Nitrogen cycling in soil under grass-clover pasture: Influence of long-term inputs of superphosphate on N mineralisation

Abstract

Phosphorus (P) is central to the productivity of grass-clover pastoral farming systems. Fertiliser P promotes the growth of clover, which provides an input of fixed N to complement that supplied from the soil (primarily mineralised N). If pasture productivity is limited by P availability, organic matter returns to the soil in excreta and plant residues will decline, which in turn may reduce the supply of N by mineralisation. We examined the effect of long-term P application to grass-clover pasture on mineralisation of N (and C). Net N mineralisation was measured in a 14-week aerobic incubation (25 °C; soil maintained at field capacity) using soils (0–15 cm depth) from a long-term (1952–2016) trial set up to quantify effects of single superphosphate (0, 188, or 376 kg−1 year−1) on productivity of irrigated, sheep-grazed pasture (no fertiliser N was applied during the trial). Although P fertilisation had only a small effect (∼10% increase) on total soil N, net N mineralisation was substantially increased (N mineralised from fertilised soil in 14 weeks was ∼1.6 times that from the unfertilised Control). In contrast, mineralisation of C was slightly greater in the Control than in fertilised soil. Nitrogen mineralisation exhibited a Mitscherlich-type relationship with available soil P, measured using the Olsen test; near-maximum mineralisation was observed at an Olsen test value of ∼10 mg P kg−1 soil. Annual in-field N mineralisation was estimated by modifying the laboratory-measured “basal” mineralisation values using temperature and moisture adjustment factors (soil temperature and moisture data were acquired from an adjacent, irrigated trial). The results confirmed that N mineralisation was the predominant source of available N in the unfertilised Control (∼160 kg ha−1 yr−1 vs ∼30 kg ha−1 yr−1 of fixed N) and that it was an important source of N for the additional dry matter grown in response to P application (N uptake increased by ∼200 kg ha−1 yr−1 in response to P fertilisation vs an increase in mineralisation of ∼100 kg ha−1 yr−1). The increase in net N mineralisation in fertilised soil was partly because immobilisation of N was less than in the unfertilised Control.