Organic carbon fractions affected by long-term fertilization in a subtropical paddy soil

Increasing evidence is showing a greater potential for carbon (C) sequestration in paddy soils than in upland soils. However, the mechanisms underlying long-term accumulation and protection of soil organic carbon (SOC) in paddy fields have not been well documented. In the present study, five soil C fractions were separated by physical fractionation in a subtropical paddy field following 27-year differential fertilization regimes (started in 1981). Results showed that, compared to the initial level, long-term rice (Oryzasativa L.) cropping increased SOC concentrations by 28.8, 30.1, 30.8, and 61.6% in the non-fertilized (CK), nitrogen (N), nitrogen-phosphorus-potassium (NPK), and NPK combined with farmyard manure (NPK + FYM) treatments, respectively. Application of FYM enhanced the formation of macroaggregates (>2,000 and 250–2,000 μm), whereas no significant differences in aggregate-size distribution were found among the CK, N, and NPK treatments. Inorganic fertilization (N and NPK) did not affect the concentration of either total SOC or any C fraction as compared with the CK, whereas application of FYM significantly increased the concentrations both in total SOC (25.5%) and in all C fractions, except coarse particulate organic matter (cPOM). Carbon in the paddy soil was dominated by free silt and clay (s + c_f) and intra-aggregate particulate organic matter within microaggregates (iPOM_m) in all treatments that accounted for 46.4–49.6% and 25.1–27.2% of the total SOC, respectively. Furthermore, the differences in C in the iPOM_m and s + c_f fractions between the CK and NPK + FYM treatments accounted for 53.2 and 38.8% of the differences in total SOC stocks, respectively. These results indicate that SOC originating from manure is stored mainly in fractions with slow turnover (i.e., iPOM_m and s + c_f), which may benefit the long-term C sequestration in paddy soils.