2.1 Study site and soil sampling
Soil was collected from experimental rice fields located in Changsha, Hunan, China (113°19′ 52″ E, 28° 33′ 04″ N). The site is characterized by a subtropical climate, with a mean annual temperature of 16.7 °C and mean annual precipitation of 1457 mm. Moist field soils were collected from the plow layer (0–20 cm) using a stainless-steel drill (diameter: 5 cm) in September, which is after the growing season in summer. The soil is derived from Quaternary Red Clay and classified as Haplic Acrisol according to the IUSS Working Group WRB (WRB, 2015). The soil received urea (80 kg ha-1 yr-1) along with biological fertilizer (livestock manure) and rice plant straw residues (6000 kg ha-1 yr-1) within a rice–rape rotation. Five soil cores were collected and thoroughly mixed; they were immediately placed in a gas-permeable plastic bag, and stored at 4 °C until analysis. A subset of soil samples was air-dried and passed through a 2-mm sieve to remove fine roots and other plant residues. The fresh soil samples were used to measure basic chemical properties such as SOC (11.27 g kg−1) and total N (TN) (1.3 g kg−1) using the potassium dichromate-concentrated sulfuric acid-phenanthroline titration method (Lu, 1999). Dissolved organic C (DOC) (69.84 mg kg−1) was analyzed using a Shimadzu TOC-VWP analyzer (TOC-VWP, Shimadzu, Japan). Soil pH (4.98) was measured using a Mettler Toledo 320 pH meter at a soil-to-water ratio (w:v) of 1:2.5.