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

Brief description

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.