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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 31 years 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 5–10 cm) 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 38 years. Lignin sequestration was accelerated relative to total SOC in the 0–5 cm soil depth but delayed in the 5–10 cm 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 (g VSC kg− 1 C) was similar to that of the primary grassland. Decreasing ratios of syringyl to vanillyl structural units reflected that after 8–19 years all remaining lignin derived primarily from the new grassland vegetation. Lignin enhances SOC accumulation only in the very surface soil (0–5 cm) but not below this depth, rendering the restoration of subsoil C stocks difficult in these ecosystems.

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 20 years of arable cropping) were sampled that had been converted to secondary pastures between 1 and 52 years 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; 0–10 cm) were fractionated to aggregates of different sizes by wet sieving (8000–2800 μm, 2800–2000 μm, 2000–500 μm, 500–250 μm, 250–53 μm and 2000 μm) 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.

Converting grassland soils to cropland changes the turnover of aggregates and their protective effects on soil organic matter (SOM). Our aim was to elucidate how rapid soil aggregation and associated SOM contents change with increased duration of long-term cropping. The study was conducted in the South African Highveld in three agro-ecosystems with different cropping durations (0–98 years) after conversion of the native grassland. Soil samples (0–20 cm; Plinthustalfs) were analyzed for aggregate size distribution (six water-stable aggregate fractions) and the C and N contents in each single fraction. Additionally, sieved and ground bulk soil was oxidized with 0.33 M KMnO 4 . With an increase of cultivation time the aggregate size distribution changed remarkably: the large macroaggregates (2000–2800 μm) and peds (> 2800 μm) broke down into finer size classes (exponential rate constant k = 0.20 year −1 ), accompanied especially by an increase of the contents of small macroaggregates (250–2000 μm) and not of the microaggregates as previously reported for other soil types. The C and N concentrations decreased in all aggregates ( k = 0.05 to 0.52 year −1 ) and the rate of decline was fastest in the peds. Most C was lost from the 250–2000 μm and 53–250 μm sized fractions (about 60%). At the primary grassland, the peds (> 2800 μm) stored 47% of total C, but less than 10% after 90 years of cropping. Parts of this C were recovered in smaller aggregate fractions. Especially the C stored in the small macroaggregates (250–2000 μm) increased from 27 ± 1% to 61 ± 1% of total C. The total loss of SOM and large aggregates went along with an accelerated loss of its C resistant to oxidation. While 40% of soil C was not oxidizable with KMnO 4 from the grassland soil, the contents of this C resistant to chemical oxidation were reduced by up to 50% as cultivation proceeded. Grinding the soil hardly changed this amount. We conclude that the loss of soil structure made also resistant SOM available for decay, which might then be difficult to replace when a restoration of the sites is anticipated.

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.