Does rice paddy management increase soil organic carbon in the warm temperate and tropical regions?

Abstract

Enhancing soil organic carbon (SOC) storage in agricultural fields is an effective option for climate change mitigation and food security. In temperate regions, rice farming under paddy is known to increase SOC stock due mainly to the reduced decomposition caused by prolonged submerged conditions compared to the cropping practices under non-flooded conditions. It remains unclear, however, if and to what extent rice paddy management increases SOC storage in warmer climate regions where decomposition is much faster. We thus examined the factors controlling the SOC stock and the net change in SOC normalized to the duration under paddy (ΔSOC) at paddy fields in warm temperate (21 sites) and tropical climatic regions (11 sites) from peer-reviewed literature which was concentrated to Asia. Using collected data (e.g., SOC content, bulk density, mean annual temperature (MAT), precipitation, management duration, soil management, crop rotation), we evaluated predictor variables that account for ΔSOC variability (i.e., mean decrease accuracy) by random forest regression. The mean paddy duration for all data (n = 170) was 22 years. Average SOC stock and ΔSOC at 0–20 cm depth were 38 ± 1.45SE Mg C ha−1 and 0.34 ± 0.04SE Mg C ha−1 yr−1, respectively, indicating that continuous paddy management had a positive effect on SOC storage in both climatic regions. SOC stock was two-fold higher and ΔSOC was four-fold higher in the warm temperate region compared to the tropical region (p < 0.05). The best predictor variables for ΔSOC were MAT followed by soil management and then paddy duration. While organic amendment including both straw and manure increased ΔSOC in both warm temperate and tropical paddies, we did not detect a positive effect of chemical fertilizer application alone or crop rotation type (e.g., rice-upland vs rice-rice). Despite high variability, ΔSOC in the warm temperate region significantly declined over time from 0.82 ± 0.10 Mg C ha−1 yr−1 (< 20 yrs) to 0.22 ± 0.06 Mg C ha−1 yr−1. While SOC stock was 50% lower, the tropical paddies showed no clear decline in ΔSOC up to 41 years (0.04–0.21 Mg C ha−1 yr−1). These results suggest that SOC accretion might have reached equilibrium after 20 years of paddy practice in the warm temperate region and, if so, ΔSOC is likely to diminish to zero in the near future. In tropical paddy fields, on the other hand, slow SOC build-up may continue for a longer term although the ΔSOC value itself is not high. While the current study showed the efficacy of paddy management on SOC build-up under the warmer climate regions, further study on the rate and qualitative changes is necessary to evaluate the trade-offs between soil fertility improvement (e.g., soil N build-up) and methane emission.