Nutrient Balances for Australian Natural Resource Management Zones

This research continues as part of the on-going activities in Australia. Using data from the International Fertilizer Association (IFA) fertilizer use by crop statistics and production and land use data from the Food and Agriculture Organization (FAO), the partial nutrient balances for N, P and K for Australian cereal, oilseed and pulses were published through the Global Partnership in Nutrient Management in 2015. However, none of these estimates includes biologically fixed N as a component of the nutrient balance, as they compare N fertilizer use with N removed in product. For a legume crop or pasture, the N fixation adds additional N to the budget that should be accounted for in nutrient budgets. In Australia, pulse crops such as field peas, faba beans, chickpea, lentil, and lupin are commonly grown in rotation with cereal and oilseed crops, and 1.8 M ha of pulses grown, which is approximately 8% of annual crop area sown. Inclusion of ways to estimate pulse N in the balance is important to determine the true balance for these rainfed farming systems.

A review of literature was undertaken and the amount of biologically N fixed can be estimated by the grain yield, the harvest index (which combined indicates peak biomass), root N fixed (estimated from shoot N fixed), shoot N concentration and the % N derived from the atmosphere. Factors of these for each species were taken from the literature to estimate fixed N based on the yield of the crop. The N balance is then calculated as the fixed N less that amount removed in the grain. Using these data, the net N fixation estimates, based on a 0.3 Harvest Index, were 24 kg N/t (chickpea), 52 kg N/t (fababean), 40 kg N/t (field pea), 31 kg N/t (lentil), 27 kg N/t (lupin), and 78 kg N/t (vetch).

Techniques are now being developed to include N fixation estimates for cover crops and crops that are ploughed into the soil as a green manure. These data are to be included in method for growers to estimate nutrient balances on their farms over time.