In China, paddy soil covers an area of 33 million hectares, accounting for 23% of the worldwide paddy area of 153 million hectares. Approximately 60% of paddy soil in China is in the subtropical region, where the dominant cultivation pattern is double-rice cropping. Over the last three decades, a large body of research has shown that paddy soil in China has sequestered a significant amount of soil organic carbon (SOC), largely because of the increased use of N fertilizers and the application of organic materials, including rice straw and manure. However, the magnitude of SOC sequestration in paddy soil varies depending on geographic regions which differ in climate, rotation system, soil background condition, and custom management. For this reason, the effects of fertilization on SOC sequestration should be reconsidered in different paddy regions of China.

Since 1986, a series of long-term field experiments have been established to investigate the effect of fertilizers (including N fertilizer, rice straw, and manure) on SOC change in subtropical paddy soil with a double-rice cropping system in China. Results of a single field experiment do not represent the regional trend of SOC change. The integration of many experiments in the subtropical region is a suitable method to evaluate the change in SOC.

Researchers in the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) employed an integrated method to estimate the effect of fertilization on paddy SOC sequestration, based on 106 paired data points from 28 long-term field experiments with double-rice cropping system in subtropical China. The fertilization schemes were selected as chemical fertilizer (NPK), NPK plus straw (NPK+s), and NPK plus manure (NPK+m).

The team found the use of NPK+m had the greatest SOC sequestration rate (0.67 Mg ha^-1 yr^-1), whereas the use of NPK+s and NPK had lower values of 0.48 ha^-1 yr^-1 and 0.30 ha^-1 yr^-1. The soil C sequestration rate decreased with the experimental time, leading to a sequestration period of 43, 65, and 55 years to reach a new equilibrium value of SOC for NPK, NPK+s and NPK+m, respectively. Besides, under the same N input condition, the treatment with N fertilizer proportionally replaced by manure (NPK+m) could enhance both rice yield and NUE by 28% and 27%, respectively, whereas the in situ rice straw incorporation (NPK+s) showed no distinct effect. Additional manure amendment on the basis of existing N fertilizer application rate did not have an effect on both rice yield and NUE. In contrast, additional rice straw incorporation decreased NUE by 24%, even though no distinct change of rice yield was found. The results indicate that application of chemical fertilizer plus manure, rather than rice straw, to paddy fields is a promising practice to enhance SOC accumulation and improve rice yield, as well as the crop N use efficiency in subtropical rice production of China.

This research was supported by the Strategic Priority Research Program-Climate Change: Carbon Budget and Related Issues of the Chinese Academy of Sciences (XDA05050505), the National Natural Science Foundation of China (41371252, 41090283 and 41001141) and the Ministry of Science and Technology (2013BAD11B02-4).

The study entitled "Lower C Sequestration and N Use Efficiency by Straw Incorporation than Manure Amendment on Paddy Soils" has been published in the March issue of Agriculture, Ecosystems and Environment, details could be found at http://www.sciencedirect.com/science/article/pii/S0167880915301778 

Contact: WU Jinshui 

E-mail: jswu@isa.ac.cn 

Institute of Subtropical Agriculture, Chinese Academy of Sciences