Quantifying nitrous oxide losses and nitrogen use efficiency in grains cropping systems on clay soils with contrasting soil carbon status and land management. Kingaroy, 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作物生产环节中最大的单项可变支出，而氮素投入更是北部谷物种植区占比最高的养分输入类型。尽管如此，从养分平衡的视角来看，种植者始终面临氮素预算赤字的问题——这主要源于多变的季节条件导致产量目标难以确定，且在降雨不可预测的情况下，通过生育期内追肥调控氮素的操作空间十分有限。这类氮素预算赤字会进一步消耗土壤养分储量，并导致未来氮肥需求量上升；与此同时，种植者在肥料方案中还需要额外供给磷（P）、钾（K）和硫（S）等其他养分，需求压力也在增加。因此，精准预测氮素需求、最大化土壤与肥料氮素储量在谷物生产中的利用效率，从而腾出现金以满足其他养分需求，同时降低环境影响，已成为明确的迫切需求。土壤测试（soil testing）信息是识别生产力养分限制因子、进而制定合理肥料方案的核心依据之一。然而，若缺乏足够稳健的校准后土壤测试-产量响应关系，无法量化施肥后的潜在产量变化，农场管理者与农技顾问便无法制定可优化生产力、养分利用效率与经济效益的肥料施用决策。这种不确定性可能导致施肥不足，进一步加剧养分储量消耗，同时造成生产力与水分利用效率下降；或是出现施肥过量的情况，降低氮素利用效率与经济效益。施肥过量的情况会因氮素的相对迁移性大幅增加场外环境影响风险，而氮淋溶与一氧化二氮（nitrous oxide，一种强温室气体）气态排放是常见的氮素流失途径。这种情况在北部种植区的典型土壤——黏质土壤中尤为令人担忧，因为该类土壤内部排水性差，且为在作物定植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，为应对气候变化而预期提升土壤碳储量的举措，会如何影响这类种植系统的氮肥需求，以及一氧化二氮的排放潜力，目前仍不明确。土壤碳与氮肥需求之间的相互作用，将是评估土壤碳固存减缓谷物生产温室气体排放有效性的核心关键。国家数据库《Making Better Fertiliser Decisions for Crops》已发现，北部区域主栽作物的土壤测试-作物响应关系存在显著缺口，目前仅有小麦拥有可明确其氮肥响应的充足数据。高粱与油菜（canola）的数据覆盖范围紧随其后，但由于土壤测试流程不统一，且配套测定项目（如用于解释生育期内氮矿化变化的土壤C）存在局限，目前这类关系的应用价值十分有限。本项目旨在填补夏播高粱的相关缺口，并扩大该区域南部油菜的数据库规模。本项目的试验将覆盖从新南威尔士州（NSW）东北部、西北部到昆士兰州（Qld）南部与中部的区域，试验点位将通过轮换试验集群的方式，覆盖不同土壤类型与种植区域。这类区域试验将与昆士兰州（Qld）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（气态与淋溶损失）以及作物氮素利用效率的详细研究。