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This paper provides values for nitrogen isotopic abundances of southern African soils and plants along land use gradients of varying aridity. The delta N-15 values of soils and plants were generally higher in sites with greater land use intensity, except in the most and site where delta N-15 decreased with land use intensity. The enrichment in N-15 with land use intensity agrees with the expected effects of grazing and cultivation on N cycling processes, including increased volatilization of ammonium, exports of plant material, and decreased N-2 fixation by the destruction of cyanobacterial soil crusts. Gross mineralization and nitrification rates were more affected by local heterogeneity in the soils than by aridity or land use. In general, C-3 plants had significantly higher delta N-15 than C-4 plants from the same location, suggesting different N use by the two plant types. This study suggests that land use intensity affects N cycling processes that may result in different and opposite changes of ecosystem delta N-15, as those observed between the most arid, and the other semi and sites analyzed. In addition to the N-15 enrichment caused by the loss of gaseous and plant N-14, changes in tree and grass cover may affect soil delta N-15 by the differential uptake of soil N with different isotopic abundances. (c) 2007 Elsevier Ltd. All rights reserved.