Soil Science Society of America Journal Dynamics of Soil Carbon Concentrations and Quality Induced by Agricultural Land Use in Central South Africa Soil Chemistry Prolonged soil cultivation has been identifed as a major cause of land degra- dation and a threat to soil quality in drought-prone environments. This study evaluated land use effects on soil carbon (C) fractions and organic C (SOC) quality across three semiarid agro-ecosystems (Harrismith, Tweespruit, and Kroonstad) in central South Africa. Soil samples were collected from crop- lands, primary and secondary grasslands at the 0- to 200-mm layer in each agro-ecosystem and analyzed for various soil C fractions. The SOC structure was characterized with 13 C nuclear magnetic resonance (NMR) spectroscopy. All analyses were done on bulk soil samples. Conversion of primary grass- lands into croplands decreased C fractions by 27 to 90% across the three agro-ecosystems, with highest losses recorded in Harrismith and Tweespruit and lowest in Kroonstad, suggesting site-specifc conditions acted together with cultivation to effect C losses. The 13 C NMR spectra revealed a slight change in SOC structural composition when O-alkyl C decreased with con- comitant increase in aromatic and alkyl C due to cultivation, with differences in the range of 1 to 11%. O-alkyl C remained almost the same in Harrismith, suggesting that lignin-derived methoxy groups were probably more domi- nant than easily decomposable carbohydrates as opposed to O-alkyl C in Tweespruit and Kroonstad. Meanwhile, reversion of cultivated soils into perennial pastures restored and even increased some soil C fractions (by 3–129%) to represent primary grasslands, especially in Kroonstad. Organic C decomposition was lower in the cultivated soils relative to virgin and restored soils. This underscores the importance of determining plant biomass compo- sition because these unusual responses were probably related to vegetation differences. Overall, this study demonstrates the potential of secondary grass- land management to rehabilitate degraded cultivated soils with implications for restoration of agro-ecosystem functions and services. Abbreviations: C EX , extractable humic substances; C HA , humic acids; C FA , fulvic acids; CWEC, cold water extractable carbon; HI, humifcation index; HWEC, hot water extractable carbon; NMR, nuclear magnetic resonance; PI, polymerization index; POXC, permanganate oxidizable carbon; SIC, soil inorganic carbon; SOC, soil organic carbon. P rolonged soil cultivation has been identified as a major precursor of degrada- tion in arid to semiarid arable lands owing to their limited phytomass pro- duction, low-erratic rainfall events and high temperatures (Lobe et al., 2001, 2002; Janzen, 2006; Von Lützow et al., 2006; Lal et al., 2015; Vázquez et al., 2016). Consequently, these arable lands are abandoned due to inflated reclamation costs or deliberately reverted to perennial pastures (also referred to as restored or secondary grasslands) to regain their original productivity (Birru, 2002; Kotzé et al., 2016). The latter was introduced at least three decades ago in the central region of South Africa in response to the government’s call to curb soil degradation. Commercial farmers in this region, in particular, Harrismith, Tweespruit and Kroonstad agro- ecosystems dominated by sandy soils with low carbon (C) stabilization capacity, were compensated to pilot this initiative. P. F. Loke* E. Kotzé C. C. du Preez Dep. of Soil, Crop and Climate Sciences Univ. of the Free State PO Box 339 Bloemfontein 9300, South Africa L. Twigge Dep. of Chemistry Univ. of the Free State PO Box 339 Bloemfontein 9300, South Africa Core Ideas • Converting grasslands to cropland deprives soils of C regardless of lability or recalcitrant nature. • Reversion into secondary grasslands can reverse soil C losses in semiarid agro-ecosystems. • Secondary grassland management improves SOC quality to resemble primary grasslands. • Relative losses and gains of soil C fractions are modulated by climatic and soil conditions. Soil Sci. Soc. Am. J. 83:366–379 doi:10.2136/sssaj2018.11.0423 Received 8 Nov. 2018. Accepted 14 Jan. 2019. *Corresponding author (lokepf@gmail.com). © 2019 The Author(s). Re-use requires permission from the publisher. Published April 29, 2019