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 13CO2 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 13C (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 13C. 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 13CO2 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. © 2019 British Society of Soil Science