Re-aggregation dynamics of degraded cropland soils with prolonged secondary pasture management in the South African Highveld

Prolonged arable cropping of subtropical sandy grassland soils resulted in substantial losses of water-stable aggregates. The objective of this study was to evaluate how rapidly and to which degree soil aggregates may be restored when degraded cropland soils are converted to secondary pasture lands. Hence, chronosequences of degraded cropland soils (more than 20years of arable cropping) were sampled that had been converted to secondary pastures between 1 and 52years ago in three agro-ecosystems of the South African Highveld. Primary grasslands as well as sites under long-term cropping served as references. The surface soils (Plinthusthalfs; 0–10cm) were fractionated to aggregates of different sizes by wet sieving (8000–2800μm, 2800–2000μm, 2000–500μm, 500–250μm, 250–53μm and<53μm). All fractions were analyzed for their respective content of soil organic carbon and total nitrogen and corrected for sand content. The results showed that reconversion of cropland into secondary pasture restored the soil structure after 9.5–18years of pasture management almost completely: the amount of sand-corrected large macroaggregates (>2000μm) approached 85–94% of the levels of the primary grasslands. The stocks of organic matter bound to large macroaggregates recovered more slowly and reached only 50% of that of the primary grassland, which, however, exhibited also a slightly elevated clay content. The carbon concentration in the aggregates did not change significantly. Thus, the increase in C stocks in the secondary pasture soils was mainly due to a rebuilding of large macroaggregates, which contained more C than the smaller-sized aggregate classes, but not more C than the respective aggregates in the degraded cropland. We conclude that only the amount of large macroaggregates was restored upon land conversion, while their protective capacity was obviously not restored and thus not strong enough to account for a full sequestration of soil organic matter.