[Response of Aggregate Distribution to Input Straw and Their Linkages to Organic Carbon Mineralization in Soils...

Development and the dynamics of stable aggregates in many soils are known to be closely related to the cycling as well as accumulation of soil organic carbon (SOC). This study explored the aggregation processes and distributions of soil organic carbon in soils developed from limestone (L), quaternary red earth (Q), granite (G), basalt (B), and tertiary red sandstone (T) subtropical China related to the addition of maize residues during 7 days and 184 days of incubation. The soils were sieved to<0.25 mm before incubation. We aimed to clarify the mechanisms underlying SOC mineralization across soils from the perspective of soil aggregate protection. Fractionation of the water stable aggregates showed that addition of maize straw promoted the formation of>2 mm and 2-1 mm aggregates, while only 1.0-0.5, 0.5-0.25 and <0.25 mm aggregates were detected in the absence maize straw. The proportion of macroaggregates as well as their stability was always higher in L, Q, and B developed soils than those in G and T developed soils. In amended soils, the accumulation of total SOC was much obvious in L, Q, and B developed soils than those in G and T developed soils, and these increases were mainly contributed by the >0.25 mm macroaggregate-associated SOC. This result indicated that>0.25 mm macroaggregates were important spots for SOC sequestration. Furthermore, the proportions of>0.25 mm macroaggregate-associated SOC were also significantly (P<0.05) higher in L, Q, and B developed soils than those in G and T developed soils, and the free light organic carbon (fLOC) followed an inverse parent material pattern as>0.25 mm macroaggregate-associated SOC. Results also demonstrated that ratios of accumulative mineralized CO₂-C to total soil organic carbon in L, Q, and B soils were significantly (P<0.05) lower than those in G and T soils. The correlation analysis further suggested that ratios of cumulative respired CO₂-C to total soil organic carbon were significantly and positively correlated (P<0.01) with the proportion of fLOC, but inversely correlated (P<0.01) with the proportion of>0.25 mm macroaggregate-associated SOC. By applying ¹³C-NMR to characterize the inherent chemical composition of soil organic carbon fractions, we noted that fLOC was more deeply decomposed than intra-aggregate light organic carbon (intra-aggregate LOC), and both the fractions were advanced decomposed in G and T developed soils, verifying enhanced protection of added maize residues inside soil aggregates. The findings of the research suggested that the parent material exerts a significant influence on SOC mineralization by controlling the formation of aggregates and location of SOC in the hierarchical structure of the soil aggregate system. We demonstrated that enhanced physical protection of SOC by forming more stable macroaggregates contributes to carbon accumulation in limestone, quaternary red earth, and basalt developed soils treated with organic amendments.