Soil organic carbon dynamics under short-term conservation agriculture cropping systems in Cambodia

Conservation agriculture (CA) constitutes a potential set of management practices to restore soil total N (STN), soil organic C (SOC) and its labile fractions (i.e., particulate organic C-POC, hotwater extractable organic C-HWEOC, permanganate oxidizable C-POXC), to increase soil enzyme activities and to enhance soil aggregation. After five years, CA averagely increased SOC stocks over CT at 0-5 cm by 10%, 20% and 18%, STN stock by 8%, 25% and 16% , POC stocks by 22%, 20% and 78%, HWEOC stocks by 61%, 55% and 53%, and POXC stocks by 23%, 21% and 32% for rice-, soybean- and cassava-based cropping systems (RcCS, SbCS and CsCS, respectively). In general, no noticeable changes in the subsoil layers were observed. When monitoring after three years, stocks of SOC fractions (i.e., mineral-associated organic C-MAOC, pyrophosphate extractable organic C-PEOC, chemically stabilized organic C-CSOC) were almost constant in each depth among land uses, except MAOC in SbCS and PEOC in CsCS at 0- 5 cm where CA showed significant effects. In contrast, ?-glucosidase activity was 18%, 28% and 49% greater in CA than in CT soils at 0-5 cm under RcCS, SbCS, CsCS, respectively, whereas arylsulfatase activity under CA was greater than CT by 36% in SbCS and 39% in CsCS. The proportions of large macroaggregates (8-19 mm) at 0-5 cm under CA averagely increased 23%, 39% and 53% in RcCS, SbCS and CsCS, respectively, and consequently increased soil aggregation indices (i.e., mean weight diameter-MWD, mean geometric diameter-MGD and aggregate stability index-ASI) compared with those under CT. On average, and across all aggregate size classes, CA accumulated SOC concentrations over CT by 11%, 7% and 6%, total N concentrations by 3%, 11% and 15% and POXC concentrations by 18%, 20% and 15% for RcCS, SbCS and CsCS, respectively, at 0-5 cm. These increases led to positive correlations between large macroaggregate-associated SOC and soil aggregation indices in 0-5 cm depth in the three cropping systems. The results of CP-MAS 13C NRM measurement showed that humic acid from soils under CT tended to have higher proportions of aliphatic C than under CT while in reverse for aromatic C. This supports the promotion of CMI under CA indicating greater lability of SOC. In conclusion, short-term CA practices in the three cropping systems increased the storage of STN, SOC and labile SOC pool and enhanced soil enzyme activities in the surface soils with potential effects in the subsoil layers through increased proportion of large macroaggregates and soil aggregation indices resulting from high and diversified biomass-C inputs and the absence of physical soil disruption. (Résumé d'auteur)