Nitrogen fertilization alters the distribution and fates of photosynthesized carbon in rice–soil systems: a 13C-CO2 pulse labeling study

Abstract

Although nitrogen (N) fertilization is widely used to increase rice yield, its impact on the distribution, transformation, and fates of photosynthetic carbon (C) in rice–soil systems is poorly understood. To address this, we quantified the C flows into various pools in a rice–soil system. Rice (Oryza sativa L.) was pulse-labeled with 13CO2 at the tillering stage. Samples were collected six times during the 26 days following labeling. We quantified the partitioned photosynthesized C into various pools using stable isotopic techniques and estimated C flows. Although the net distribution of assimilated C to belowground pools did not change, N fertilization promoted C assimilation in aboveground biomass. C allocation into soil was enhanced by N fertilization during early growth, but decreased during late growth. N fertilization induced higher mass-specific rhizodeposition (per unit root dry weight) and its turnover rate compared with the unfertilized system. However, with higher microbial turnover, the daily C allocation from roots to soil was similar at both fertilization levels. Although total C input into soil is enhanced by N fertilization, its further fate is N fertilization independent, thus leading to a net accumulation of C input in rice paddy soil similar to that observed unfertilized soil.