Quantifying nitrous oxide losses and nitrogen use efficiency in grains cropping systems on clay soils with contrasting soil carbon status and land management. Kingsthorpe, Queensland, 2013 - 2014

Fertiliser application is the largest single variable expense for grain growers when producing a crop, with Nitrogen inputs by far the largest nutrient input in the northern grains region. Despite this, growers are consistently running negative N budgets from a nutrient balance perspective, largely in response to uncertain yield targets due to variable seasonal conditions and limited opportunity to manage N by in-season inputs in situations where rainfall is unpredictable. These negative N budgets further erode soil reserves and will result in higher fertilizer N demand in future, although growers are also being faced with increased need to supply other nutrients (P, K and S) in the fertilizer program. There is therefore a clear need to accurately predict N demands, and to maximise the efficient use of soil and fertilizer N reserves to produce grain, freeing up cash to meet other nutrient demands while also minimizing environmental impacts. Soil testing information is one of the key factors needed to identify nutrient limits to productivity and subsequently devise a fertilizer program. However, without calibrated soil test – yield relationships that are robust enough to quantify likely yield response to added fertilizer, farm managers and advisors are not able to make fertilizer decisions that will optimize productivity, nutrient use efficiency or profitability. This uncertainty can result in under-application and further erosion of nutrient reserves, as well as declining productivity and water use efficiency, or alternately over-application with reduced N use efficiency and profitability. The latter situation greatly increases the environmental risks of off-site impacts due to relative mobility of N, with nitrogen leaching and gaseous loss of nitrous oxide (a potent greenhouse gas) common loss pathways. The latter is of particular concern in clay soils that characterise the northern cropping region, due to the combination of poor internal drainage and extended fallowing to allow build-up of soil water and mineral N reserves prior to crop establishment. It is also unclear how desired increases in soil carbon in response to climate change will influence nitrogen fertiliser demands in these cropping systems, as well as the potential for nitrous oxide emissions. The interaction between soil carbon and nitrogen fertiliser requirements will be pivotal in any assessment of the effectiveness of soil carbon sequestration to abate greenhouse gas emissions from grain production. The national database "Making Better Fertiliser Decisions for Crops" has identified some significant gaps in soil test-crop response relationships for major crops in the northern region, with the only crop with a reasonable quantity of data defining fertilizer N response being wheat. The coverage for sorghum and canola are the next most prominent, but these relationships are currently of limited value due to inconsistencies with soil testing procedures and limitations with accompanying measurements like soil C (to explain variation in-season N mineralization). The purpose of this project is to address the gaps for summer sorghum, and expand the database for canola in the southern parts of the region. It will have trials spread from northeast and northwest NSW to southern and central Qld, with locations based on rotating clusters of trials through different soils and districts. These regional trials will complement work conducted at core sites in Qld, where detailed studies of soil N dynamics, environmental losses (gaseous and leaching) and crop N use efficiency will be conducted.

肥料施用是谷物种植者进行作物生产时最大的单项可变支出，而氮素（Nitrogen）投入是北部谷物产区最主要的养分输入项。尽管如此，从养分平衡的视角来看，种植者始终面临负氮素预算的问题，这在很大程度上源于季节条件多变导致产量目标不确定，且在降雨不可预测的情况下，难以通过季内追肥来调控氮素水平。这类负氮素预算会进一步消耗土壤养分储量，并将导致未来对氮肥的需求攀升；与此同时，种植者还需要在肥料方案中补充其他养分（磷、钾和硫）。因此，精准预测氮素需求、最大化利用土壤与肥料氮素储量以生产谷物的需求迫在眉睫——此举既能释放资金以满足其他养分需求，又能最大限度降低环境影响。
土壤测试信息是明确生产力受哪些养分限制，并据此制定肥料方案的关键因素之一。但若缺乏经过校正、足以量化肥料添加后潜在产量响应的可靠土壤测试-产量关系，农场管理者与农技顾问便无法做出能够优化生产力、养分利用效率或经济效益的肥料施用决策。这种不确定性可能导致氮肥施用量不足，进一步加剧养分储量消耗，同时导致生产力与水分利用效率下降；或是反之，施用量过高则会降低氮素利用效率与经济效益。
鉴于氮素具有相对较强的移动性，后者（施用量过高）会大幅增加氮素淋失与一氧化二氮（nitrous oxide，一种强效温室气体（greenhouse gas））气态挥发这类常见流失途径带来的非点源环境风险。这种情况在北部种植区典型的黏质土壤中尤其令人担忧，因为这类土壤内部排水性差，且为了在作物播种前积累土壤水分与矿质氮储量，往往需要进行长时间休耕。
此外，在气候变化背景下，人们期望通过提升土壤碳（soil carbon）含量来改善这些种植系统，但目前尚不清楚这将如何影响氮素肥料需求，以及一氧化二氮排放的潜在变化。土壤碳与氮肥需求之间的相互作用，将是评估土壤碳封存以缓解谷物生产温室气体排放有效性的核心因素。
国家数据库"Making Better Fertiliser Decisions for Crops"已明确，北部产区主要作物的土壤测试-作物响应关系存在显著缺口：目前仅有小麦拥有足够数量的数据来界定氮肥施用响应关系。高粱与油菜的数据覆盖量紧随其后，但由于土壤测试程序不一致，以及配套的土壤碳（用于解释季内氮素矿化变化）测量存在局限，当前这些关系的参考价值有限。
本项目旨在填补夏播高粱的相关数据缺口，并扩大该产区南部油菜的数据库覆盖范围。试验将分布在从新南威尔士州（New South Wales, NSW）东北部、西北部到昆士兰州（Queensland, Qld）南部与中部的区域，试验点位将根据不同土壤与产区的轮换试验集群进行设置。这些区域试验将与昆士兰州核心试验点的研究形成互补，后者将开展土壤氮素动态、环境损失（气态挥发与淋失）以及作物氮素利用效率的详细研究。