Nitrate leaching after cut grass/clover leys as affected by time of ploughing

Abstract. Nitrate leaching after one year of a cut grass/clover ley was measured in two succeeding years to investigate how the postponing of ploughing leys from early to late autumn or spring, in combination with spring or winter cereals affected leaching of nitrate. The experiment was conducted as three field trials, two on a coarse sandy soil and one on a sandy loam soil. For calculation of nitrate leaching, soil water samples were taken using ceramic suction cups. The experiments started in spring in a first year ley and ended in spring three years later. Total nitrate leaching for the three year periods for each trial ranged between 160–254 and 189–254 kg N/ha on the coarse sand and 129–233 kg N/ha on the sandy loam. The results showed that winter wheat ( Triticum aestivum L.) did not have the potential for taking up the mineralized N in autumn after early autumn ploughing of grass/clover leys, and that the least leaching was generally found when ploughing was postponed until spring, and when winter rye ( Secale cereale L.) was grown as the second crop rather than spring barley ( Hordeum vulgare L.). Nevertheless, leaching was generally high in the winter period even when winter rye was grown. On these soil types ploughing out should be postponed, whenever possible, to spring. Crop systems that maximize the utilization of mineralized N and thereby minimize nitrate leaching need to be further developed. Based on N balances, the data were further used to estimate the biological N fixation by the clover.     

Yield and Quality of Mixed Seed Crops of Timothy ( Phleum pratense ) and Clovers ( Trifolium spp.) in an Organic Cropping System

Seed production of timothy ( Phleum pratense L.) in mixed crops with alsike ( Trifolium hybridum L.), white ( T. repens L.) or red ( T. pratense L.) clover was evaluated in an organic cash crop system. No fertilizer was applied except for household compost in the sowing year. While the first year crops of the alsike clover/timothy and red clover/timothy leys were harvested for clover seed, the first year crop of the white clover/timothy ley and all second year crops were harvested for timothy seed. The botanical composition of the alsike clover/timothy crop averaged 89/11 in the first ley year, and 8/92 in the second year. The corresponding yields averaged 443 kg ha -1 clover seed and 849 kg ha -1 timothy seed, respectively. Second year seed yields of timothy from mixtures with red and white clover were, in turn, 7% higher and 44% lower than from the corresponding mixture with alsike clover. First year seed yields of alsike clover from the alsike clover/timothy combination, and first year yields of timothy from the timothy/white clover combination were mostly rejected because of insufficient purity. In contrast, first year seed yields of red clover from the red clover/ timothy combination, and second year yields of timothy from all combinations, never failed to met the international requirement of maximum 1% contamination of one particular species in certified seed.     

Carbon allocation to roots,rhizodeposits and soil after pulse labelling: a comparison of white clover (<Emphasis Type="Italic">Trifolium repens</Emphasis> L.) and perennial ryegrass (<Emphasis Type="Italic">Lolium perenne</Emphasis> L.)

Organically managed farm areas in Denmark are expanding and typically contain clover-grass leys that are known to stimulate accumulation of organic matter in arable soils. We compared the C allocation to roots and soil from clover and grass, and determined for how long assimilated C remained mobile in these plant-soil systems. Pots with perennial ryegrass, white clover or a mixture of both were pulse-labelled with ¹⁴CO₂, and harvested for analyses after 4, 11, 20, and 30 days. ¹⁴C losses by shoot respiration stopped within 4 days and after this incubation time the input of assimilated ¹⁴C to below-ground compartments was greater in grass (52%) than in clover (36%). During the next 4 weeks, ¹⁴C allocation below ground increased in grass (up to 75% at day 30), but remained constant in clover (37% at day 30). In the grass/clover mixture, the below-ground fraction increased to 50% at day 30. In clover, ¹⁴C was incorporated sooner into stable plant and soil pools and less was released in rhizodeposition than in grass. This was confirmed by the ¹⁴C in the soil microbial biomass that decreased fastest in the clover treatment. Root-derived C compounds of clover probably decomposed faster than those from grass. The larger size and specific activity of the soil microbial biomass in the mixed treatment suggested a stimulating effect of the two plant species on substrate utilisation by the microbial community. This study showed that a 2- to 3-week distribution period is needed before sampling for quantitative estimates of C allocation.     

Response of three cereal crops in continuous arable or ley-arable rotations to fertilizer nitrogen and soil nitrogen at Rothamsted's Woburn Ley-arable experiment

Rothamsted's Woburn Ley-arable experiment, started in 1938 on a sandy loam soil, provides valuable real-world data on the effects of all-arable and ley-arable rotations. In this study, six rotations were compared from 1973 to 2001. Two had 3-year arable “treatment” crops, two had 3-year leys, and two had 8-year leys; the leys being all-grass given fertilizer nitrogen (Ln3 and Ln8), or grass/clover (Lc3 and Lc8). Here, we present the yields of two test crops, winter wheat (1981–2000) followed by spring barley (1982–1991) or winter rye (1997–2001) in each of the six rotations, and their response to four rates of fertilizer N and soil N. From fitted yield/N response curves, we show that maximum wheat yields were least (7.10 t ha⁻¹) in the AB rotation, slightly higher, but not significantly so (7.65 t ha⁻¹) following Ln leys but significantly higher (8.12 t ha⁻¹) following Lc leys. Significantly less fertilizer N (30 kg ha⁻¹) was needed to achieve the higher yields following Lc leys. Yields of the second cereal following the leys were 0.3–0.8 t ha⁻¹ higher than those in the AB rotation; these increases were not statistically significant. However, significantly less fertilizer N, 26–38 kg N ha⁻¹, was required to achieve those yields. There was no difference found between the type of ley. The initial benefit of the Lc leys was short-lived. If leys are to be introduced into mainly arable farming systems, they may need to be subsidized to make them financially viable.     

Nitrous oxide emissions, cereal growth, N recovery and soil nitrogen status after ploughing organically managed grass/clover swards

The period after ploughing of grass–clover leys within a ley‐arable rotation is when nitrogen accumulated during the ley phase is most vulnerable to loss. We investigated how ploughing date and timing of cessation of grazing before ploughing affected nitrous oxide (N₂O) losses of the first cereal crop. Ploughing dates were July and October for a winter wheat pilot study and January and March for spring barley in the main experiment. Timings of cessation of grazing (main experiment only) were October, January and March. Spring barley yield, nitrogen uptake and soil mineral nitrogen were also assessed. A separate large‐scale laboratory incubation was made to assess the effect of temperature and rainfall on nitrous oxide emissions and nitrate leaching under controlled conditions. Nitrous oxide emissions in the 1‐ to 2‐month period after autumn or spring ploughing, or sowing were typically between 20 and 150 g N ha^?1 day^?1 and increased with temperature and rainfall. Tillage for crop establishment stimulated N₂O emissions with up to 2.1 kg N ha^?1 released in the month after spring tillage. Cumulative nitrous oxide emissions were greatest (～8 kg ha^?1 over 17 months) after cessation of grazing in March before March ploughing, and lowest (～5.5 kg ha^?1) after cessation of grazing in January before January ploughing. These losses were 1.2–3.9% of the N inputs. In the laboratory study, winter ploughing stimulated nitrate leaching more than nitrous oxide emissions. The optimum time of ploughing appears to be early spring when the cold restricts nitrogen mineralization initially, but sufficient nitrogen becomes available for early crop growth and satisfactory N offtake as temperature increases. Early cessation of grazing is advantageous in leaving an adequate supply of residues of good quality (narrow C:N ratio) for ploughing‐in. Restricting tillage operations to cool, dry conditions, being aware of possible compaction and increasing the use of undersown grass–clover should improve the sustainability of organic farming.     

Seasonal variation in field-saturated hydraulic conductivity in two swelling clay soils in Sweden

A simple method (the inversed auger hole method) for measuring field-saturated hydraulic conductivity (K_fs) was investigated. Measurements were carried out in the spring, summer and autumn at three depths in two Swedish clay soils (Ultuna and Limsta, with clay contents of 45–60%0 and 65–80%, under barley and grass/clover ley respectively). Seasonal fluctuations in K_fs at Limsta were more pronounced, and were observed deeper in the profile. This was attributed primarily to larger structural changes due to a higher capacity for swell/shrink (normal shrinkage over the available water range) and an earlier drying up of the soil under grass/clover ley. It was shown that the measured K_fs values were strongly correlated with the total inter aggregate (macro-) porosity (e_t), estimated from a simple model of soil shrinkage. Combining the data from both soils, a single power-law relation was adequate (r= 0.73) to describe the variations in K_fs with e_t     

Cutting regime determines allocation of fixed nitrogen in white clover

Leguminous leys are important sources of nitrogen (N), especially in forage-based animal production and organic cropping. Models for estimating total N₂ fixation of leys—including below-ground plant-derived N (BGN)—are based on grazed or harvested leys. However, green manure leys can have different proportions of above-ground plant-derived N (AGN) and BGN when subjected to different cutting regimes. To investigate the effects of cutting on N distribution in white clover, a pot experiment was carried out using ¹⁵N techniques to determine N₂ fixation, N rhizodeposition and root C and N content of cut and uncut white clover (Trifolium repens L. cv. Ramona) plants. Percentage N derived from air (%Ndfa) was lower in uncut (63%) than in cut (72%) plants, but total Ndfa was not significantly affected by cutting. The higher reliance on N₂ fixation in cut plants was thus counterbalanced by lower biomass and total N content. With BGN taken into account, total plant-derived N increased by approximately 50% compared with AGN only. Cutting did not affect the proportion of BGN to standing shoot biomass N after regrowth, but decreased the proportion of BGN to total shoot biomass production during the entire growth period. Thus, estimates of N fixation in green manure leys should consider management practices such as cutting regime, as this can result in differences in above- and below-ground proportions of plant-derived N.     

Effects of White Clover Inclusion on Turf Characteristics,Nitrogen Fixation,and Nitrogen Transfer from White Clover to Grass Species in Turf Mixtures

Abstract

An irrigated field trial was conducted to test the effects of white clover in three turfgrass species (perennial ryegrass, Kentucky bluegrass, and creeping bentgrass) on color, clipping yield, and botanical composition and to estimate nitrogen (N)₂ fixation and N transfer from white clover to associated turfgrass species under different N‐fertilization conditions in 1999–2002.

Nitrogen fertilizers significantly increased color ratings in all observations. Grass–white clover mixtures had better color ratings than pure grass at all sampling dates and seasonal averages in unfertilized conditions. Fertilized pure grass plots yielded significantly more than control plots in all turfgrass species. Nitrogen fertilization did not affect clipping yield greatly in turfgrass–white clover mixtures. Nitrogen application significantly decreased white clover percentage in the harvested clippings in second and third year.

Nitrogen fertilization increased tissue N concentration positively in all turfgrass species grown alone. In contrast, N fertilization did not greatly affect tissue N concentration of either turfgrass species or white clover in the mixtures. Nitrogen fixation of white clover was estimated as 24.6, 30.7, and 33.8 g m^?2 year^?1 in perennial ryegrass, Kentucky bluegrass, and creeping bentgrass, respectively. The total estimated N₂ fixation gradually decreased with increasing N fertilization. Nitrogen transfer from white clover to the associated turfgrass varied from 4.2 to 13.7% of the total N that the white clover fixed annually.     

Nitrogen leaching and herbage production on intensively managed grass and grass-clover pastures on sandy soil in Finland

The main aim of this study was to compare the N leaching from grass fertilized with 220 kg N ha⁻¹ and grass‐clover pastures receiving no fertilization during three grazing years and a renewal year, in a 4‐year ley rotation. The other aim was to compare the herbage and milk production of these pastures. The study was conducted on a lysimeter field; five lysimeters (size 10 × 10 m) were assigned to each treatment. Automated drinking water outlets for the cows were located on one of the lysimeters on each treatment. The amount of leachate was recorded and composite samples were analysed for total N, NO₃‐N, NH₄‐N and soluble organic N (SON). The number of grazing cows was adjusted according to the herbage mass production. The amount of milk was measured. The total input of N to the area was 290 and 215 kg N ha⁻¹ year⁻¹ for grass and grass‐clover treatments, respectively. The total N leaching during grazing years was 17 and 9 kg N ha⁻¹ from grass and grass‐clover treatments, respectively. Renewal of the sward increased N leaching in both treatments, up to 60 and 40 kg total N ha⁻¹ in grass and grass‐clover treatments, respectively. During the grazing years 96% of the leached N was in the form of NO₃‐N, but during the renewal year the proportion of NO₃‐N was lowered to 89% and the rest was in the form of SON. The total amount of N in the surface runoff was 3–5 kg ha⁻¹ year⁻¹. As high N fertilizer rates per application are a common practice in Finland, short‐term grass‐clover pastures can be considered environmentally beneficial when compared with intensively fertilized grass pastures in comparable circumstances.     

Microbial biomass, microbial diversity, soil carbon storage, and stability after incubation of soil from grass–clover pastures of different age

A laboratory incubation study with clover grass pasture soils of seven different ages (0, 1, 2, 3, 4, 5, and 16 production years) was carried out to determine initial soil carbon (C) and nitrogen (N) stocks and potentials for greenhouse gas emissions (N₂O and CO₂). Compared with the soil from the recently established pasture, an increase of total soil C and N was observed along with pasture age. Greenhouse gas emissions were low and not significantly different among the soils from younger pastures (0–5 years), but especially N₂O emissions increased markedly in the soil from 16-year-old grass–clover. Low emissions might mainly be due to an early C limitation occurring in the soils from younger pastures, which was also corroborated by decreasing levels of cold water-extractable C and early shifts within the microbial community. However, higher emissions from the old pasture soil were offset by its increase in total soil C. A longer ley phase without soil disturbance may therefore be beneficial in terms of overall C sequestration in systems with temporary grass–clover swards.     

Nitrous oxide emissions and methane oxidation by soil following cultivation of two different leguminous pastures

 Nitrous oxide (N₂O) emissions and methane (CH₄) consumption were quantified following cultivation of two contrasting 4-year-old pastures. A clover sward was ploughed (to 150–200 mm depth) while a mixed herb ley sward was either ploughed (to 150–200 mm depth) or rotovated (to 50 mm depth). Cumulative N₂O emissions were significantly greater following ploughing of the clover sward, with 4.01 kg N₂O-N ha^–1 being emitted in a 48-day period. Emissions following ploughing and rotovating of the ley sward were much less and were not statistically different from each other, with 0.26 and 0.17 kg N₂O-N ha^–1 being measured, respectively, over a 55-day period. The large difference in cumulative N₂O between the clover and ley sites is presumably due to the initially higher soil NO₃ ^– content, greater water filled pore space and lower soil pH at the clover site. Results from a denitrification enzyme assay conducted on soils from both sites showed a strong negative relationship (r=–0.82) between soil pH and the N₂O:(N₂O+N₂) ratio. It is suggested that further research is required to determine if control of soil pH may provide a relatively cheap mitigation option for N₂O emissions from these soils. There were no significant differences in CH₄ oxidation rates due to sward type or form of cultivation. Received: 1 November 1998     

Effect of three contrasting onion (Allium cepa L.) production systems on nitrous oxide emissions from soil

 N₂O emissions were measured from three contrasting onion (Allium cepa L.) production systems over an 8.5-month period. One system was established on soil where a clover sward had 3 months earlier been ploughed in (ploughed clover site). This production system followed conventional production management practices. The other two systems were established on soil where a mixed herb ley had 3 months earlier been either ploughed or rotovated. These last two production systems followed the guidelines of the International Federation of Organic Agriculture Movements (IFOAM). Cumulative N₂O emissions were significantly greater from the ploughed clover site compared to the ploughed ley site (3.8 and 1.6 kg N₂O-N ha^–1, respectively), while cumulative N₂O emissions from the ploughed ley and rotovated ley sites were not significantly different from each other. Emissions from all sites were dominated by episodes of high N₂O flux activity following seedbed preparation and drilling, when soil water suction (SWS) was shown to be the rate-controlling variable. The decline in the N₂O fluxes after these peak emissions followed clear exponential relationships of the form F=Ae^{– kt} (r≥0.91), where F is the daily flux and A is the y-intercept. First-order decay constants (k) during these periods of declining N₂O fluxes (corresponding to half-lives of 2.6–3.0 days) were not significantly different in magnitude from the first-order rate constants that characterised the increasing SWS. Gross differences in cumulative emissions between the clover and ley sites were attributed to the influence of differing soil pHs at the two sites on the N₂O:(N₂O+N₂) ratio in the denitrification products. It also appeared that fertiliser applications to the clover site had both direct and indirect effects on N₂O emissions by: (1) enhancing N₂O emissions via potential nitrification, (2) increasing the NO₃ ^– supply for enhanced N₂O emissions via denitrification, and (3) influencing the N₂O:(N₂O+N₂) ratio by lowering soil pH and increasing NO₃ ^– concentrations. Onion crop yields were greater at the clover site, mainly due to the higher density of planting made possible under a conventional production philosophy. Expressing the yield on the basis of net N₂O emissions, 23 t onions kg^–1 N₂O-N was obtained from the ploughed clover, which was double that obtained for the two systems based on the ley site. However, when the N₂O emissions from the cultivation of the soils prior to the sowing of the onions was included, all three systems produced a similar yield per kilogram of N₂O-N emitted, averaging 10 t kg^–1. Received: 6 January 1999     

A mixture of grass and clover combines the positive effects of both plant species on selected soil biota

The introduction of N₂-fixing white clover (Trifolium repens) in grassland is a management measure that may contribute to sustainable grassland systems by making them less dependent on inorganic fertilizers. However, little is known about the impact of this measure on soil biota and ecosystem services. We investigated earthworms, nematodes, bacteria and fungi in an experiment in which white clover-only and a mixture of grass and white clover without fertilization were compared with grass-only with and without fertilization.In comparison with grass-only, white clover-only had a lower total root biomass and a lower C/N-ratio in the above- and below-ground plant biomass. These plant characteristics resulted in a lower bacterial biomass, a lower fungal biomass, a higher proportion of bacterivorous nematode dauerlarvae, a lesser proportion of herbivorous nematodes and a greater abundance of earthworms in clover-only.The quantity and quality (C/N-ratio) of the above- and below-ground plant biomass in the mixture of grass and white clover (20–30% clover in the DM) was comparable with grass fertilized with 150 kg N ha⁻¹ of inorganic fertilizer. Differences between these treatments might show specific clover effects in the grass–clover mixture on soil biota other than quantity and C/N-ratio of the litter. However, the only differences were a higher proportion of bacterivorous nematode dauerlarvae and a different nematode community composition in grass–clover.The soil structure in white clover-only showed a higher proportion of angular blocky elements, a lower penetration resistance, a higher number of earthworm burrows, a higher potential N-mineralization and respiration than the soil in grass-only. This suggests that clover stimulates the ecosystem services of water infiltration and supply of nutrients, but is less conducive to soil structure maintenance. The grass–clover mixture differed from grass-only in a higher respiration and from clover-only in a higher percentage of soil crumbs. We suggest that when clover is introduced in grassland to reduce the reliance on inorganic fertilizer, the mixture of grass and clover maintains the positive impact of grass roots on soil structure and increases the supply of nutrients via the soil food web. Thus, a grass–clover mixture combines the agronomic benefits of the two plant types.     

Is the productivity of organic farms restricted by the supply of available nitrogen?

Abstract. This paper reviews information from the literature and case studies to investigate whether productivity in organic systems is restricted by the supply of available N during the major phases of crop growth. Organic systems have the potential to supply adequate amounts of available N to meet crop demand through the incorporation of leys, N rich cash crop residues and uncomposted manures. However, this is seldom achieved because leys are only incorporated once every few years and organically produced crop residues and manures tend to have low N contents and slow mineralization rates. N availability could be improved by delaying ley incorporation until spring, applying uncomposted manures at the start of spring growth, transferring some manure applications from the ley phase to arable crops, preventing cover crops from reaching a wide C:N ratio and better matching crop type with the dynamics of N availability.     

Size and activity of the soil microbial biomass under grass and arable management

 The effects of 5 years of continuous grass/clover (Cont grass/clover) or grass (Cont grass) pasture or 5 years of annual grass under conventional (Ann grass CT) or zero tillage (Ann grass ZT) were compared with that of 5 years of continuous barley (LT arable) on a site which had previously been under arable crops for 11 years. For added comparison, a long-term grass/clover pasture site (LT past) nearby was also sampled. Soil organic C (C_org) content followed the order LT arable=Ann grass CT<Ann grass ZT<Cont grass=Cont grass/clover<LTpast. Trends with treatment for microbial biomass C (C_mic), basal respiration, flourescein diacetate (FDA) hydrolytic activity, arginine ammonification rate and the activities of dehydrogenase, protease, histidase, acid phosphatase and arylsulphatase enzymes were broadly similar to those for C_org. For C_mic, FDA hydrolysis, arginine ammonification and the activities of histidase, acid phosphatase and arylsulphatase, the percentage increase caused by 5 years of continuous pasture (in comparison with LT arable) was 100–180%, which was considerably greater than that for organic C (i.e. 60%). The microbial metabolic quotient (qCO₂) was higher for the two treatments which were mouldboard ploughed annually (LT arable and Ann grass CT) than for the undisturbed sites. At the undisturbed sites, C_org declined markedly with depth (0–15 cm) and there was a similar stratification in the size and activity of C_mic and enzyme activity. The microbial quotient (C_mic/C_org) declined with depth whilst qCO₂ tended to increase, reflecting a decrease in the proportion of readily available substrate with depth. Received: 7 July 1998     

The amount,but not the proportion,of N2 fixation and transfers to neighboring plants varies across grassland soils

ABSTRACT

Biological nitrogen fixation (BNF) is an important nitrogen source for both N₂-fixers and their neighboring plants in natural and managed ecosystems. Biological N fixation can vary considerably depending on soil conditions, yet there is a lack of knowledge on the impact of varying soils on the contribution of N from N₂-fixers in mixed swards. In this study, the amount and proportion of BNF from red clover were assessed using three grassland soils. Three soil samples, Hallsworth (HH), Crediton (CN), and Halstow (HW) series, were collected from three grassland sites in Devon, UK. A pot experiment with ¹⁵N natural abundance was conducted to estimate BNF from red clover, and the proportion of N transferred from red clover to the non-N₂ fixing grass in a grass-clover system. The results showed that BNF in red clover sourced from atmosphere in the HH soil was 2.92 mg N plant^?1, which was significantly lower than that of the CN (6.18 mg N plant^?1) and HW (8.01 mg N plant^?1) soils. Nitrogen in grass sourced from BNF via belowground was 0.46 mg N plant^?1 in the HH soil, which was significantly greater than that in CN and HW soils. However, proportionally there were no significant differences in the percentage N content of both red clover and grass sourced from BNF via belowground among soils, at 65%, 67%, 65% and 35%, 27%, 31% in HH, CN, and HW, respectively. Our observations indicate that the amount of BNF by red clover varies among grassland soils, as does the amount of N sourced from BNF that is transferred to neighboring plants, which is linked to biomass production. Proportionally there was no difference among soils in N sourced from BNF in both the red clover plants and transferred to neighboring plants.     

Catch crop strategy and nitrate leaching following grazed grass-clover

Cultivation of grassland presents a high risk of nitrate leaching. This study aimed to determine if leaching could be reduced by growing spring barley (Hordeum vulgare L.) as a green crop for silage with undersown Italian ryegrass (Lolium multiflorum Lam.) compared with barley grown to maturity with or without an undersown conventional catch crop of perennial ryegrass (Lolium perenne L.). All treatments received 0, 60 or 120 kg of ammonium‐N ha^?1 in cattle slurry. In spring 2003, two grass‐clover fields (3 and 5 years old, respectively, with different management histories) were ploughed. The effects of the treatments on yield and nitrate leaching were determined in the first year, while the residual effects of the treatments were determined in the second year in a crop of spring barley/perennial ryegrass. Nitrate leaching was estimated in selected treatments using soil water samples from ceramic cups. The experiment showed that compared with treatments without catch crop, green barley/Italian ryegrass reduced leaching by 163–320 kg N ha^?1, corresponding to 95–99%, and the perennial ryegrass reduced leaching to between 34 and 86 kg N ha^?1, corresponding to a reduction of 80 and 66%. Also, in the second growing season, leaching following catch crops was reduced compared with the bare soil treatment. It was concluded that the green barley/Italian ryegrass offers advantages not only for the environment but also for farmers, for whom it provides a fodder high in roughage and avoids the difficulties with clover fatigue increasingly experienced by Danish farmers.     

Denitrification losses of nitrogen fertilizer applied to winter wheat following ley and arable rotations as estimated by acetylene inhibition and 15N balance

We studied the fate of 222 kg N ha^?1 applied in spring as K¹⁵NO₃ to winter wheat test crops which followed either continuous arable cropping (Arable) or a rotation in which a 3-year grass/clover ley preceded the wheat (Ley). Denitrification losses (measured by an acetylene-inhibition method) of over 1 kg N ha^?1 d^?1 were measured for short periods following heavy rain in mid-May. However the generally dry and cool weather resulted in accumulated losses by denitrification between fertilizer application and anthesis equivalent to only 5.3% and 3.6% (±2%) of the applied N for the arable and ley treatments respectively. The smaller loss from the ley was despite this treatment containing more inorganic N and available carbon. ¹⁵N balance indicated that, at anthesis, 1.5% and 11.5% (± 7%) of the labelled N was lost from the arable and ley treatments respectively. Given the precision of the 15N and the acetylene-inhibition methods, the results are not significantly different. However, the larger difference between methods for losses from the ley treatment may be an underestimate because ¹⁵N balance does not measure losses of unlabelled N. These were probably very small on the arable treatment but could have increased total N loss by 25% to c. 32 kg ha^?1 on the ley treatment compared with the 8 kg ha-1 measured as denitrified. Such a large difference is unlikely to be an error but was probably due to ammonia volatilization from this crop which was severely infected by mildew. The results were thus a poor test of the acetylene-inhibition method, but revealed another loss process which could be significant in some situations.     

Residual effect and nitrate leaching in grass‐arable rotations: effect of grassland proportion,sward type and fertilizer history

A key point in designing grass‐arable rotations is to find the right balance between the number of cultivations and the length of the grass phase. In a field experiment, we investigated the effect of cropping history (grazed unfertilized grass–clover and fertilized [300 kg N per hectare] ryegrass, proportion of grassland and previous fertilizer use) on crop growth and nitrate leaching for 2 years following grassland cultivation. In the final year, the effect of perennial ryegrass as a catch crop was investigated. The nitrogen fertilizer replacement value (NFRV) of grassland cultivation was higher at 132 kg N per hectare in the rotation with 75% grassland compared with on average 111 kg N per hectare in rotations with 25 and 38% grassland and the NFRV of ryegrass in the rotation was higher than that of grass–clover. Nitrate leaching following cultivation was not affected by the proportion of grassland in the crop rotation or sward type. However, there was a considerable effect of having a ryegrass catch crop following the final barley crop as nitrate leaching was reduced from 60 to 9 kg N per hectare. When summarizing results from the crop rotations over a longer period (1997–2005), management strategy adopted in both the grassland and arable phases appeared to be the primary instrument in avoiding nutrient losses from mixed crop rotations, irrespective of grass proportion. In the arable phase, the huge potential of catch crops has been demonstrated, but it is also important to realize that all parts of the grass‐arable crop rotations must be considered potentially leaky.     

生草对渭北旱地苹果园土壤有机碳组分及微生物的影响

在渭北旱地苹果园行间播种毛苕子（Vicia villosa）、白三叶（Trifolium repens）、黑麦草（Lolium perenne）和早熟禾（Poa pratensis）,以清耕为对照,对0100 cm土层的土壤有机碳各组分及微生物群落功能多样性进行研究。结果表明,行间生草可显著增加040 cm土层土壤的总有机碳（TOC）、颗粒有机碳（POC）、轻质有机碳（LFOC）、易氧化有机碳（ROC）、可溶性有机碳（DOC）和微生物量碳（MBC）,豆科牧草毛苕子和白三叶的各有机碳含量总体上高于禾本科牧草黑麦草和早熟禾。其中在020 cm土层中,豆科牧草的TOC含量平均每年增加约1.2 g/kg,禾本科牧草每年增加约0.9 g/kg。生草处理的微生物群落碳源利用率（AWCD）、微生物群落Shannon 指数（H）和微生物群落丰富度指数（S）均高于清耕处理,其中豆科牧草的微生物活性更高。因此,生草可以提高土壤有机碳各组分的含量、土壤微生物群落碳源利用率、微生物群落的丰富度和功能多样性,豆科牧草毛苕子和白三叶提高效应更加明显。