Publisher Summary This chapter reviews the cycling of nutrients within pasture soils. The major emphasis is on the central role of the grazing animal in influencing soil fertility, particularly in the dung and urine patches. In addition, the progress is reviewed that has been made in modeling the cycling of nutrients in pasture systems. Nutrients are partitioned differently between dung and urine, with K being excreted mainly in urine; P, Ca, and Mg are excreted principally in dung and N and S are excreted in both forms. Nutrients returned in excreta can be in inorganic and organic forms, depending on the particular nutrient being considered. For some nutrients (e.g., P and S) significant mineralization of ingested organic forms occurs during passage through the animal, whereas much of the N is excreted in the readily available organic urea form. From the present knowledge of the major nutrient inputs and losses for the grazed pasture system and an understanding of the pathways of nutrient flux within the system and some key measurements, simple mass balance nutrient models for various pasture systems can be constructed. Such simple models have been used to calculate site-specific maintenance fertilizer requirements of pastures based on the amount of nutrient required to replace losses in the soil (e.g., through fixation and leaching) and losses by animal transfer and in animal products.

Nutrient cycling and soil fertility in the grazed pasture ecosystem

Nitrate (NO3
−) leaching from agriculturalproduction systems is blamed for the rising concentrations ofNO3
− in ground- and surface-waters around the world.This paper reviews the evidence of NO3
− leachinglosses from various land use systems, including cut grassland, grazed pastures,arable cropping, mixed cropping with pasture leys, organic farming,horticultural systems, and forest ecosystems. Soil, climatic and managementfactors which affect NO3
− leaching are discussed.Nitrate leaching occurs when there is an accumulation ofNO3
− in the soil profile that coincides with or isfollowed by a period of high drainage. Therefore, excessive nitrogen (N)fertilizer or waste effluent application rates or N applications at the wrongtime (e.g. late autumn) of the year, ploughing pasture leys early in the autumn,or long periods of fallow ground, can all potentially lead to highNO3
− leaching losses. N returns in animal urine havea major impact on NO3
− leaching in grazed pastures.Of the land use systems considered in this paper, the potential for causingNO3
− leaching typically follow the order: forest< cut grassland < grazed pastures, arable cropping < ploughing ofpasture < market gardens. A range ofmanagement options to mitigate NO3
− leaching isdescribed, including reducing N application rates, synchronizing N supply toplant demand, use of cover crops, better timing of ploughing pasture leys,improved stock management, precision farming, and regulatory measures. This isfollowed by a discussion of future research needs to improve our ability topredict and mitigate NO3
− leaching.

Nitrate leaching in temperate agroecosystems: sources, factors and mitigating strategies

Emission Of NOx (principally NO) and N2O from soils is reviewed with particular emphasis placed on the atmospheric and ecological implications of this source. The photochemistry of these species in the atmosphere is summarized as well as the methods available for the determination of fluxes. Processes which produce and consume both NO and N2O in soils are principally microbiological in nature and are linked directly and indirectly with the chemical and physical factors that control gaseous transport through the soil medium. Linkages among these processes occur over many different temporal and spatial scales which makes interpretation of the available data difficult. A summary of results from laboratory and field studies shows that considerable spatial and temporal variability exists in the emissions. This variability can be related to factors such as temperature, water content, soil composition, nutrient availability, vegetation, disturbances (e.g., burning, agricultural practices), and others. Because NOx and N2O play central roles in many important environmental problems, there is a need for accurate estimates of the magnitude of the soil source, but the large degree of variability in the existing data makes extrapolation highly uncertain. To overcome this uncertainty, models are required which can simulate the processes responsible for production, consumption, and transport of these species at all relevant temporal and spatial scales. Integrated field studies will also be required to validate the model results.

NOx And N2O Emissions From Soil

European grassland-based livestock production systems face the challenge of producing more meat and milk to meet increasing world demands and to achieve this using fewer resources. Legumes offer great potential for achieving these objectives. They have numerous features that can act together at different stages in the soil–plant–animal–atmosphere system, and these are most effective in mixed swards with a legume proportion of 30–50%. The resulting benefits include reduced dependence on fossil energy and industrial N-fertilizer, lower quantities of harmful emissions to the environment (greenhouse gases and nitrate), lower production costs, higher productivity and increased protein self-sufficiency. Some legume species offer opportunities for improving animal health with less medication, due to the presence of bioactive secondary metabolites. In addition, legumes may offer an adaptation option to rising atmospheric CO2 concentrations and climate change. Legumes generate these benefits at the level of the managed land-area unit and also at the level of the final product unit. However, legumes suffer from some limitations, and suggestions are made for future research to exploit more fully the opportunities that legumes can offer. In conclusion, the development of legume-based grassland–livestock systems undoubtedly constitutes one of the pillars for more sustainable and competitive ruminant production systems, and it can be expected that forage legumes will become more important in the future.

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Potential of legume-based grassland–livestock systems in Europe: a review

Low input legume-based agriculture exists in a continuum between subsistence farming and intensive arable and pastoral systems. This review covers this range, but with most emphasis on temperate legume/grass pastures under grazing by livestock. Key determinants of nitrogen (N) flows in grazed legume/grass pastures are: inputs of N from symbiotic N2 fixation which are constrained through self-regulation via grass/legume interactions; large quantities of N cycling through grazing animals with localised return in excreta; low direct conversion of pasture N into produce (typically 5–20%) but with N recycling under intensive grazing the farm efficiency of product N: fixed N can be up to 50%; and regulation of N flows by mineralisation/immobilisation reactions. Pastoral systems reliant solely on fixed N are capable of moderate-high production with modest N losses e.g. average denitrification and leaching losses from grazed pastures of 6 and 23 kg N ha−1 yr−1. Methods for improving efficiency of N cycling in legume-based cropping and legume/grass pasture systems are discussed. In legume/arable rotations, the utilisation of fixed N by crops is influenced greatly by the timing of management practices for synchrony of N supply via mineralisation and crop N uptake. In legume/grass pastures, the spatial return of excreta and the uptake of excreta N by pastures can potentially be improved through dietary manipulation and management strategies. Plant species selection and plant constituent modification also offer the potential to increase N efficiency through greater conversion into animal produce, improved N uptake from soil and manipulation of mineralisation/immobilisation/nitrification reactions.

Nitrogen cycling in low input legume-based agriculture, with emphasis on legume/grass pastures

The model simulates the cycling of N in grassland systems grazed by beef cattle and predicts the annual amount of N in liveweight gain, and the amounts lost through ammonia volatilization, denitrification and leaching, on the basis of fertilizer application and soil and site characteristics. It aims to provide a better understanding of the way in which these various factors interact in their influence on N transformations. The model has been programmed to run on IBM-compatible personal computers and responds rapidly to changes in input parameters.

D. C. Whitehead

Papers

A model to predict transformations and losses of nitrogen in UK pastures grazed by beef-cattle