Sample Pretreatment Affects the Distribution of Organic Carbon in Aggregates of Tropical Grassland Soils

Soil aggregation is an important mechanism for the stabilization of soil organic C (SOC). The distribution of SOC among aggregate classes depends on sample pretreatment and on the applied method of aggregate fractionation. We examined the effect of soil pretreatment (air‐dried vs. field‐moist soil) on the yield and SOC content of aggregate size fractions (8000–2000, 2000–250, 250–53, and <53 μm). Aggregate size fractions were separated by wet sieving tropical soils of different parent material and mineralogy (volcanic ashes dominated by short‐range‐order aluminosilicates and marine Tertiary sediments dominated by smectitic clays), which were used as pastures for 13 to 50 yr after deforestation. In addition, the proportion of pasture‐ and forest‐derived SOC in the aggregate fractions was determined using the 13 C/ 12 C isotope ratio. In volcanic ash soils, there was no clear effect of soil pretreatment on the distribution of aggregates into aggregate size classes. Furthermore, the SOC concentration and proportion of pasture‐derived SOC of aggregates within each size class did not differ across treatments. In smectitic clay soils, however, the two pretreatments resulted in distinct differences in the distribution of dry matter yield and also of SOC among the aggregate fractions. Wet sieving of dry soil led to a separation of macroaggregates rich in pasture‐derived SOC, whereas wet sieving of moist soil isolated microaggregates with high contents of pasture‐derived SOC. This implies that soil organic matter plays a major role in both the formation and the stabilization of macroaggregates and in the early stage of microaggregate formation in sedimentary soils but not in volcanic ash soils.