Managing wheat stubble as an effective approach to sequester soil carbon in a semi-arid environment: Spatial modelling

Abstract

Attention to farm management practices that enhance soil organic carbon (SOC) stock is increasing because of the latter's importance of soil fertility, crop production and the global carbon cycle. Sequestering atmospheric carbon dioxide (CO2) as SOC has potential feedback to climate change. Spatial modelling of the effects of wheat stubble incorporation by tillage on SOC storage was studied in a semi-arid rainfed wheat cropping system, using the Agricultural Production Systems Simulator (APSIM). The model was validated against a long-term (1979–2004) experiment and yielded a Ratio of Performance to Deviation (RPD) of 1.6 and R2 of 0.63, indicating a moderate accuracy in predicting SOC turnover. In the Liverpool Plains and the southern slopes of NSW, SOC at top 30cm layer is in a higher range of 40–50tha−1, while from the southern west plains to the northern plains extending to the New England tablelands, SOC is in a lower range of 25–35tha−1. It is notable that SOC in the central slopes is also in the lower range of 25–35tha−1. There is large simulated variation to changes in SOC from stubble management under the current climate that ranges from 0 to –200kgha−1year−1 when 100% of the wheat stubble is removed. When 100% of the wheat stubble is 100% incorporated, the changes in SOC become positive, from 0 to 200kgha−1year−1. The SOC change patterns associated with the rates of wheat stubble incorporation are similar under a projected future climate. However, as future temperatures rise, less SOC will be sequestered. For example, under the 100% removal of wheat stubble, the averaged SOC is decreased by 126±40kgha−1yr−1 under the current climate, while under the 18 GCM projected climate (2049–2098), the reduction is 135±15kgha−1yr−1. In contrast, when 100% wheat stubble is incorporated into the soil, the averaged SOC is increased by 100±34kgha−1yr−1 under the current climate, while under the 18 GCM projected climate, the averaged SOC is increased by 80±23kgha−1yr−1. To maintain the current level of SOC in the south-western wheat growing region (lower rainfall) of the state 20–40% wheat stubble is required to be incorporated into soil, compared to that in the north-eastern area (high rainfall), where the rate is about 40–60%. Across the actual wheat growing area in NSW, the decreased SOC with the 100% removal of wheat stubble results in 3.90±1.23Mt CO2 emissions per year under the current climate. Under the 18 GCM projected climate, the mean emission per year is 4.06±0.50Mt CO2 if 100% wheat stubble is removed from field. In contrast, when 100% wheat stubble is incorporated into soil, the amount of increased SOC will reduce the atmospheric CO2 emissions by 3.29±1.11Mtyr−1 under the current climate or by the mean of 2.68±0.77Mtyr−1 under the GCM projected climate. There is a clear trend to theoretically decrease CO2 emissions with the increased incorporation of wheat stubble.