Modelling soil organic carbon 2. Changes under a range of cropping and grazing farming systems in eastern Australia

The level of soil organic carbon (SOC) that is attained under agriculture largely depends upon rates of carbon input and its decomposition under various agronomic practises such as stubble (crop residue) management and fertiliser application. In this study, we used the APSIM-Wheat and APSIM-Agpasture models to simulate changes in SOC in a range of crop and pasture management systems across nine locations in eastern Australia. We explored the extent to which various crop and pasture management options affect changes in SOC from a sub-tropical to a temperate environment. Specifically, we examined how nitrogen fertilisation, stubble management and stocking rate affect SOC and what strategies might be employed by farmers to increase SOC sequestration across eastern Australia. We modelled a continuous cropping regime, a continuously grazed pasture and a mixed cropping and pasture rotation. Under continuous cropping higher nitrogen application and higher amounts of stubble incorporation increased the SOC levels at all locations. At Roma, the northern-most site, there was little additional gain in SOC from increasing N above 70kgNha^-1 whereas most other sites showed benefits above 70kgNha^-1. The biggest factor in boosting SOC under cropping was the level of stubble incorporation. At all but one site, continuously grazed pasture generally resulted in SOC increases over the 60years. However, increasing stocking rate decreased the rates of SOC changes at all sites. Crop-pasture rotations show that the impacts of even 4years of pasture is likely to be significant in reducing declining SOC at low nitrogen application during cropping phases. N fertilisation and stubble incorporation reduced the impact of stocking rate by reducing the decline in SOC. The difference in SOC changes between nine sites across eastern Australia was largely described by mean temperature and rainfall but high temperature strongly interacted with management practises (stocking rate, N application and residue incorporation) to reduce the sequestration of C despite favourable rainfall. Our results indicate that a mean annual temperature higher than about 20°C can switch a soil from net sink into a net source of atmospheric CO₂ if other factors affecting soil carbon changes such as stubble incorporation, stocking rate and site rainfall are constant.