The impact of soil management on aggregation, carbon stabilization and carbon loss as CO2 in the surface layer of a Rhodic Ferralsol in Southern Brazil

Soil aggregation and organic carbon accumulation are two intrinsically linked phenomena. Soils under natural vegetation and conservation tillage systems generally have higher aggregation indices and total organic carbon (TOC) stocks in the surface layers than soils under conventional management (ploughing). From the point of view of the emission of carbon dioxide (CO2), C stabilization and loss in the surface layer is relevant. The objective of this study was to investigate the effect of no-tillage (NT) and conventional tillage (CT) on TOC stabilization and loss through soil aggregation in the topsoil. Soil aggregation, TOC stocks, and fluxes of CO2 of samples from a Rhodic Ferralsol under NT and CT in a long-term field experiment in Southern Brazil were measured. A natural forest site was also evaluated as reference to the management sites. Emissions of CO2 were measured in laboratory incubation experiment on bulk soil samples and on soil aggregate size fractions, previously separated by dry sieving, in intact and destroyed (crushed to <0.250 mm) state. The soil under NT had larger aggregates and larger proportion of the soil in greater aggregate size classes than CT. Total organic carbon stocks were higher under NT both in bulk soil samples and macroaggregates than under CT. Under laboratory conditions the bulk soil samples from NT showed higher emission rates of CO2 (CO2BS = 18.3 kg C ha−1 h−1) than from CT (CO2BS = 2.3 kg C ha−1 h−1) due to the overall higher TOC stocks. The TOC that was lost by CO2 emission due to the oxidation of readily decomposable macroaggregate-protected SOM in NT (ΣΔCO2 = 79.4 kg C ha−1 h−1) was, however, also higher than under CT (CT: ΣΔCO2 = 29.1 kg C ha−1 h−1). The TOC stabilized by macroaggregation in NT was also more than the TOC lost by CO2 emission from the bulk soil (ΣΔCO2-CO2BS = 61.1 kg C ha−1 h−1), and the difference between these two was higher under NT than under CT (ΣΔCO2-CO2BS = 26.8 kg C ha−1 h−1), showing that NT in fact accumulates more TOC in the soil by protecting it within the macroaggregates. The natural forest had higher TOC stock and emission rates than the tilled soils, however it also stabilized more TOC. Thus, no-tillage in its effect on carbon stabilization, is between the natural ecosystem and CT, representing a soil management system that seems to be an efficient on the way path to the recuperation of soil after conventional management systems.