Tony J. van der Weerden

Nitrous oxide emissions from agricultural soils in New Zealand—A review of current knowledge and directions for future research

Should international protocols be ratified, New Zealand will become legally committed to limit its greenhouse gas emissions The three major greenhouse gases are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) Agricultural soils are generally considered to be the main source of N2O emissions in New Zealand, but production estimates to date are surrounded by great uncertainty This paper reviews our current understanding of agricultural N2O emissions, and suggests directions for future research needs by evaluating the default emission factors of the 1996 Revised Guidelines for National Greenhouse Gas Inventories of the Intergovernmental Panel on Climate Change (IPCC) for the New Zealand situation The emission factors calculated for New Zealand agricultural soils are generally within the range of the 1996 IPCC default values, but the limited amount of research data available hampers a full evaluation of the appropriateness of these factors for New Zealand More long‐term studies are needed to refine our emission factors, particularly those for animal urine and dung returned to pasture Application of the IPCC methodology to New Zealand identifies herbivore excrement as the single largest potential source of anthropogenic N2O emissions (about 50% of the total emission) In addition, research is also required for indirect sources of N2O, because only limited overseas data, and none from New Zealand, are available The 1996 IPCC methodology does not account for variations in climatic and soil physical conditions, which are known to affect N2O emissions In the longer term, development of robust process‐based models, coupled with spatial and temporal data sets of the major drivers of N2O emissions, may therefore be a useful approach for obtaining national emission estimates for New Zealand This will require long‐term monitoring of N2O emissions, under various land uses and on a national network of sites

Influence of pore size distribution and soil water content on nitrous oxide emissions

Nitrous oxide (N2O) emissions from agricultural soils have been estimated to comprise about two-thirds of the biosphere’s contribution of this potent greenhouse gas. In pasture systems grazed by farmed animals, where substrate is generally available, spatial variation in emissions, in addition to that cause by the patchiness of urine deposition, has been attributed to soil aeration, as governed by gas diffusion. However, this parameter is not readily measured, and the soil’s water-filled pore space (WFPS) has often been used as a proxy, despite gas diffusion in soils depending on the volumetric fractions of water and air. With changing water content, these fractions will reflect the soil’s pore size distribution. The aims of this study were: (i) to determine if the pore size distribution of two pastoral soils explains previously observed differences in N2O emissions under field conditions, and (ii) to assess the most appropriate soil water/gas diffusion metric for estimating N2O emissions. The N2O emissions were measured from intact cores of two soils (one classified as well drained and one as poorly drained) that had been sampled to a depth of 50 mm beneath grazed pasture. Nitrogen (N, 500 kg N/ha) was applied to soil cores as aqueous nitrate solution, and the cores were drained under controlled conditions at a constant temperature. The poorly drained soil had a larger proportion of macropores (23.5 v. 18.7% in the well-drained soil), resulting in more rapid drainage and increased pore continuity, thereby reducing the duration of anaerobicity, and leading to lower N2O emissions. Emissions were related to three soil water proxies including WFPS, volumetric water content (VWC), and matric potential (MP), and to relative diffusion (RD). All parameters showed highly significant relationships with N2O emissions (P < 0.001), with RD, WFPS, VWC, and MP accounting for 59, 72, 88, and 93% of the variability, respectively. As VWC is more readily determined than MP, the former is potentially more suitable for estimating N2O emission from different soils across a range of time and space scales under field conditions.

Addition of straw or sawdust to mitigate greenhouse gas emissions from slurry produced by housed cattle: a field incubation study.

The use of housed wintering systems (e.g., barns) associated with dairy cattle farming is increasing in southern New Zealand. Typically, these wintering systems use straw or a woodmix as bedding material. Ammonia (NH) and greenhouse gas (GHG) emissions (nitrous oxide [NO] and methane [CH]) associated with storage of slurry + bedding material from wintering systems is poorly understood. A field incubation study was conducted to determine such emissions from stored slurry where bedding material (straw and sawdust) was added at two rates and stored for 7 mo. During the first 4 mo of storage, compared with untreated slurry, the addition of sawdust significantly reduced NH and CH emissions from 29 to 3% of the initial slurry nitrogen (N) content and from 0.5 to <0.01% of the initial slurry carbon (C) content. However, sawdust enhanced NO emissions to 0.7% of the initial slurry-N content, compared with <0.01% for untreated slurry. Straw generally had an intermediate effect. Extending the storage period to 7 mo increased emissions from all treatments. Ammonia emissions were inversely related to the slurry C:N ratio and total solid (TS) content, and CH emissions were inversely related to slurry TS content. Mitigation of GHG emissions from stored slurry can be achieved by reducing the storage period as much as possible after winter slurry collection, providing ground conditions allow access for land spreading and nutrient inputs match pasture requirements. Although adding bedding material can reduce GHG emissions during storage, increased manure volumes for carting and spreading need to be considered.

Preface from the editors

In December 2016, scientists, students, regional council staff, consultants, industry and other interested parties gathered in Queenstown to discuss soil and land management. The theme for discussion was ‘Soil, a Balancing Act Down Under’. As the theme suggests, this was the Joint conference of the NZ Society of Soil Science and Soil Science Australia. Over 330 papers were presented, to an audience of around 500 people. Half of the audience travelled from Australia. Our conference focused on the role of soil in our societies where a careful balance must be reached between many complex and often competing land use, productivity and environmental aspirations. We posed the question ‘what is required from soil scientists now and in the future that will enable society to reach the “balance”?’. This special issue of NZJAR contains a cross section of papers from the conference that were voted by the attendees and the science committee to be of greatest relevance to the conference topic. Included in this issue are insights from Curran-Cournane et al. and Mackay et al. on NZ land use and evaluation, plus useful perspectives of land use in Victoria, Australia, from Dahlhaus et al. and Cann et al.We learn about the importance of soil clays from Jock Churchman, the current elemental surveys in New Zealand from Rattenbury et al., and the demands on forestry soils from Clinton. Finally, but not least, Wakelin et al. take us on a mind boggling journey into the world of soil microbiology, and the opportunities that exist for land-based industries. The conference discussions drew attention to the increasing pressure placed on our soils which requires us to refine current practices and identify new approaches to the way we manage land. A key challenge for the future is to find the balancing point between food production, water quality and greenhouse gas emissions. This means not only research, but also communication of alternatives to society. Clear and concise information from the soil science community will help fuel solutions that support economic viability, environmental protection and social acceptability. Our land practitioners are well educated and utilise a number of technologies to aid their business. They are demanding information from the soil science community that will help them run productive and environmentally sound businesses. New tools and techniques are emerging that better inform crop fertiliser requirements, water placement, etc. and also soil resistance and resilience to anthropogenic and natural pressures that exacerbate soil erosion or impact on eco-system service attributes. It is important that we remain abreast of challenges and provide both a leading and supportive role to communities and industry. To visualise and tackle large complex issues, there is a growing need for soil scientists to work amongst diverse collaborative groups and maintain a strong connection with practitioners but also strengthen the community ‘connection’ to the land. Supporting our young graduates will help ensure the relevance of their research, sense of belonging in the research communities and enable them to provide valuable input and insight. The wider conference encompassed sessions on effective nutrient management, improved decision making, paedology within landscapes, water management and the fate of nitrogen, to name a few. Soil and resource management came through as a common theme throughout