Microbial residues as indicators of soil restoration in South African secondary pastures

摘要

Prolonged intensive arable cropping of semiarid grassland soils in the South African Highveld resulted in a significant loss of C, N and associated living and dead microbial biomass. To regenerate their soils, farmers converted degraded arable sites back into secondary pastures. The objective of this study was to clarify the contribution of microorganisms to the sequestration of C and N in soil during this regeneration phase. Composite samples were taken from the topsoils of former arable land, namely Plinthustalfs, which had been converted to pastures 1-31 years ago. Amino sugars were determined as markers for microbial residues in the bulk soil and in selected particle-size fractions. The results showed that when C and N contents increased during the secondary pasture usage, the amino sugar concentration in the bulk soil (0-5 cm) recovered at similar magnitude and reached a new steady-state level after approximately 90 years, which corresponded only to 90% of the amino sugar level in the primary grassland. The amino sugar concentration in the clay-sized fraction recovered to a higher end level than in the bulk soil, and also at a faster annual rate. This confirms that especially the finer particles contained a high amount of amino sugars and were responsible, thus, for the restoration of microbially derived C and N. The incomplete recovery of amino sugars in bulk soil can only in parts be attributed to a slightly coarser texture of secondary grassland that had lost silt through wind erosion. The soils particularly had also lost the ability to restore microbial residues below 5 cm soil depth. Overall, the ratios of glucosamine to muramic acid also increased with increasing duration of pasture usage, suggesting that fungi dominated the microbial sequestration of C and N whereas the re-accumulation of bacterial cell wall residues was less pronounced. However, the glucosamine-to-muramic acid ratios finally even exceeded those of the primary grassland, indicating that there remained some irreversible changes of the soil microbial community by former intensive crop management.