulted from applying small amounts of fertilizer N in February. It was much less successful in simulating the effects of later, larger applications of N, mainly because the mineral N measured in the soil did not seem to respond to these applications. The uptake of N by the crops and their production of dry matter were simulated very well in some cases and rather less so in others.     

Potassium uptake and requirement in organic grassland farming

Use of mineral fertilizers is restricted in organic farming. The aim of the present paper was therefore to study whether potassium (K) limits yields in Norwegian organic grasslands. The K status in soil and herbage on 26 organic farms was investigated, and the response to K application in six fertilization experiments was explored. Further, the relationship between soil K analyses and K release from soil was examined. K application to grassland on the investigated farms was generally low, giving negative field K balances on 23 of the farms. The soils were classified as low or intermediate in readily available K (K_AL) on 23 of the farms. The mean K concentration for herbage samples from the first cut on these farms was 18.0 g K kg⁻¹ dry matter. In fertilization experiments, K application increased the K concentration in herbage. However, there was no significant effect on yield, even when K concentration in herbage on plots without K application was low. The lack of significant yield response to K application can be explained by low amounts of crop-available nitrogen (N). There was a tendency for increased plant uptake from reserve K with increasing values of acid soluble K (K–HNO₃) in soil. Separate K analyses of timothy (Phleum pratense) and red clover (Trifolium pratense) revealed that red clover showed better competitiveness for K than timothy in leys where N supply was limited.     

Measured and simulated nitrogen dynamics in winter wheat and a clay soil subjected to drought stress or daily irrigation and fertilization

The temporal dynamics of N in above- and below-ground parts of winter wheat and the dynamics of soil mineral-N were measured in the field in four treatments in wheat and a grass ley (L). The wheat treatments were: control (C), drought (D), daily irrigation (I), and daily irrigation and fertilization (IF). Nitrogen (20 g m^–2) was supplied as single doses in spring in C, D, and I, and according to a logistic N uptake function in IF. L, which was under establishment, was irrigated and fertilized in the same way as IF, but the total amount applied was only 5.6 g N m^–2. A soil nitrogen simulation model, SOILN, was used to combine crop and soil N data with measured litter decomposition rates and other major parts of the nitrogen cycle to calculate annual N budgets, based on daily model calculations. The dynamic patterns of crop N uptake and soil mineral N were similar in C, D, and I, although different in magnitude, but different in IF. Plant N uptake in C, D, and I was almost nil after anthesis, whereas it continued in IF until harvest. Generally, simulated soil mineral N levels (0–90 cm) agreed reasonably well with measurements on a yearly time scale, whereas their short-term dynamics were less well described by the simulations. We tested the hypothesis that the short-term variations were due to processes not included in the model,i.e., the loss of recently taken up plant N via roots during the growing season, and microbial N immobilization and remineralization processes induced by root-derived carbon. A simulated input to the soil of 150 g C m^–2 in IF, mimicking root-derived C, resulted in an improved agreement between simulated and measured short-term mineral N dynamics. Because of irrigation, net N mineralization of soil organic material in I and IF was about twice that in C and D, while that in L was about three times higher due to irrigation and high soil temperatures. Simulated N leaching during the following winter was highest in L, followed by I, IF, C and D. Measurements and simulations of N amounts in the system indicated that daily fertilization decreased N leaching compared with single-dose fertilization.     

Mulch N recycling in green manure leys under Scandinavian conditions

Nitrogen (N) recycling to the regrowth of mulched red clover (Trifolium pratense L.) and mulched mixed red clover/perennial ryegrass (Lolium perenne L.) leys was determined in field experiments during three consecutive years using ¹⁵N-labelled shoot material. Nitrogen recycling was greater in the pure clover stands than in the mixed stands in the beginning of the growing season, but increased successively in the mixed stands so that it was similar (14–15.5%) in both stands at the end of the season. This recycling of N from the mulch led to increased biomass accumulation but did not alter stand composition in the mixed stands. Mulch-derived N was incorporated into the soil organic N in both pure clover and mixed stands which thus contributed to building up soil fertility. An approximately similar proportion of N remained unaccounted for in mulched pure clover and mixed stand leys and presumably represented gaseous losses. To exploit the benefits of green manure leys in the humid temperate zone while minimising the negative environmental impact, these should be harvested rather than mulched.     

Nitrogen mineralization,nitrate leaching and crop growth following cultivation of a temporary leguminous pasture in autumn and winter

A field experiment was conducted to investigate the effect of timing and method of cultivation of a 3-year old ryegrass/white clover pasture on subsequent N mineralization, NO ₃ ^- -N leaching, and growth and N uptake of a wheat crop in the following season. The size of various N pools and decomposition of¹⁴C-labelled ryegrass material were also investigated. Cultivation method (mouldboard or chisel ploughing) generally had no significant effect on the accumulation of mineral N in the profile in the autumn or on the amount of NO ₃ ^- -N leached over winter.¹⁴C measurements suggested that initial decomposition rate of plant material was faster from May than March cultivation treatments. Despite this, overall net mineralization of organic N (of soil plus plant origin) increased with increasing fallow period between cultivation and leaching. The total amounts of mineral N accumulated in the soil profile before the start of leaching were 139, 119 and 22 kg N ha^–1 for the March, May and July cultivated soils respectively. Cumulative leaching losses over the trial calculated from soil solution samples were 78, 40 and 5 kg N ha^–1 for the March, May and July cultivated soils respectively. Differences in N mineralization over the season were generally not reflected by changes in amounts of potentially-mineralizable soil N (as measured by extraction or laboratory incubation) or levels of microbial biomass during the season. The amount of mineral N in the profile in spring increased with decreasing fallow period. This was reflected in an approximately 15% and 25% greater grain yield and N uptake respectively by the following wheat crop in plots cultivated in July rather than in March.     

Efficiency of fall-applied urea for barley: Influence of date of application

Fall application of N fertilizers is often inferior to spring application for increasing yields of spring-sown cereal grains. The objective of this study was to determine the influence of date of application on efficiency of fall-applied N. Fall application dates were related to recovery of fall-applied N as mineral N in soil in spring, and related to yield and N uptake for spring-sown barley. Urea at a rate of 50 or 56 kg N ha^–1 was incorporated into the soil to a depth of 10 cm. There were 2 or 3 application dates in the fall and one in the spring at sowing. Linear regression indicated recovery of fall-applied N as soil mineral N in spring increased from 30% with urea added on 19 September to 79% with addition on 6 November, but the predictability was low (r = 0.54^**). Increase in grain yield, expressed as relative efficiency of fall- versus spring-applied N, was only 23% on 19 September but rose to 76% by 6 November (r = 0.68^**). Results were similar for N uptake in grain. Other approaches to predicting the relative efficiency of fall- versus spring-applied N for yield increase were based on fall soil temperature at 5 cm depth, instead of fall calendar date. Soil temperature on the day of N application gave inferior correlation (r = –0.55^**), but the use of number of days from application to first day of 0°C soil temperature gave a fairly close correlation (r = –0.77^**). Soil degree-days accumulated from application to first day of 0°C soil temperature gave a similarly close correlation (r = –0.78^**). In all, the efficiency of fall-applied urea was markedly increased by delaying the application into the late fall; and calendar date, number of days or soil degree-days from application to soil freezing all predicted the efficiency fairly well.(Contribution No. 599)     

Aboveground nitrogen in relation to estimated total plant uptake in maize and bean

The main objective of this field study was to estimate the total plant uptake of soil mineral N in maize (Zea mays L.) and common bean (Phaseolus vulgaris L.) grown in crop rotations under different N content in Nicaragua. Secondary objectives were to estimate the fraction of the measured soil mineral N content taken up in this way, and to determine how the measured N in plant aboveground parts was related to the total mineral N uptake. A large variation in N content was obtained by using data from fertilisation experiments. Plant total N uptake was estimated as the residual N in a mass balance calculation of soil mineral N. Mineral N content in the top 0–0.3 m soil layer in the field cultivations and in tubes isolated from root uptake, and N content in aboveground plant parts were measured every 30 days. Estimated plant total uptake of soil mineral N varied considerably (2.5–14 g N m⁻² 30 day⁻¹) over periods and N treatments. The range of variation was similar for maize and bean. The fraction of the soil mineral N that was taken up by the plant daily varied more in maize (about 0.03–0.12 day⁻¹) than in bean (about 0.05–0.08 day⁻¹). Our results suggest that monthly changes in N in aboveground plant parts were linearly related to plant total N uptake during the same period. Aboveground plant N constituted between about 55% and 80% of total uptake of soil mineral N in maize depending on period within season, whereas for bean it was more constant and smaller (about 40%).     

Decomposition in the field of residues of oilseed rape grown at two levels of nitrogen fertilisation. Effects on the dynamics of soil mineral nitrogen between successive crops

The decomposition of oilseed rape residues of different quality and its effects on the mineral N dynamics of the soil in the period between crops were studied in situ. The residues studied were obtained by growing an oilseed rape crop at two levels of N fertilisation, 0 and 270 kg N ha^-1. The study was carried out using two types of experiment: field plots and cylinders filled with disturbed soil and inserted into the soil. The decomposition of the residues was followed using an approach involving the dynamics of both carbon and nitrogen, the parameters measured being the CO₂ emitted from the soil, the soil mineral N content, the C present in soluble form or in the form of microbial biomass, and the C and N present in the form of plant residues.The two residues studied, of similar biochemical composition, and differing only in their N content, were rapidly mineralised: approximately 50% of the carbon in the residues was decomposed during the first two months following incorporation into the soil. The carbon mineralised in the form of CO₂ was largely related to the C present in the residues, no relationship having been found with the C present in soluble form or in the form of microbial biomass.Calculation of net N mineralisation from the residues using a model of mineralisation and leaching has provided evidence of an immobilisation phase for soil mineral N, during the first steps of residues decomposition. Labelling the high-N residues with ¹⁵N has moreover enabled us to demonstrate the low availability of the organic N from this residue, 20.8% of the organic N being mineralised in the course of 18 months of experimentation. Eventually, only the highest-N content residue resulted in a mineral N surplus in the soil, equivalent to 9 kg N ha^-1, by comparison with the control soil. Finally, this study has provided good evidence of the complementarity between the two experimental methods. The cylinders of disturbed soil gave a precise measurement of the decomposition of the residues, especially by means of monitoring soil respiration. The field plots were used to monitor the dynamics of soil mineral N which were calculated with the aid of a mathematical model of mineralisation and leaching of nitrogen in the presence and absence of residues.     

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.     

Modelling nitrogen dynamics in soil–crop systems with HERMES

Model runs with HERMES were performed over entire crop rotation cycles for two experimental sites on loamy and sandy soils in Germany with differently managed plots. The model was able to simulate soil water and nitrogen contents on the sandy plots of Müncheberg with an index of agreement (IA)  >0.8 and  >0.69. Crop growth and N-uptake was simulated well with IA values  >0.89 and  >0.75, respectively. For the loess site in Bad Lauchstädt model results for above-ground biomass, storage organ and N-uptake agreed well with observations over a 4-year rotation with IA values of 0.93, 0.94 and 0.71, respectively. Soil mineral nitrogen was significantly overestimated on the cropped plot (IA = 0.45) as well as on the black fallow plot (IA = 0.65) using the default initialization of the decomposable nitrogen pools from the organic matter content. Equilibration of the pools, using data from a neighbouring long term experiment, improved soil mineral nitrogen simulation to an IA of 0.72 for the cropped and 0.91 for the fallow plot. The long term model performance was investigated using data from 1903 to 2002 of four differently managed plots in Bad Lauchstädt. Soil organic carbon, derived from simulated nitrogen pools, showed acceptable results for the unfertilized plot, but a distinct underestimation on plots with farmyard manure application. Simulated historical winter wheat and potato yields were distinctly overestimated during the initial 50 years. Therefore, an adoption of crop parameters for older varieties is necessary. The index of agreement of 0.9 indicates that the annual weather impact on yield fluctuations was correctly reflected.     

The influence of tillage and cropping-intensity on cereal response to nitrogen,sulfur, and phosphorus

Efficient fertilizer use is a prerequisite for achieving optimum crop yield while avoiding environmental contamination. Cereal response to nitrogen (N), sulfur (S), and phosphorus (P) were determined for 6 years under differing tillage [conventional-till (CT) vs. no-till (NT)] and intensity of cropping (cereal/fallow vs. cereal/cereal). Semidwarf white winter wheat (Triticum aestivum L.) alternated yearly with either fallow or spring cereal [barley (Hordeum vulgare L.) or spring wheat] on a Typic Haploxeroll soil in a 415 mm rainfall zone. Fertilizer treatments were no fertilizer (None), N only (N), N plus S (NS), and N plus S plus P (NSP). Average application rate, when applied, was 109 kg N, 18 kg S, and 11 kg P ha^–1. Average cereal yield without fertilizer was 1.82 t ha^–1. Nitrogen increased grain yield in 6 of 6, S in 4 of 6, and P in 3 of 6 years, with P and S response significant the remaining years at the 10% probability level. Average yield increases were 1.11 t ha^–1 for N, 0.93 t ha^–1 for S, and 0.47 t ha^–1 for P. The NT/CT yield ratio was 0.60, 0.75, 0.93, and 0.95 with None, N, NS, and NSP addition, respectively, indicating that N and S deficiency were more severe in no-till. Limited increase in the NT/CT ratio with P addition indicated that P deficiency was less affected by tillage. Winter wheat always yielded less under NT than CT regardless of fertility, whereas spring cereals reached equality when fertilized with NSP. Annually-cropped wheat yielded 52, 67, 89, and 90% of wheat after fallow with None, N, NS, and NSP, respectively. Thus N and S, but not P, deficiency was more intense with increased frequency of cropping. Adequate fertility was a prime prerequisite for efficient yield in all systems.     

Spatial variation of nitrogen mineralization as a guide for variable application of nitrogen fertilizer to cereal crops

The site specific management of variable rate nitrogen (N) fertiliser application to crops is a cost-effective system that optimises outputs and reduces environmental impact. However, its implementation requires information on the spatial variability of soil and crop variables and, especially, of the N supply from the soil, measured as the available N and the N mineralized from organic matter. The objective of this study was to obtain the spatial structure of the variation of net N mineralization, within the field scale in a cereal cropping system, in order to improve site specific N management. A nested sampling survey was conducted in the field using scales of variation at 1.5, 4.5, 13.5, 40.5 and 121.5 m, arranged in hierarchical order with n = 96 samples. Samples were collected in autumn and spring and N mineralization measured by aerobic incubation. The components of variance of the N mineralized were calculated using residual maximum likelihood and used to produce an approach to the variogram. The within-field spatial variation was almost all (92–93%) encompassed by the scales of variation measured, all occurring within 40.5 m in both seasons. However, there was a significant amount of fine scale variation at 1.5 m in autumn and 4.5 m in spring. These results will guide future spatial sampling of the N supply, and soil monitoring in general.     

The effect of nitrogen catch crop species on the nitrogen nutrition of succeeding crops

Ten widely different plant species were compared for their ability to reduce soil mineral nitrogen levels in the autumn and their ability to improve the nitrogen nutrition of the succeeding crop. The species included monocots and dicots, crops that survived the winter (persistent) or were winter killed (non-persistent) as well as legumes and non legumes. Their ability to reduce soil mineral nitrogen content was dependent on both root depth and persistency of the crops in the autumn. For non-persistent catch crops most of the mineralization of plant nitrogen occurred during the winter, and for some of these so early as to allow leaching of some mineralized nitrogen. For persistent crops most of the mineralization occurred shortly after incorporation in the spring. The effect of the catch crops on nitrogen uptake by the succeeding barley crop varied from 13 to 66 kg N ha^–1 and the differences between the crops could not be related to any single character, but to a combination of root depth, persistency, plant nitrate accumulation, and depletion of the soil mineral nitrogen pool in spring.     

Simulation of the nitrogen balance in the soil and a winter wheat crop

A simulation model for winter wheat growth, crop nitrogen dynamics and soil nitrogen supply was tested against experimental data. When simulations of dry matter production agreed with measurements, nitrogen uptake was simulated accurately. The total amount of soil mineral nitrogen as well as the distribution of mineral nitrogen over the various soil layers were generally simulated well, except for experiments in which fertilizer was applied late in spring. In these experiments, applied nitrogen disappeared because it could not be accounted for by the model. Some explanations for this disappearance are briefly discussed.     

Effects of nitrification inhibitors and time and rate of slurry and fertilizer N application on silage maize yield and losses to the environment

Field experiments with silage maize during eight years on a sandy soil in The Netherlands, showed that dicyandiamide (DCD) addition to autumn-applied cattle slurry retarded nitrification, thus reducing nitrate losses during winter. Spring-applied slurry without DCD, however, was on average associated with even lower losses and higher maize dry matter yields.Economically optimum supplies of mineral N in the upper 0.6 m soil layer in spring (EOSMN), amounted to 130–220 kg ha^–1. Year to year variation of EOSMN could not be attributed to crop demand only. According to balance sheet calculations on control plots, apparent N mineralization between years varied from 0.36 to 0.94 kg ha^–1 d^–1. On average, forty percent of the soil mineral N (SMN) supply in spring, was lost during the growing season. Hence, the amounts of residual soil mineral N (RSMN) were lower than expected. Multiple regression with SMN in spring, N crop uptake and cumulative rainfall as explanatory variables, could account for 79 percent of the variation in RSMN.Postponement of slurry applications to spring and limiting N inputs to economically optimum rates, were insufficient measures to keep the nitrate concentration in groundwater below the EC level for drinking water.     

Nitrogen dynamics following grain legumes and subsequent catch crops and the effects on succeeding cereal crops

The effects of faba bean, lupin, pea and oat crops, with and without an undersown grass-clover mixture as a nitrogen (N) catch crop, on subsequent spring wheat followed by winter triticale crops were determined by aboveground dry matter (DM) harvests, nitrate (NO₃) leaching measurements and soil N balances. A 2½-year lysimeter experiment was carried out on a temperate sandy loam soil. Crops were not fertilized in the experimental period and the natural ¹⁵N abundance technique was used to determine grain legume N₂ fixation. Faba bean total aboveground DM production was significantly higher (1,300 g m^?2) compared to lupin (950 g m^?2), pea (850 g m^?2) and oat (1,100 g m^?2) independent of the catch crop strategy. Faba bean derived more than 90% of its N from N₂ fixation, which was unusually high as compared to lupin (70–75%) and pea (50–60%). No effect of preceding crop was observed on the subsequent spring wheat or winter triticale DM production. Nitrate leaching following grain legumes was significantly reduced with catch crops compared to without catch crops during autumn and winter before sowing subsequent spring wheat. Soil N balances were calculated from monitored N leaching from the lysimeters, and measured N-accumulation from the leguminous species, as N-fixation minus N removed in grains including total N accumulation belowground according to Mayer et al. (2003a). Negative soil N balances for pea, lupin and oat indicated soil N depletion, but a positive faba bean soil N balance (11 g N m^?2) after harvest indicated that more soil mineral N may have been available for subsequent cereals. However, the plant available N may have been taken up by the grass dominated grass-clover catch crop which together with microbial N immobilization and N losses could leave limited amounts of available N for uptake by the subsequent two cereal crops.     

Uptake of fertilizer and soil nitrogen by ryegrass swards during spring and mid-season

Double-labelled¹⁵N ammonium nitrate was used to determine the uptake of fertilizer and soil N by ryegrass swards during spring and mid-season. The effects of water stress (40% of mean rainfall v 25 mm irrigation per 25 mm soil water deficit) and the rate of application of N in the spring (40 v 130 kg ha^–1) on the recovery of 130 kg N ha^–1 applied in mid-season were also evaluated. Apparent recovery of fertilizer N (uptake of N in the fertilized plot minus that in the control expressed as a percentage of the N applied) was 95 and 79% for fertilizer N applied in the spring at rates of 40 and 130 kg ha^–1, respectively. Actual recovery of the fertilizer N assessed from the uptake of¹⁵N was only 31 and 48%, respectively. The uptake of soil N by the fertilized swards was substantially greater than that by the control. However, the increased uptake of soil N was always less than the amount of fertilizer N retained in or lost from the soil. Broadly similar patterns for the uptake of fertilizer and soil N were observed during mid-season. Uptake of N in mid-season was highest for swards which received 40 kg N ha^–1 in the spring and suffered minimal water stress during this period. Application of 130 kg N ha^–1 in spring reduced the uptake of N in mid-season to an extent similar to that arising from water stress. Only 1.8 to 4.2 kg ha^–1 (3 to 10%) of the N residual from fertilizer applied in the spring was recovered during mid-season. Laboratory incubation studies suggested that only a small part of the increased uptake of soil N by fertilized swards could be attributed to increased mineralisation of soil N induced by addition of fertilizer. It is considered that the increased uptake of soil N is partly real but mostly apparent, the latter arising from microbially mediated exchange of inorganic¹⁵N in the soil.