Some clarification of the impacts of grassland intensification on food production, nitrogen release, greenhouse gas emissions and carbon sequestration: using the example of New Zealand
Anthony J. Parsons, John H. M. Thornley, Susanne Rasmussen and Jacqueline S. Rowarth, 2016, Some clarification of the impacts of grassland intensification on food production, nitrogen release, greenhouse gas emissions and carbon sequestration: using the example of New Zealand, CAB Reviews 2016 11, No. 054
We used an established, process-based model of the dynamics of carbon (C) and nitrogen (N) cycling between plants, soils and animals in grazed temperate pastures to clarify expectations of how some major components of intensification affect the outcomes (short-term and long-term) of alternative systems for food production and environmental impact. We use the example of New Zealand, due to its clear recent history of intensification, the level of concern and hence nature of research undertaken there. A transition from low-input drystock, to dairy (lactation) systems with higher N inputs in fertilizer, and/or C and N inputs in supplements, reveals how at the same intermediate N input, food yield (per ha) can be doubled, while environmental N release halved, with minimal impact on C sequestration. We stress the sources of sustained changes in N release (e.g. nitrate/ nitrous oxide) are altered inputs (fertilizer/supplements) and less so animal numbers in response to these. Much of the increased efficiency is due to ‘improved’ N partitioning in lactating (cf. dry) animals. A reversion to dry-stock (or ‘de-stocking’) therefore offers greater environmental challenge, unless N inputs decline accordingly. Responses to supplements (being a source of C) re-inforce how the driving limitation to the grazed ecosystem is C capture per ha, and we highlight the need for a renewed focus on fundamental research on plant C uptake per ha per unit of N input. We offer a graphical method for visualizing the outcomes of options, and their trade-offs, with implications for policy and future research direction.
Keywords: Intensification, Carbon sequestration, Nitrogen use efficiency, Supplements, Dairy production
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