Potential impacts of climate change on soil organic carbon and productivity in pastures of south eastern Australia

Increasing soil organic carbon (SOC) stocks is an often-mentioned option to mitigate greenhouse gas emissions. However, increasing carbon stocks in agricultural soils is difficult and the ability of soils to store carbon as the climate changes is uncertain. This is due to many interacting factors, including those that vary spatially, contributing to organic matter inputs and decomposition rates. We used two models, the Sustainable Grazing Systems whole-farm system model (SGS) and the RothC soil carbon model, to investigate the potential impacts of climate change on SOC stocks in pastures in a temperate, winter-dominant rainfall region of south eastern Australia. A wide range of possible future climates were simulated from 2017 to 2090 at two sites, each with two soil types. Results demonstrate that projected rainfall, the factor with the most variability between climate scenarios, was the primary source of uncertainty in SOC response. Dry climate projections resulted in lower SOC content than other scenarios. The two models were similar in their projected trends, but the RothC model generally gave larger percent changes in soil carbon over the simulation period and a larger range of responses due to changes in site characteristics, particularly clay content. Sustainable stocking rates were determined by the whole-farm system model based on climate, pasture production, and maintaining minimum dry matter coverage. Wet future climates lead to increased production that supported increases in stocking and increases in SOC stocks. While soil carbon accumulated at slower rates or remained steady under dry projections, lower production meant this was accompanied by decreased average 2070–2090 stocking rates, which approached zero by 2090 on the low-rainfall site. This highlights an important interaction between SOC and grazing management. The results demonstrate the extent of the uncertainty associated with soil carbon trading for farmers and the need for adaptation options that allow farms to remain sustainable and productive as the climate changes.