Tracking the photosynthesized carbon input into soil organic carbon pools in a rice soil fertilized with nitrogen

Abstract

Replenishment of soils with carbon (C) produced during photosynthesis plays an important role in global C cycling. Nitrogen (N) fertilization is critical for rice production, but its effects on the deposition of photosynthesis-derived C into soil C pools is poorly understood. To address this, we used continuous 14C-labeling to quantify the deposition of photosynthesis-derived C into various soil organic pools in a rice-soil system. Rice (Oryza sativa L.) was continuously supplied with 14C-labeled CO2 (14C-CO2) for 36 days, with increasing N fertilizer rates (0 [N0], 10 [N10], 20 [N20], or 40 mg N kg−1 soil [N40], respectively). Rice shoot and root biomass significantly increased following N fertilization. The amount of photosynthesis-derived C converted into soil organic carbon (14C-SOC) was proportional to the soil N concentration, and accounted for 8.0–19.3 % of rice biomass C. The 14C-SOC content was positively correlated with the rice root biomass, suggesting that N increased root exudation of photosynthesis-derived C. The amounts of 14C-labeled C in the dissolved organic carbon (14C-DOC) and in the microbial biomass carbon (14C-MBC), as proportions of 14C-SOC, were 3.9–7.8 and 6.6–24.0 %, respectively. The 14C-DOC, 14C-MBC, and 14C-SOC as proportions of total DOC, MBC, and SOC were 9.7–11.6, 6.9–10.6, and 0.37–1.71 %, respectively. Nitrogen fertilization promotes deposition of photosynthesis-derived C into SOC pools in a rate-dependent manner. However, the 14C-MBC as a proportion of both 14C-SOC (14C-MBC/14C-SOC) and MBC (14C-MBC/MBC) increase during rice growth at lower N concentrations.