Objective  Fertility and organic carbons in soil as affected by long-term post-harvest incorporation of spent rice straws on a typical red earth, two-cropping rice field in southern China were studied for better farming management.

  Method  On the lots, treatments with applications of no fertilizer (CK), chemical fertilizer (NPK), or spent rice straws+chemical fertilizer (M) were implemented. After the late season, 2^nd crop of rice was harvested in 2020, samples in the tillage layer (0-20 cm) at the test lots were collected to determine the organic carbons (OC), microbial biomass carbon (MBC), nitrogen, and other fertility indicators in the soil.

  Result   The long-term incorporation of straw discards by plowing them back into the field along with chemical fertilizer significantly improved the soil fertility. The M treatment altered the OC composition with significantly increased free particulate carbon (FPOC) and soluble particulate carbon (DOC), over CK (P＜0.05). The carbon pool in soil was enriched more than NPK treatment, especially, the content of granular OC of 2.32 g·kg⁻¹ was significantly higher under M treatment than 1.94 g·kg⁻¹ under NPK. Compared with CK, the MBC increased by 21.94% under NPK, and even higher, by 25.98%, under M. In addition, M also raised the nutrients, such as alkali-hydrolyzable nitrogen (AN), available phosphorus (AP), and available potassium (AK), in the soil at the two-crop rice fields.

  Conclusion  By turning discarded straws into the rice field after harvest along with chemical fertilization, the carbon pool, OC composition, nutrients in the soil, as well as farming management, were significantly improved. Aside from AP, pH and total phosphorus (TP) were the other important factors affecting the carbon pool in the rice field. With the incorporation of spent straws as described in this article AP in soil benefitted as well.