Impact of agricultural management practices on the nutrient supply potential of soil organic matter under long-term farming systems

Soil organic matter (SOM) has the potential to supply substantial quantities of nutrients [i.e nitrogen (N), phosphorus (P) and sulphur (S)] for plant uptake. Yet there is little understanding of the impact of management on the nutrient supply potential in soils (particularly, P and S). To quantify N, P and S availability from SOM, surface soils (0–10 cm) were collected from 14 management practices across three long-term (16–46 years) experimental sites under semi-arid (Luvisol), Mediterranean (Luvisol) and sub-tropical (Vertisol) environments in Australia. The practices comprised conventional (CT) and reduced tillage (RT) with mixed farming, no-till with continuous cropping (NT), and perennial pasture (PP) in the semi-arid Luvisol, while in a Mediterranean direct-drilled continuous cropping system, stubble was either retained (SR) or burnt (SB). Practices on the Vertisol comprised a factorial combination of CT, NT, SR, SB with either 0 (0N) or 90 kg urea-N ha⁻¹ (90N) in a continuous cropping system. Soils were incubated under controlled soil moisture and temperature, and cumulative organic C mineralised (C_min), and net available N, P and S were measured over 126 days. In the semi-arid Luvisol, CT and/or RT showed significantly higher C_min and net available N, P and S than NT and PP. In the Mediterranean Luvisol, C_min and net available P were not influenced by stubble management. In the Vertisol, CTSR (cf. CT-SB and NT-SR/SB) with or without N fertilisation significantly increased C_min, and CT-SR and/or -SB with N fertilisation (cf. CT-SR/SB without N fertilisation and NT-SR and/or -SB with or without N fertilisation) significantly increased net available N and P. This study found a continuous release of net available N (11–49 kg N ha⁻¹ over 126 days) across all management practices, whereas, the release of available P and S was evident only during the first 30 days (6–74 kg P ha⁻¹ , −4 to 22 kg S ha⁻¹ ), after which microbial immobilisation or clay fixation of P and S predominated, particularly in the Vertisol. In conclusion, the results indicate that SOM is a ready source of plant available P and S (in addition to N), and tillage and stubble retention generally enhanced SOM mineralisation and nutrient release, which varied with soil type.