Nitrogen isotope composition of soils, C3 and C4 plants along land use gradients in southern Africa

This paper provides values for nitrogen isotopic abundances of southern African soils and plants along land use gradients of varying aridity. The δ15N values of soils and plants were generally higher in sites with greater land use intensity, except in the most arid site where δ15N decreased with land use intensity. The enrichment in 15N 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 N2 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, C3 plants had significantly higher δ15N than C4 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 δ15N, as those observed between the most arid, and the other semi arid sites analyzed. In addition to the 15N enrichment caused by the loss of gaseous and plant 14N, changes in tree and grass cover may affect soil δ15N by the differential uptake of soil N with different isotopic abundances.