Nitrous oxide emissions from ruminant urine: science and mitigation for intensively managed perennial pastures
Introduction
Grasslands are a major terrestrial ecosystem that occupy about one-quarter of the Earth’s land area [1]. Intensification of grassland, through the manipulation of primary productivity by plant and animal breeding, increased stocking rates and increased fertiliser-N inputs, increases reactive-N losses resulting in negative environmental impacts [2]. Such intensification of grassland ecosystems, which has been projected to increase further [2], confronts the need to mitigate greenhouse gas (GHG) emissions [3]. Nitrous oxide (N2O) is both a potent GHG and an ozone (O3) depleting molecule [4]. The atmospheric N2O concentration has continued to increase by 0.73 ppb y−1 for several decades [5]. Since agriculture is the dominant anthropogenic source of N2O, this rate is likely to be exacerbated due to the future predictions for feed/food, and the use of agricultural N fertilisers, and climate warming [5,6]. Previously, it has been determined that livestock grazing contributes >10% of the global annual N2O emissions [7]. Nitrogen deposited onto pasture soil at rates in excess of plant demand and the immobilization capacity of soil microorganisms will result in N being transformed into a reactive N species (Nr) and ultimately lost from the pasture soil system (Figure 1): for example, as nitrate (NO3−), dissolved organic-N (DON), ammonia (NH3) and nitric oxide (NO). Surplus N may also be lost as environmentally benign atmospheric nitrogen (N2). For this reason, inputs of N to intensively managed grassland ecosystems (deposition, biologically fixed N, fertilisers and manures) should ideally balance outputs (exported product). On the other hand, large parts of, for instance, the Eurasian steppe, have been heavily grazed without returns of nutrients due to overnight confinement of animals in feedlots and utilization of animal faeces as fuel. In some ecosystems this imbalance between N inputs and outputs has resulted in soil degradation and even desertification through soil carbon (C) loss [8,9]. Since N inputs are relatively low within extensively grazed ecosystems so too are the accompanying N2O emissions [e.g. Ref. 10]. Consequently, mitigation of N2O emissions from grazed pasture systems is strongly focused on intensively managed systems and is dependent on an improved understanding of how soil chemical and biophysical factors interact with Nr to produce N2O emissions, which is in turn a function of the C, N and other nutrient cycles that occur within such pasture soils. This article focuses on synthesising recent knowledge of the drivers of N2O emissions from soils and associated mitigation strategies within intensively grazed pasture soils with a focus on ruminant excreta.
Section snippets
Ruminant urine-N deposition and transformation in grazing systems
Via photosynthesis and the uptake of N compounds, plants integrate C and N, growing pasture that is ultimately consumed by ruminants. Following forage ingestion ruminants uncouple N from C. The C losses from ruminants occur via enteric methane (CH4) production, respiration (carbon dioxide, CO2), export of product such as milk, and the excretion of urine and dung on pasture [2]. Losses of N from ruminants occur via milk production, and the excretion of urine and dung. Emissions of N2O are
Inorganic-N transformations
Pasture uptake and soil cation exchange sites within the soil compete for NH4+ reducing the NH4+ pool available for NH3 oxidising bacteria (AOB), archaea (AOA) or comammox bacteria (complete nitrifiers) [19]. However, it is the AOB that dominate the nitrifier response to elevated inorganic-N concentrations following ruminant-urine deposition onto pasture soils [20] which is consistent with the now recognized niche differentiation of AOB, AOA, and comammox bacteria [21••,22]. Such niche
Soil physical conditions and soil organic matter content
Clearly, a fundamental driver determining the microbiology responsible, and the substrate available, for N2O production in pasture soils is the aerobic status of the soil: this depends on soil pore structure, which is in turn a function of texture and bulk density, soil water content, and oxygen consumption rates. Soil WFPS has often been used to describe the potential for soil N2O emissions. However, recent studies have demonstrated that strong relationships exist between N2O emissions and
Conclusions
Recent research has advanced our understanding of the microbial pathways transforming inorganic-N in pasture systems, and the resulting N2O emissions. However, this knowledge needs to be applied in the context of the temporal soil physical and chemical dynamics recognized as occurring in pasture ruminant urine patches. Chemical effects pertain especially to the effects of ammonia on nitrification, while soil physical conditions determine soil aeration status and ensuing microbial pathways.
Funding
This article evolved from a workshop titled ‘Climate Change, Reactive Nitrogen, Food Security and Sustainable Agriculture’ held at the Karlsruhe Institute of Technology in Garmisch-Partenkirchen, Germany, on 15–16 April 2019, and which was sponsored by the OECD Co-operative Research Programme: Biological Resource Management for Sustainable Agricultural Systems whose financial support made it possible for the authors to participate in the workshop.
Conflict of interest statement
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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