Labile organic carbon fractions in the rhizosphere contribute to nitrogen and phosphorus uptake in rice under long-term crop rotations and nitrogen application

The turnover of labile organic carbon (C) in the rhizosphere is rapid and affects the soil nutrient supply capacity. However, the mechanism by which rhizosphere labile organic C influences nutrient uptake is still unclear. The rhizosphere soils of summer rice among four upland-paddy rotation treatments of rice-winter fallow (RF), rice-potato with rice straw mulch (RP), rice-wheat (RW), and rice-Chinese milk vetch (RC) were characterized, and plant nitrogen (N) and phosphorus (P) uptake were quantified at the various growth stages. The results showed that labile organic C fractions differed at various growth stages. Readily oxidizable C was higher prior to than post the flowering stage; dissolved organic C was 118–151 % higher at the flowering stage than at the other stages, while microbial biomass C gradually decreased over the growth period. In addition, rotations with cover crops in winter crops and the N fertilizer input were conducive to increasing labile organic C and nutrient supply, and enzyme activities generally in the order of RP > RC > RW > RF. The RP treatment increased labile organic C fractions by 24–76 % and amount of available N and P in soil by 48–160 %. The effects of rotation and N treatments on labile organic C, N and P were associated with changes in the structure and composition of bacterial communities (RDA = 63–81 %). The structural equation model showed that the effects of rotation and N treatments on plant N and P uptake were partially regulated by labile organic C fractions (R2 = 0.6–0.9) which in turn correlated with soil nutrient supply and enzyme activities. The concentrations of readily oxidizable C and microbial biomass C at early growth stages, as well as particulate and dissolved organic C in the late growth stages, showed promise as indicators for optimizing rotations and N application in the field. Future research should focus on validation of these indicators across a wider range of soil types and climatic conditions, and the correlation between these indicators and crop productivity should be investigated for further understanding the potential impacts on agricultural sustainability and soil management.