Physical and chemical stabilization of soil organic carbon along a 500-year cultived soil chronosequence originating from estuarine wetlands: Temporal patterns and land use effects

How soil carbon is stabilized during centuries of cultivation and how this would be influenced by land use largely remain unclear. Here the relative role of physical and chemical stabilization mechanisms in agricultural soil organic carbon (SOC) accumulation were studied by fractionation of paddy/upland cropland soils along a 500-year soil chronosequence created by intermittent reclamation of estuarine wetlands. In unreclaimed wetlands, about 50% of SOC was chemically-stabilized by binding to Fe/Al oxyhydrates (mainly amorphous Fe) and 30% by unknown forms of chemical association with silt/clay particles. Physical stabilization of SOC by soil aggregation was negligible in wetlands. After conversion of wetlands to croplands, SOC rapidly declined within the first 16 years and then recovered slowly with cultivation time. Chemical mechanisms still dominated SOC stabilization processes during 500 years of cultivation, but the contribution of Fe/Al-bound and Ca-bound carbon to total SOC decreased with time. The contribution of physically stabilized carbon (i.e. microaggregate-occluded particulate organic carbon, iPOM) to SOC kept around 16% in croplands even when microaggregate contents increased from 8.83% to 30.52% between 16 and 500 years. The iPOM fraction was not closely related to microaggregate formation but to free coarse particulate organic matter, a carbon fraction indicative of inputs of plant materials. Consistently higher SOC density in paddy soils than in upland soils was observed along the chronosequence, which could be accounted for by higher contents of physical and chemical carbon fractions in paddy fields. The higher physically-stabilized carbon of paddy soils probably resulted from larger stubble return rather than from stronger soil aggregation given similar contents of microaggregates between the two cropland types. Notably, in both paddy and upland soils, carbon concentrations of intra-microaggregate silt/clay particles were consistently higher than those of free silt/clay particles. An implication was that despite the small proportion (<20% here) of physically-stabilized carbon to total SOC in croplands, soil aggregation could promote chemical SOC stabilization by creating intimate interactions between occluded carbon and soil minerals within aggregates. To conclude, during five centuries of soil cultivation, chemical carbon stabilization was the dominant mechanism controlling SOC accumulation of paddy/upland croplands but physical occlusion of carbon by aggregation might have promoted chemical stabilization of SOC.