Turnover of nitrogen from components of lupin stubble to wheat in sandy soil

Abstract

The rate of decomposition of 15N-labelled lupin (Lupinus angustifolius) stubble and the use of mineralised 15N by wheat were determined in field experiments on a deep loamy sand previously cropped to lupin. In one experiment, leaf, stem, and pod (pod-valve) components were applied separately to mini-plots that were either left unplanted or subsequently planted to wheat. In the second experiment, leaf and stem components, each of either low or high N concentration, were applied separately to mini-plots which were subsequently planted to wheat. Soil was recovered in layers to a maximum depth of 1 m and subsequently analysed for 15N in NH + 4 , NO-3 , and total N. The net mineralisation of stubble 15N was estimated from the decrease in soil organic 15N (total 15N – inorganic 15N), and the uptake of 15N by wheat was measured periodically. All treatments were characterised by the high retention of lupin stubble 15N in the soil organic matter. Between 9 and 34% of stem and pod 15N, and 19–49% of leaf 15N, was mineralised within a 10-month period. From these data the annual net mineralisation of a typical lupin stubble was estimated at 25–42 kg N/ha, an N benefit similar to that estimated from agronomic trials. Wheat uptake of lupin-stubble 15N ranged from 9 to 27%. Of the stubble components, only the leaf contained sufficient quantities of mineralisable N to be an important source of N for wheat. At wheat maturity in the first experiment, losses of stubble 15N ranged from 13% (leaf) to 7% (stem). In the second experiment, losses of 15N were only observed from the high N treatments (leaf 8%, stem 15·5%). Stubble component chemistry appeared to affect net mineralisation and plant uptake differently. Across both experiments, annual net mineralisation best correlated (R = 0·69) with the N concentration of the stubble components. Wheat N uptake was strongly positively correlated with polysaccharide content (R = 0·89) but negatively correlated with lignin content (R = – 0·79). Although large quantities (58 and 98 kg N/ha) of soil-derived inorganic N were found in the root-zone (–1·0 m) of wheat sown after lupins, and attributed to the decomposition of lupin root systems and surface residues prior to the establishment of each experiment, it is concluded that the short-term decomposition of lupin stubble 15N results in a modest release of inorganic N. Consequently, the primary value of lupin stubble in the N economy of lupin : cereal rotations is to replenish the soil organic N reserve.