The soils of rice paddies, which cover an area of 165 million ha worldwide, hold great potential for expanded C sequestration. The soil organic carbon pools in agricultural systems are significantly affected by the input of crop residues and rhizodeposits. Furthermore, autotrophic soil microbes that assimilate CO₂ contribute to C sequestration in paddy soil. Since C inputs promote microbial activity and native SOC decomposition, their quantity and quality influence microbe-mediated decomposition processes. Therefore, the quantification of different C substrates deposited in paddy soils and their respective effects on native SOC require further investigation.

Recently, researchers in the Institute of Subtropical Agriculture,Chinese Academy of Sciences(ISA) employed ¹³C labelling techniques and two end-member mixing model to investigate the fate and priming effect of organic C from different sources by conducting a 300 days incubation study with four different ¹³C-labelled substrates: rice shoots (Shoot-C), rice roots (Root-C), rice rhizodeposits (Rhizo-C), and microbe-assimilated C (Micro-C).

The team found that both rhizodeposits and microbe-assimilated C could reduce native SOC decomposition and might more effectively contribute to the stability and sequestration of soil C than rice shoots and roots. Root-C-treated soils exhibited the highest rate of C mineralization, followed by Shoot-C-, Rhizo-C-, and Micro-C-treated soils, whereas the opposite trend was observed for mean residence times. By the end of 300-d incubation, Shoot-C-treated soils exhibited higher total mineralization and positive PEs, whereas Root-C failed to exhibit a significant priming effect. However, the total C contents of Rhizo-C- and Micro-C-treated soils were higher than those of untreated soil, no significant differences in cumulative C emissions were observed. Given that about 0.3% and 0.1% of the cumulative C emission were derived from labelled Rhizo-C and Micro-C. They concluded that the soil organic C-derived emissions were lower in Rhizo-C- and Micro-C-treated soils than in untreated soil. This suggested that rhizodeposits and microbe-assimilated C could be used to reduce the mineralization of native soil organic C and to effectively improve soil C sequestration.

The contrasting behaviour of the different photosynthesized C substrates suggested that recycling rice roots in paddies was more beneficial than recycling shoots and demonstrated the importance of increasing rhizodeposits and microbe-assimilated C in paddy soils via nutrient management.

This research was supported by funding from the National Natural Science Foundation of China (41430860, 41371304), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB15020401), the Open Foundation of Key Laboratory of Agro-ecological Processes in Subtropical Region, the Chinese Academy of Sciences Institute of Subtropical Agriculture (ISA2015101), and the Recruitment Program of High-End Foreign Experts of the State Administration of Foreign Experts Affairs, awarded to Georg Guggenberger (GDT20154300073).

The study entitled "Fate of rice shoot and root residues, rhizodeposits, and microbe-assimilated carbon in paddy soil-Part1: ecomposition and priming effect" has been published in Biogeosciences, details could be found at http://www.biogeosciences.net/13/4481/2016/ 

Contact: GE Tida and WU Jinshui

E-mail: gtd@isa.ac.cn; jswu@isa.ac.cn

Institute of Subtropical Agriculture, Chinese Academy of Sciences