Divergent effects of legume cover crops on microbial necromass accumulation at surface versus subsurface soils in subtropical paddy fields.

Legume cover crops are widely used to increase soil organic carbon (SOC) sequestration in agricultural ecosystems. Microbial necromass play important roles in SOC buildup and sequestration. However, how legume cover crops impact microbial necromass accumulation and its contribution to SOC remain poorly understood in paddy soils, particularly in deeper subsurface soils. Here, we conducted a three-year field experiment to evaluate the effect of different legume cover crops (milk vetch (RM) and hairy vetch (RH)) on microbial necromass accumulation at three depth increments down to 60 cm. We used amino sugar as microbial necromass biomarker. Our results showed that legume cover crops increased microbial necromass accumulation in surface layer (0-20 cm) by 1.4 % and 20.8 % in RH and RM treatments, respectively. However, this positive effect diminished with soil depth and became negative in the 40-60 cm layer, with 22.8 % and 48.0 % lower in RH and RM treatments than fallow control, respectively, possibly due to intensified N limitation in subsurface soils. This was supported by the observation that cover crops combined with N fertilizer promoted greater necromass accumulation at both 20-40 cm and 40-60 cm layers as compared with no-fertilizer cover crop treatment. Our findings demonstrate that cover crops exerted contrasting effect on microbial necromass accumulation along soil depth with stronger depletion in deeper subsurface soils. Our work underscores the need for full considering microbial necromass and SOC changes throughout a certain soil depth when evaluating the potential of crop management practices to optimize soil C sequestration.