Soil organic carbon mineralization rates in aggregates under contrasting land uses

Abstract

Measuring soil organic carbon (SOC) mineralization in macro-aggregates (250–2000μm), micro-aggregates (250–53μm) and the <53μm fraction helps to understand how spatial separation of SOC inside soil aggregates regulates its dynamics. We hypothesized that (i) compared with macro-aggregates SOC mineralization rate of micro-aggregates would be slower, (ii) adsorption of SOC on <53μm fraction decreases the SOC mineralization rate, and (iii) land use has a significant influence on SOC decomposition rate. To test these hypotheses we collected topsoil from Dermosol (Acrisols in FAO Soil Classification) sites under three contrasting land uses namely native pasture (NP), crop–pasture rotation (CP) and woodland (WL). Macro-aggregates, micro-aggregates and the <53μm fraction were separated from bulk soil by wet sieving. The three aggregate size ranges were then incubated for six months and CO2 evolution was measured at different time intervals. The chemically stable SOC of <53μm fraction of macro-aggregates, micro-aggregates and the <53μm fraction (separated by wet sieving) was measured by oxidation of SOC with 10% H2O2. On average, cumulative mineralization, Cmin (gCO2–Ckg−1 aggregate) of the <53μm fraction, was 28% lower than that of macro-aggregates and micro-aggregates. However, SOC mineralized (SOCmin) was similar in all size fractions. The size of slow SOC pool (percent of SOC concentration in aggregates) was also significantly higher in the <53μm fraction and ranged from 58 to 96%, across aggregate sizes. However, the chemically stable SOC (percent of SOC concentration in aggregates) was significantly higher in macro-aggregates and micro-aggregates than that of the <53μm fraction. Mean residence time (MRT) of slow SOC pool (MRTs) was higher in the <53μm fraction than for either macro-aggregates or micro-aggregates. Among the land uses NP had higher SOCmin compared with CP and WL. In conclusion, the insignificant difference in SOCmin, slow SOC pool sizes and MRTs between macro-aggregates and micro-aggregates indicated that SOC mineralization rate and thus the protection of SOC was similar in both macro-aggregates and micro-aggregates.