Short-Term Impacts of Tillage Practices on Soil Carbon Profiling Under a Dryland Farming System.

Short-term impacts of tillage practices on soil carbon profiling under a Dryland farming system

Promil Mehra¹ Jack Desbiolles², Binoy Sarkar¹, Nanthi Bolan^1,3,4, Risha Gupta⁵

¹ Centre for Environmental Risk Assessment and Remediation, University of South Australia, Mawson Lakes,   South Australia 5095, Australia, ² Barbara Hardy Institute, University of South Australia, Mawson Lakes, South Australia 5095, Australia, ³ Cooperative Research Centre for Contamination Assessment and Remediation of the Environment, PO Box 486, Salisbury, South Australia 5106; ⁴ Global Centre for Environmental Risk Assessment, University of New Castle, NSW, Australia ⁵ Phonomics and Bioinformatics Research Centre, School of Information and Mathematical Sciences

Cropping management practices through tillage and crop residue retention can affect the sequestration potential of carbon in soils under rainfed systems. While most studies have focussed on such comparatively easily predictable long term impacts, lesser information is available on the profiling of soil carbon (C) at the onset of cropping system change. This short-term impact might be small but significant in terms of calculating the soil C budget under an individual cropping system. This study evaluated the short-term impact (Year 1, Year 2) of wheat cropping under no-tillage (NT) relative to conventional tillage (CT) with three application rates (0, 2.5,5 t/ha) of cereal (barley – first year, wheat – second year) straw mulch on the status of C levels: total carbon (TC), total organic carbon (TOC), and C fractions including coarse particulate organic carbon (cPOC), fine POC (fPOC), mineral associated organic carbon (MAOC) within two soil layers: i) shallow layer (0-2.5, 2.5-5, 5-10cm), and ii) deep layer (0-10, 10-20, and 20-30 cm). Irrespective of the tillage treatments, shallow layers had significant effect (P<0.001) on the net TC and TOC (14% and 5%, respectively) than deep layers. On the contrary, within the shallow layers, TC and TOC values were significantly higher (P<0.001) under NT (TC: 37%, 13%, 14%) and (TOC: -0.2%, 9%, 9%) than CT regardless of the mulching treatments. To elucidate the short-term impact further, different soil C fractions were measured in the shallow layers. In comparison to CT, the amount of different C fractions (cPOC, fPOC, MAOC) were significantly different (P<0.05) in the shallow layer under NT treatment (12%, -22%, 8%) and (33%, 15%, -4%) respectively in 0 and 5 t/ha mulching treatment. Similarly, the amount of hot-water extractable C (HWC), which is considered to be easily decomposable or labile C, in the shallow layer of NT treatment were two folds greater than CT system, which indicated that less disturbance (NT) and mulching enabled the build-up of  labile organic C compounds in soil. In comparison to the deep layer, the increase in net carbon stock in the shallow layer (18.4%, 13.9%, and -1.7%, respectively in three sub-depths without mulch) under NT system signified the stratification of more C in the shallow layer than CT treatment. Therefore, soil C budgeting study under short-term rainfed cropping system should carefully consider the C stock and fractions in the shallow depth range along with the deeper depth.