Grass-clover Leys Research Articles

Combining agro-ecological functions in grass-clover mixtures

Grass-clover mixtures show many benefits for sustainable agriculture. In the Netherlands, organic farmers often work together in a so-called partner farm concept, with the aim to close nutrient cycles on a regional level. In this system, arable farms grow one-year grass-clover leys, as fodder for a livestock farm, in exchange for, e.g., manure. This practice could also be used in the transition of conventional farms towards a more circular regenerative and nature inclusive agriculture. In the current experiment we assessed the effect of a range of grass (Lolium perenne: Lp, Lolium multiflorum: Lm) and clover (Trifolium pratense: Tp and Trifolium repens: Tr) monocultures and mixtures on both below- and aboveground parameters in light of benefits for livestock and arable farms, and biodiversity. The grass monocultures showed good weed suppression, high root density, and especially Lp had a positive effect on soil structure. Clover, on the other hand, showed high herbage dry matter yield (particularly Tp) and Nitrogen (N) yield, and Tr showed high digestibility. Moreover, clover had a positive effect on the soil mineral N, and earthworm abundance tended to be higher in the clover monocultures. When (some of) the four species were combined in grass-clover mixtures, they combined the positive effects of the species and often even outperformed the (best) monocultures. We concluded that grass-clover mixtures increased agro-ecological functions.

Forbs differentially affect soil microbial community composition and functions in unfertilized ryegrass-red clover leys

Increasing plant diversity in agroecosystems is proposed to enhance multiple ecosystem services. Adding selected forbs such as caraway (Carum carvi L.) and plantain (Plantago lanceolata L.) to productive grass-clover mixtures can further enhance forage yields, root biomass, uptake of mineral nutrients and improve animal performance. Yet, it remains unclear whether and how adding these forbs to grass-clover mixtures can influence soil microbial communities and associated soil carbon (C) and nitrogen (N) cycling. Based on a three-year grassland experiment varying in species diversity and composition with and without fertilizer application, we determined soil microbial community composition and functions related to C and N cycling under laboratory incubations. Results showed that inclusion of caraway modified soil microbial community composition by enhancing fungal-to-bacterial phospholipid fatty acids of the ryegrass-red clover mixture. Adding plantain to the ryegrass-red clover mixture increased the relative decomposition rate of the labile C pool, but not of the recalcitrant C pool. Yet, β-glucosidase activity and net N mineralization were unchanged due to inclusion of either forb. Moreover, fertilization with cattle slurry generally weakened these forb-induced changes in soil microbial properties. These findings demonstrate that adding selected forbs to unfertilized grass-clover leys can modify soil microbial community composition and associated C and N cycling, implying a potential for promoting long-term soil C sequestration through enhanced fungi-to-bacteria ratio, but a limited role in improving soil N fertility.

Long-term effects of grass-clover leys on the structure of a silt loam soil in a cold climate

Abstract Grass/legume leys are commonly included in the crop rotation in mixed farming systems in cold or humid regions in order to sustain the supply of nutrients and maintain soil fertility. Leys are also known to sequester soil organic carbon and also improve the structural stability and mechanical properties of soil. However, few studies have investigated the long-term effects of ley rotations on the architectural properties of the structural pore space in soil. We investigated the effects of grass/clover leys on soil structure in the topsoil and upper subsoil of a silt loam in a long-term field trial established in 1956 at Offer in northern Sweden. This experiment includes four treatments with varying proportions of ley (1, 2, 3 or 5 years) in 6-year rotations. We used X-ray tomography to quantify topsoil structural pore space at a resolution of 65 μm in the first year of arable cropping following the ley break, a few weeks after sowing in spring. Earthworm populations were quantified by both hand-sorting and chemical extraction, while near-saturated infiltration was measured as a proxy for soil structure in the upper subsoil. In the topsoil, the treatments with a greater proportion of ley had larger organic carbon contents, smaller bulk densities and larger porosities. However, effects of crop rotation on the pore space were limited to pores smaller than 65 μm, as no treatment effects were found for the volume, size distribution, connectivity or complexity of the X-ray imaged pore space, even though the grass-clover leys promoted larger numbers and biomass of topsoil-dwelling earthworm species. Furthermore, no positive effects of grass/clover leys on organic carbon content and soil structure were found in the subsoil. The macropore infiltration capacity in upper subsoil was generally very small (overall median value of 3 mm h−1) indicating a lack of functional macroporosity below plough depth. Consistent with these results, no individuals of deep-burrowing earthworm species were found at the site, while previous observations showed only shallow rooting, both of which are attributed to the cold climate and poor subsoil drainage.

Forbs enhance productivity of unfertilised grass-clover leys and support low-carbon bioenergy

Intensively managed grasslands are dominated by highly productive grass-clover mixtures. Increasing crop diversity by inclusion of competitive forbs may enhance biomass production and sustainable biofuel production. Here we examined if one or all of three forbs (chicory, Cichorium intybus L.; caraway, Carum carvi L.; plantain, Plantago lanceolata L.) included in ryegrass-red clover mixtures enhanced above- and below-ground productivity, and assessed their biofuel potentials, based on a three-year experiment with and without fertilisation as cattle slurry. We determined herbage yield, standing root biomass, and estimated methane energy output and greenhouse gas (GHG) emissions per energy unit using life cycle assessment. Results showed that plantain-containing grass-clover mixtures significantly increased herbage yield, while chicory- or caraway-containing mixtures maintained similar yields to the grass-clover mixture. Standing root biomass of the grass-clover mixture was enhanced by inclusion of caraway and plantain, with that of plantain further enhanced by fertilisation. The highest methane energy output was achieved in plantain-containing grass-clover mixtures. All unfertilised mixtures achieved the 60% reduction in GHG emissions compared to fossil fuel, whereas all fertilised mixtures did not meet the 60% reduction target. These findings suggest that including competitive forbs such as plantain in grass-clover mixtures enhances productivity, supporting low-carbon footprint bioenergy production.

Profitability of controlled traffic in grass silage production – economic modelling and machinery systems

Controlled traffic farming (CTF) systems aim to reduce soil compaction by restricting machinery field traffic to permanent traffic lanes. Grass-clover silage production is generally associated with intensive field traffic, resulting in reduced silage clover content. If CTF can increase yield and clover content in grass-clover leys, this would reduce the need for grain and expensive protein concentrate in dairy cow feed rations. A mixed integer programming model was developed to evaluate the potential profitability of CTF in a dairy farm context. Existing field trial data were used to calculate the expected yield outcome of CTF, based on reductions in trafficked area. The results revealed that CTF increased profitability by up to €50/ha. Total machinery costs are likely to increase on converting to CTF, but variable machinery costs are likely to decrease.

Effect of organic grass-clover silage on fiber digestion in dairy cows

There are differences in grass-clover proportions and chemical composition between herbage from primary growth (PG) and regrowth (RG) in grass-clover leys. Mixing silages made from PG and RG may provide a more optimal diet to dairy cows than when fed separately. We tested the hypotheses that increasing dietary proportions of grass-clover silage made from RG compared with PG would increase digestion rate of potentially degradable NDF (pdNDF), and increase ruminal accumulation of indigestible NDF (iNDF). Eight rumen cannulated Norwegian Red cows were used in two replicated 4×4 Latin squares with 21-day periods. Silages were prepared from PG and RG of an organically cultivated ley, where PG and RG silages were fed ad libitum in treatments with RG replacing PG in ratios of 0, 0.33, 0.67 and 1 on dry matter basis in addition to 8 kg concentrate. We evaluated the effect of the four diets with emphasis on rumen- and total tract fiber digestibility. Increasing RG proportions decreased silage intake by 7%. Omasal flow of pdNDF decreased, whereas iNDF flow increased with increasing RG proportions. Increasing RG proportions decreased rumen pool sizes of NDF and pdNDF, whereas pool sizes of iNDF and CP increased. Increasing RG proportions increased digestion rate of NDF, which resulted in greater total tract digestion of NDF. Pure PG diet had the highest calculated energy intake, but the improved rumen digestion of NDF by cows offered 0.33 and 0.67 of RG leveled out milk fat and protein yields among the three PG containing diets.

Apparent nitrogen fertilizer replacement value of grass–clover leys and of farmyard manure in an arable rotation. Part I: grass–clover leys

AbstractApparent nitrogen fertilizer replacement values of grass–clover leys (NFRVGCL) and farmyard manure (NFRVFYM) were studied in a long‐term (24 years) experiment. This paper reports the results for grass–clover leys (GCL). Five rates of farmyard manure (FYM) and four rates of fertilizer nitrogen (N) were applied to two arable rotations (RotA – arable+silage maize; RotB – arable+GCL). Polynomial response surfaces fitted to dry matter (DM) yield and N offtake in the three arable crops were used to assess NFRVGCL by interpolation, at high (Method H) and low (Method L) N fertilizer rates, always comparing RotA and RotB at equal FYM rates. In Expt1 (started in 1980), arable crops in RotB required 188, 246, 270, 295 and 312 kg N/ha less fertilizer per cycle than in RotA, to match the DM yields found at highest fertilizer rate in RotA (Method H). These values refer to FYM rates of 0, 20, 40, 60 and 80 t/ha per cycle, respectively. Corresponding fertilizer savings in Expt2 (started in 1981) were 287, 323, 341, 346 and 337 kg N/ha per cycle. These NFRVGCL values represent savings of 50–83% (Expt1) or 77–92% (Expt2) in N fertilizer applied to the arable phase. Slightly lower NFRVGCL was found by Method L. Extra N offtake in RotB over RotA was a poor estimator of NFRVGCL, showing much lower values and different responses to FYM and fertilizer than NFRVGCL. Mixed leys enable large fertilizer savings in arable rotations.

Apparent nitrogen fertilizer replacement value of grass–clover leys and of farmyard manure in an arable rotation. Part II: farmyard manure

AbstractThe apparent nitrogen fertilizer replacement value of farmyard manure (NFRVFYM) was studied in a long‐term experiment on a loamy sand soil at Grabow, Poland. The experiment combined five rates of FYM with four nitrogen (N) fertilizer rates in two rotations (RotA and RotB) started in 1980 (Expt1) and, in parallel, 1981 (Expt2). Rotations consisted of potatoes, winter wheat and spring barley, followed by silage maize (RotA) or grass–clover ley (GCL, RotB). We analysed six consecutive cycles. NFRVFYM was much larger in RotB than RotA, due to high N supply from GCL residues combined with likely deficiencies in potassium (K) and/or phosphorus (P), causing weak crop responses to N fertilizer in RotB relative to FYM effects. Devoid of non‐N effects, ‘true’ (as opposed to apparent) NFRVFYM was estimated at 2.0 kg fertilizer‐N per tonne of FYM (at equal N offtake), or 2.7 kg/t (at equal DM yield). These values correspond with equivalencies of 0.37 and 0.50 kg fertilizer‐N per kg FYM‐N. RotB required 25 kg P and 350 kg K/ha/cycle more input than RotA to compensate for higher P and K offtake in GCL compared to maize. The study highlights differences between approaches to assess NFRVFYM, giving larger values at high compared with low N fertilizer rate, and larger values at low compared with high FYM rate. These contrasts reflect drift in the ratio between the agronomic N‐use efficiencies of FYM and fertilizer, respectively, and in the ratio between the apparent recoveries of N from these sources.

Yield and herbage quality from organic grass clover leys—a meta-analysis of Norwegian field trials

A meta-analysis based on experiments in organically cultivated grasslands in Norway was conducted to quantify the effects of management factors on herbage yield and feed quality. A dataset was collected that included 496 treatment means from experiments in five studies carried out at eight locations with the latitude range of 58.8 to 69.6 N between 1993 and 2010. We tested the effect of harvesting system (two vs. three cuts annually), plant developmental stage at the first cut, growth period (temperature sum) and the herbage clover proportion. Plant maturity at the first cut and herbage clover proportion explained to a large extent herbage yield and quality of the first cut and annual yield. The timing of the first cut influenced also the yield and herbage quality of the second cut. The analysis confirmed the importance of legumes performance for herbage yield and quality from grasslands in organic production. Estimated annual herbage DM yield harvested at standardized plant development stage and at average clover proportion was 9 % higher in the two—compared to the three-cut system. The crude protein concentration and in vitro dry matter digestibility was 17 and 3 % higher and the NDF concentration 7 % lower in the annual herbage from the three-cut than from the two-cut system, respectively. The empirical equations developed in this study may be applied to explore different options for grassland management as basis for ration and production planning and in scenario analysis of economic performance of individual and model farms. The equations do also reveal in numeric terms the trade-offs in management practice between high yields, yield digestibility, NDF and crude protein content in organic forage production relying on red clover N2 fixation as the engine in the system.

Nitrate leaching in maize after cultivation of differently managed grass-clover leys on coarse sand in Denmark

When grass-clover leys have been ploughed nitrate leaching may increase. However, management of leys before or after ploughing may affect the leaching risk. We examined the effect of cultivating a six year ley, which the last two years had been treated differently (grazing only; spring cut followed by grazing, and spring and autumn cuts with grazing during the summer season only) on nitrate leaching in maize with/without a catch crop (per. ryegrass) and with/without slurry application (135kg total-Nha−1). An unfertilized barley harvested for silage with a catch crop of Italian ryegrass was used as reference. Shortening the grazing season in the grass-clover ley phase did not affect leaching after ploughing. In the following maize, the use of a perennial ryegrass catch crop in a high-yielding maize crop was not able to reduce nitrate leaching significantly, although leaching and maize yield tended to be lower with the catch crop. In unfertilized treatments both maize yields and nitrate leaching were significantly lower compared with treatments with slurry application. The unfertilized barley undersown with Italian ryegrass reduced leaching much more than unfertilized maize with perennial ryegrass as an undersown catch crop, despite yields of barley and ryegrass being less than half of the dry matter yields of unfertilized maize. The experiment illustrates that growing maize after ploughing of grass-clover leys without environmental consequences is difficult.

Enchytraeids as indicator of soil quality in temporary organic grass-clover leys under contrasting management: A feasibility study

One objective in organic farming is to sustain the quality of the soil resource. Because enchytraeids are an important soil faunal component, they stand as bioindicators of soil quality. We tested this candidature in a field experiment on loamy sand soil with 1- and 4-year old grass-clover leys subject to contrasting management regimes (cutting, slurry injection, grazing). Enchytraeid density, biomass and species composition was determined in 0–18 cm soil sampled in October, and again in March and May before and after slurry application, respectively. For soils retrieved in October, ley age had no consistent impact on enchytraeid density and biomass. Injection of slurry significantly affected the enchytraeid community at one sampling only but tended to sustain higher enchytraeid abundance in 1-year old leys throughout the year. One persistent feature was a larger enchytraeid community in the autumn than in the spring, regardless of management regime. Compared with leys used for cutting, grazed leys had consistently lower enchytraeid density and biomass. This was ascribed to soil compaction induced by heifer trampling. The number of species ranged from 11 to 18 in the autumn and from 7 to 13 in the spring with no distinct relation to management. Simpson's index of diversity did not differ among sampling dates and management regimes, including ley age. Although the enchytraeid community to some extent reflected differences in management regime, the patchy distribution and management induced interactions among soil physical, chemical and biological properties suggest that enchytraeid abundance is not a feasible stand-alone indicator of management impacts on soil quality in temporary grass-clover leys but may candidate as one of several biological key parameters in more comprehensive soil quality assessment schemes.

Nitrate leaching and residual effect in dairy crop rotations with grass–clover leys as influenced by sward age, grazing, cutting and fertilizer regimes

Intensive dairy farming, with grass-arable crop rotations is challenged by low N use efficiency that may have adverse environmental consequences. We investigated nitrate leaching and N fertility effects of grass–clover leys for five years in two organic crop rotations with different grassland proportions (33 and 67%) and five grassland managements in terms of cutting, grazing, fertilization and combinations thereof. In grass–clover, the combination of fertilization and grazing caused excessive leaching (average 60kg Nha−1) but leaving out either fertilization or full-time grazing substantially reduced leaching losses to on average 23kg Nha−1. There was no linear relationship between sward age and nitrate leaching. The annual N surplus of the grasslands was only weakly related to nitrate leaching (R2=0.05, P<0.01). The residual effect of grassland cultivation was highest following grazing and fertilization and lowest without fertilization or cutting-only management and was not influenced by grassland age. The arable part of the mixed crop rotation was nitrate leaky where crop coverage in autumn was insufficient. Nitrate leaching following the crops may roughly be divided into four groups: (1) low leaching (<10kg Nha−1) consisting of the barley wholecrop undersown with Italian ryegrass, (2) medium–low leaching (10–20kg Nha−1) consisting of barley with grass–clover undersown and the 1-yr-old grasslands, (3) medium leaching (25–50kg Nha−1) consisting of barley–pea and 2-4-yr-old grasslands with the 2-yr-old leaching the most (36–46kg Nha−1), and (4) high leaching (>50kg Nha−1) with lupin and maize, where especially maize was consistently high in all five years (average 81kg Nha−1). Great care should be taken during all phases of the dairy crop rotation where grasslands cause considerable build-up of fertility. With due care and the best management practice, nitrate leaching losses may be reduced to low levels.

The energy efficiency of organic agriculture: A review

AbstractGrowing populations and a constrained fossil-manufactured energy supply present a major challenge for society and there is a real need to develop forms of agriculture that are less dependent on finite energy sources. It has been suggested that organic agriculture can provide a more energy efficient approach due to its focus on sustainable production methods. This review has investigated the extent to which this is true for a range of farming systems. Data from about 50 studies were reviewed with results suggesting that organic farming performs better than conventional for nearly all crop types when energy use is expressed on a unit of area basis. Results are more variable per unit of product due to the lower yield for most organic crops. For livestock, ruminant production systems tend to be more energy efficient under organic management due to the production of forage in grass–clover leys. Conversely, organic poultry tend to perform worse in terms of energy use as a result of higher feed conversion ratios and mortality rates compared to conventional fully housed or free-range systems. With regard to energy sources, there is some evidence that organic farms use more renewable energy and have less of an impact on natural ecosystems. Human energy requirements on organic farms are also higher as a result of greater system diversity and manual weed control. Overall this review has found that most organic farming systems are more energy efficient than their conventional counterparts, although there are some notable exceptions.

Maximizing the nitrogen efficiency of a prototype mixed crop-livestock farm in The Netherlands

This paper describes the year-over-year improvement of the nitrogen (N) efficiency in a prototype mixed farm system through the implementation of a coherent set of ecotechnological adjustments. This farm, the former APMinderhoudhoeve, was located on a marine clay loam soil in Oostelijk Flevoland, the Netherlands, reclaimed from the sea in the late 1950s. The designed farm structure was representative for the Dutch soil-bound agriculture in the late 1990s in terms of food crops’ choice and average level of milk production per ha of farmland. Key management strategies were the inclusion of grass-clover leys in the 7-year crop rotation, restricted grazing of the dairy herd and protein-poor and fibre-rich cattle diets including cereal straw. The farm components animal-manure-soil-crop were analysed in-depth in order to evaluate N flows and soil organic matter (SOM) dynamics at the farm system level, covering a period of six years. In its final experimental state, farm N use efficiency, expressed as the proportion of purchased crop fertilizers and cattle feed that was sold as exported products (crops, milk and cattle), was as high as 73%, Besides, total environmental losses were as low as 42kg N ha−1 yr−1. A scenario analysis revealed that these losses could be further minimized to 30kg N ha−1 yr−1 in the stabilization phase, of which one-third as leached nitrate and a somewhat higher fraction as volatilized ammonia. The remaining losses represented mainly denitrified harmless N2 from the stored animal slurry and farmyard manure. Soil N accumulated at an average rate of 89kg N ha−1 yr−1, whereas SOM showed a positive trend but with fluctuations from year to year which were negatively related to the annual rainfall amount. Despite the lowered protein content in the cow diets, average annual milk production increased from 8100 tot 8700kg cow−1. This could be mainly ascribed to a lower cow replacement rate due to less animal health problems, leading to an increased average lactation number of the dairy herd. Overall, the obtained environmental side effects on the prototype farm in its final experimental state were already below the targets set by the European Union with respect to the Nitrate Directive for the year 2020. It is concluded that when the best ecotechnological means are combined in a balanced mixed farm system a multiple win situation is attained in terms of food production capacity, cattle health and environmental quality.

Long-Term Effect of Different Carbon Management Strategies on Water Flow and Related Processes for Three Loamy Soils

The decline in organic matter of arable land, induced and accelerated by modern agriculture, has been identified as a threat to sustained soil quality. In this article, we studied strategies to counter this decrease by building up soil organic carbon (SOC) levels in the soils using several approaches. They included return of organic matter, such as straw, animal manure, and slurry, to the soil and diverse crop rotations with, for example, grass-clover leys. We used three field sites in Denmark with different C repletion strategies to assess and quantify the effect of these approaches on preferential flow and loss of colloids during heavy irrigation events. The field sites were all under long-term management and therefore represent up to 30 years of pairwise different management strategies. One field in each field pair was managed with a more C-repleting strategy (HighC) than the other (LowC). Only small differences in SOC contents were identified, and none of the management strategies had succeeded in building up SOC pools large enough to saturate the soil with C. Only at one field site was the content of water-dispersible colloids lower in the HighC than the LowC treatment. Preferential flow patterns showed a rapid breakthrough of irrigation water but only little or negligible effect in reducing the risk of colloid loss and chemicals leaching from the root zone, despite 20 to 30 years of different management strategies with this aim. Previous studies reported in the literature have emphasized high SOC contents and grass ley soils to significantly reduce nonequilibrium water flow. Our results thus indicate the need for more effective management options than those addressed in this study.

Nitrogen fixation and transfer in grass-clover leys under organic and conventional cropping systems

Symbiotic dinitrogen (N2) fixation is the most important external N source in organic systems. Our objective was to compare symbiotic N2 fixation of clover grown in organically and conventionally cropped grass-clover leys, while taking into account nutrient supply gradients. We studied leys of a 30-year-old field experiment over 2 years in order to compare organic and conventional systems at two fertilization levels. Using 15N natural abundance methods, we determined the proportion of N derived from the atmosphere (PNdfa), the amount of Ndfa (ANdfa), and the transfer of clover N to grasses for both red clover (Trifolium pratense L.) and white clover (Trifolium repens L.). In all treatments and both years, PNdfa was high (83 to 91 %), indicating that the N2 fixation process is not constrained, even not in the strongly nutrient deficient non-fertilized control treatment. Annual ANdfa in harvested clover biomass ranged from 6 to 16 g N m−2. At typical fertilizer input levels, lower sward yield in organic than those in conventional treatments had no effect on ANdfa because of organic treatments had greater clover proportions. In two-year-old leys, on average, 51 % of N taken up by grasses was transferred from clover. Both, organically and conventionally cropped grass-clover leys profited from symbiotic N2 fixation, with high PNdfa, and important transfer of clover N to grasses, provided sufficient potassium- and phosphorus-availability to sustain clover biomass production.

Drycore Appears to Result from an Interaction between Rhizoctonia solani and Wireworm (Agriotes ssp.)—Evidence from a 3-Year Field Survey

Drycore is an important quality deficiency in Europe especially in organic potato production and after grass clover leys. The drycore symptom is attributed to Rhizoctonia solani Kühn (teleomorph: Thanatephorus cucumeris (Frank) Donk). In the framework of a 3-year survey (2001–2003), data concerning potato quality, crop rotation, management, and site parameters were analysed from 185 potato fields. The hypothesis was tested that injuries on potatoes caused by wireworms facilitate the penetration of R. solani and favour the formation of drycore. Analysis of variance showed a significant influence of wireworm damage, seed quality, and grass clover leys in the crop on the level of drycore damage. On fields which had both a low occurrence of black scurf on the seed tubers and a low occurrence of wireworm damage at harvest, significant drycore damage was never observed. The relative risk for drycore damage on tubers was significantly higher if black scurf or wireworm damage was on the same tuber. In contrast, no higher risk for drycore was observed on tubers with slug damage. Abiotic factors like farm manure application, organic matter content, texture, and pH of the soil also had no significant influence on the level of drycore. Thus, the wounding of potatoes by wireworm could be confirmed as the major variable for drycore. The mode of action has to be clarified under controlled conditions.

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

AbstractThe 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.

Reply to Short Comment on: 'Nitrogen mineralisation in relation to previous crops and pastures'

In a recent paper we reported rapid nitrogen mineralisation in soil where lucerne and a mixture of the perennial grasses Phalaris aquatica and Dactylis glomerata had been growing (Angus et al. 2006). We were surprised that pastures containing perennial grasses led to higher mineralisation rates than pure lucerne pastures, because we expected that pure lucerne would fix more N and its residue would release mineral N more rapidly than a lucerne+ perennial grass mixture. In their comment on this paper, Luxhoi and HauggaardNielsen (2007, this issue) found the results to be unsurprising and offered 2 explanations. They speculated that pastures containing perennial grass had recovered fertiliser N, some of which was later mineralised. They also speculated that the presence of grass may have reduced the level of soil mineral N and, hence, stimulated symbiotic N2-fixation by lucerne. The first speculation is inappropriate as no N fertiliser was applied to the pastures in our experiments, reflecting normal practice for pastures growing on mixed cropping–livestock farms in Australia. The only fertiliser applied was phosphatic. The second speculation is probably unjustified; N2-fixation by the different pastures was not compared in the season when mineralisation was measured. However, in a previous season at the same experiment, N2-fixation by pure lucerne was substantially greater than by the lucerne+ perennial grass mixture (Peoples et al. 1998).While itwas reasonable forLuxhoi andHauggaard-Nielsen (2007) to presume that the proportion of lucerne N obtained from biological fixation might be increased by grass competition, this was not reflected in the earlier measurements of Peoples et al. (1998), and any gain would be more than offset by the reduction in lucerne growth. Luxhoi and Hauggaard-Nielsen (2007) also point out that similar results to ours were reported from an experiment in Denmark by Hauggaard-Nielsen et al. (1998). We accept that the results were similar, but the nature of the experiments was so different that any similarity may have been coincidental. They reported high rates of net mineralisation in soil after ungrazed mixed grass–clover leys that had received no N fertiliser, but which contained 87 kg/ha of above-ground N at the time of incorporation in spring. Our experimental system consisted of grazed pastures, all with negligible above-ground biomass in autumn when they were cultivated before sowing. Their experiment may have received significant input of mineral

Response of organically managed grassland to available phosphorus and potassium in the soil and supplementary fertilization: field trials using grass–clover leys cut for silage

Effective use and recycling of manures together with occasional and judicious use of supplementary fertilizing materials forms the basis for management of phosphorus (P) and potassium (K) within organic farming systems. Replicated field trials were established at three sites across the UK to compare the supply of P and K to grass-clover swards cut for silage from a range of fertilizing materials, and to assess the usefulness of routine soil tests for P and K in organic farming systems. None of the fertilizing materials (farmyard manure, rock phosphate, Kali vinasse, volcanic tuff) significantly increased silage yields, nor was P offtake increased. However, farmyard manure and Kali vinasse proved effective sources of K to grass and clover in the short to medium term. Available P (measured as Olsen-P) showed no clear relationship with crop P offtake in these trials. In contrast, available K (measured by ammonium nitrate extraction) proved a useful measurement to predict K availability to crops and support K management decisions.