Lignin dynamics in secondary pasture soils of the South African Highveld

Abstract

Reseeding grasses into degraded cropland and using it as permanent secondary pasture may combat desertification and promote sequestration of soil organic carbon (SOC). Our objective was to assess the contribution of lignin and grass-derived carbon to the rates of SOC accrual upon such land-use change. We sampled secondary pastures of different age (1 to 31years old), established on former degraded cropland at three agro-ecosystems in the semi-arid Highveld of South Africa; old degraded croplands and primary grassland used for cattle grazing served as controls. Topsoil samples (0–5 and 5–10cm) were characterized by their natural 13C and 15N abundances and by their contents of lignin-derived phenols (VSC) after alkaline CuO oxidation. The results showed that the δ13C signature in secondary pasture soil was variable and not a sensitive indicator of land restoration in these sites. Soil δ15N values declined from 8 to 6‰ with increased duration of secondary pasture use and approached the values of the primary grassland, suggesting that N-use efficiency improved. The VSC contents increased in the surface soils and reached a new steady-state equilibrium after 38years. Lignin sequestration was accelerated relative to total SOC in the 0–5cm soil depth but delayed in the 5–10cm soil depth, reflecting different amounts of depth-dependent inputs of plant debris into the pastures. Even if absolute VSC contents did not reach the respective concentrations in the primary grassland, the contribution of VSC to total SOC (gVSCkg−1 C) was similar to that of the primary grassland. Decreasing ratios of syringyl to vanillyl structural units reflected that after 8–19years all remaining lignin derived primarily from the new grassland vegetation. Lignin enhances SOC accumulation only in the very surface soil (0–5cm) but not below this depth, rendering the restoration of subsoil C stocks difficult in these ecosystems.