Sensitivity of Labile Soil Organic Carbon Pools to Short-Term Conservation Agriculture Production Systems in Cambodia.

Conservation agriculture (CA) constitutes an effective tool to increase soil C sequestration although rigorous empirical evidence from Southeast Asia is still limited. The objective of this study was to quantify short-term impacts of CA (5 years) on labile SOC pools in a Cambodian Latosol under rice-, soybean- and cassava-based cropping systems (RCS, SCS and CCS, respectively). The field trials were initiated in 2009 and there were four treatments in each cropping system consisting of (i) conventional tillage (CT); (ii) no-till (NT) systems (one year frequency pattern of main crops); and (iii) and (iv) NT systems (bi-annual rotations of main crops with maize). The tillage and crop rotations had significant on SOC and N stocks in SCS and CCS and an increasing trend was observed in RCS. On average, NT increased SOC stocks over CT at the 0-5 cm depth by 10%, 20% and 18% and N stocks by 8%, 25% and 16% for RCS, SCS and CCS, respectively. Lower SOC stocks in the subsoil layers under NT than CT were consistently observed in the three cropping systems. Similarly, greater hot-water extractable C (HWEOC) stocks of 61%, 55% and 53%, and permanganate oxidizable C (POXC) stocks of 23%, 21% and 32% in NT than in CT soils under RCS, SCS and CCS, respectively, at 0-5 cm soil layer were apparent. When comparing to reference vegetation (RV), the order of SOC, HWEOC and POXC levels were RV > NT > CT at 0-5 cm in the three cropping systems. In conclusion, our results corroborate the concept that short-term CA impacts on SOC dynamics are detected in the topsoil layer and the potential to increase SOC in the subsoil layers when deep-rooting cover crops are included in the crop rotations. The labile SOC pools (i.e. HWEOC and POXC) could be served as sensitive indicators in short-term changes in agricultural management practices.