Effect of reduced mineral fertilization (NPK) combined with green manure on aggregate stability and soil organic carbon fractions in a fluvo-aquic paddy soil

Abstract

A 10-year field experiment with different inorganic fertilizer rates, NPK (F; 100 %, 80 % and 60 %) supplemented with Chinese milk vetch, Astragalus sinicus L. (MV; 15 t ha−1, 22.5 t ha−1, 30 t ha−1 and 37.5 t ha−1) as green manure was conducted in a fluvo-aquic paddy soil in Southern China. The study aimed to assess soil water stable aggregates (WSA) and their stability indices, aggregate associated SOC and physical protection mechanisms of SOC within aggregate associated density fractions (fLOC, c-iPOC, f-iPOC and mSOC) at 0−20 cm soil depth. The results showed that soil macroaggregation was significantly improved (P < 0.05) with combined application of NPK and green manuring, by 76.7 % and 10.1 % with 37.5t + F80, and by 80.6 % and 12.5 % with 30t + F60, compared to the control (CK) and F (100 % NPK) treatments, respectively. Aggregate stability indices such as mean weight diameter (MWD) and geometric mean diameter (GMD) were increased under 37.5t + F80 (51.4 % and 22.7 %) and 30t + F60 (54.3 % and 22.7 %) treatments compared to CK. Bulk SOC concentration was positively correlated with MWD (R2 = 0.62) and GMD (R2 = 0.59). Compared to CK, bulk soil OC was significantly increased by 31.0 % with 22.5 t + F80, 38.8 % with 37.5 t + F80 and by 18.0 % with 30 t + F60. With reduced mineral fertilization combined with green manure (MV), SOC concentrations of the macroaggregates (0.25−2 mm), microaggregates (0.053−0.25 mm) and silt + clay fraction (<0.053 mm) were 18.0 %, 11.2 % and 6.8 % higher than CK. The highest proportion of OC added with MV residues was distributed in mSOC fraction isolated from macroaggregates (0.25−2 mm) and microaggregates (0.053−0.25 mm). However, c-iPOC0.25−2 mm and f-iPOC0.053−0.25 mm fractions were the most sensitive indicators of the increased SOC, suggesting a shift of SOC towards microaggregates and thereby long-term SOC sequestration within the microaggregates through physical protection mechanisms. Moreover, f-iPOC was positively correlated (slope = 0.051; R2 = 0.17) with bulk soil OC indicating that f-iPOC was main SOC accumulating fraction within the microaggregates. It can be concluded that reduced mineral fertilization combined with green manuring (especially, 22.5 t + F80, 37.5 t + F80 and 30 t + F60) could be an efficient strategy to improve soil structure and intra-aggregate SOC sequestration in fluvo-aquic paddy soil.