Spatial Variations in Organic Carbon Pools and Their Responses to Different Annual Straw Return Rates in Surface Paddy Soils in South China

To identify the effects of straw return on different organic carbon pools in surface paddy soils (0–20 cm), a total of 33 soil samples under different annual straw return rates (SRr) was collected, and then the samples were analyzed based on a 100-day incubation. The data from acid hydrolysis-incubation experiments were fitted to a three-pool first-order kinetics model that divided soil organic carbon (SOC) into active (Ca), slow (Cs) and resistant (Cr) pools. The results showed that the mean pool sizes of Ca, Cs, and Cr were 0.27, 10.26, and 13.46 g·kg−1, representing a mean of 1.35%, 41.91%, and 56.74% of the total SOC (TOC), respectively. The SOC pools in the surface paddy soils in Dongxiang had a small Ca pool but had longer mean residence times of the Ca and Cs pools than those in other regions in China. The three carbon pools were less affected by the paddy soil type but showed obvious spatial variations. The SRr contributed a strong positive effect on the variability of Cs and Cr, especially on Cs variability, while it had very little effect on Ca variability. Soil available nitrogen dominated the variability in TOC and Cr compared to the other soil properties. Therefore, the Cs pool is more sensitive than the other carbon pools to long-term straw return.