Effect of nitrogen fertilizer on rice photosynthate allocation and carbon input in paddy soil

The photosynthate carbon (C) released in the rhizosphere plays a crucial role in C sequestration, microbial activities and nutrient availability in soil. Nitrogen (N) fertilization modifies the allocation and dynamics of photosynthates in paddy rice systems, but these effects depend on plant growth stages. Rice ( Oryza sativa L.) plants were pulse labelled with 13 CO 2 at the tillering, elongation, heading and grain‐filling stages with 0 and 225 kg N ha −1 fertilizer. The plants and soil were sampled shortly after each pulse labelling and at harvest. Relative 13 C (as % of assimilated C) in the roots and rhizosphere soil was largest at the early growth stage (tillering) and subsequently decreased. At harvest, 68% of the rhizodeposited C remained in bulk soil without N fertilizer, which corresponded to 6.2% of the net assimilated 13 C. The absolute amount of net belowground C input (root + rhizodeposition) by rice was 268 and 468 kg C ha −1 under 0 and 225 kg N ha −1 fertilizer, of which rhizodeposition accounted for 60 and 40%, respectively. We concluded that N fertilization raised the belowground C input by rice mainly by increasing root biomass rather than by rhizodeposition. Highlights Rice photosynthesis‐derived carbon (C) was quantified in soil by multiple pulse labelling with 13 CO 2 Young rice plants allocated more assimilates into the soil compared to mature plants Nitrogen deficiency led to greater C retention in bulk soil than in the rhizosphere Nitrogen fertilization increased the net belowground C input mainly with larger root biomass