Rice is harvested annually from 165 million hectares of land worldwide and provides the basic food for nearly half the world's population. In addition, the paddy ecosystem can sequester more C than upland soils, slowing the increase in atmospheric CO^₂ concentration. 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. Hence, studying the processes of rice C distribution and transformation, and quantifying the amount of photosynthesis-derived C inputs into the soil are critical activities required to increase our understanding of global C cycling and the ecological functions of paddy ecosystems. However, there have been few direct studies of the interactions between C assimilation by photosynthesis and C cycling in rice-soil systems.

To address this, the research group from the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) continuous use ¹⁴C-labeling technology to quantify the deposition of photosynthesis-derived C into various soil organic pools in a rice-soil system under different N fertilization application.

The researchers found that 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. Rice growth with N fertilization increased the inputs of photosynthesis-derived C into soil, indicating the release of root exudates.

The results also showed that the amounts of ¹⁴C-labelled 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.

In a word, N promotes deposition of photosynthesis-derived C into SOC pools in a rate-dependent manner. However, the amounts of ¹⁴C-MBC increase during rice growth at low N concentrations. This information increases our understanding of the role of below-ground biomass in the storage of SOC in flooded rice-soil systems.

Both reviewers and field editor think the experiment presented in this study interesting and useful. They think this study presents novel work that enhances our understanding of the allocation of below ground C in common cultivation practices and greatly improves upon pulse labelled reports. Continuous labelling is a great advantage of this study, as the most previous experiments were done by pulse labelling. Therefore, the results presented in this manuscript are very interesting and useful.

This study was supported financially by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB15020401), the National Natural Science Foundation of China (41301275; 41090283).

This study entitled "Tracking the photosynthesized carbon input into soil organic carbon pools in a rice soil fertilized with nitrogen." has been online in Plant and soil. (DOI: 10.1007/s11104-014-2265-8). The full-text can be download at http://link.springer.com/article/10.1007/s11104-014-2265-8.