Nitrogen fixation in summer-grown soybean crops and fate of fixed-N over a winter fallow in subtropical sugarcane systems

Abstract

Legumes including soybeans (Glycine max L.) can provide substantial nitrogen (N) inputs into cropping systems when grown as a part of a rotation. However, in the wet subtropics where land is fallowed for 4–6 months after soybean crops before planting of sugarcane (Saccharum L. spp. hybrids), climatic conditions over winter can be conducive to rapid mineralisation of N from residues with consequent N losses through nitrate leaching or denitrification processes. Using 15N natural abundance methodology, we estimated N2 fixation in 12 summer-grown soybean crops in the Australian wet subtropics, and tracked the fate of soybean residue-N from brown manure crops (residue from plants at late pod-filling left on the soil surface) using 15N-labelled residue in three of these fields over the winter fallow period. Disregarding two poor crops, N2 fixation ranged from 100–290 kg N ha–1 in shoots at mid pod-filling, equating to 170–468 kg N ha–1 including estimated root N contributions. Following the winter fallow, 61 and 68% of soybean residue-N was recovered in clay and peat soils respectively, to 0.9 m depth at one location (Coraki) but only 55% of residue-N could be accounted for to 0.9 m depth in a sandy soil at another location (Ballina). In addition, around 20% of the recovered 15N at this site was located at 0.3–0.6 m depth in the soil profile. Our results indicate that substantial loss of soybean residue-N can occur during winter fallows in the wet subtropics, suggesting that winter cover crops may be necessary to retain N in fields and minimise losses to the environment.