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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 δ 15 N values of soils and plants were generally higher in sites with greater land use intensity, except in the most arid site where δ 15 N decreased with land use intensity. The enrichment in 15 N 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 δ 15 N 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 δ 15 N, as those observed between the most arid, and the other semi arid sites analyzed. In addition to the 15 N enrichment caused by the loss of gaseous and plant 14 N, changes in tree and grass cover may affect soil δ 15 N by the differential uptake of soil N with different isotopic abundances.

Se presentan los resultados obtenidos a partir del análisis de isótopos estables de carbono (C), nitrógeno (N) y azufre (S), realizados sobre muestras de restos humanos (cabello y uña) y sobre fibras de origen animal (camélidos). Las muestras proceden de un contexto funerario asociado con un asentamiento residencial agropastoril (sitio Punta de la Peña 9), localizado en la localidad de Antofagasta de la Sierra, Catamarca, Puna Meridional Argentina. Los resultados son discutidos en el marco del conjunto de evidencias contextuales que se poseen para el sitio y de los modelos generales que se manejan, en relación con las estrategias de subsistencia, en el área de estudio y para el momento cronológico particular asociado con las muestras (ca. 1500 años AP). Además de la incidencia en la dieta humana de productos de origen agrícola, como el maíz, se plantea que el análisis de isótopos en muestras animales propone una línea complementaria para el abordaje de problemáticas que trascienden la información paleodietaria de las poblaciones humanas. Entre éstas, destacamos aquellas vinculadas con el complejo y multivariado proceso de domesticación de camélidos en los andes meridionales y/o con las estrategias particulares desplegadas por la actividad pastoril.

The mean annual rainfall in southern Africa is found to explain over half of the observed variance in the stable nitrogen (N) isotopic signatures of C3 vegetation in southern Africa (r2=0.54, P

We determined key photosynthetic gas exchange parameters, and their temperature dependence, in dominant woody plants at four savanna sites on a moisture gradient in Botswana, southern Africa. Leaf stable carbon and nitrogen (N) isotope and morphological measures were made concurrently. Sampling of these predominantly non-N-fixing species took place during an exceptional rainfall season, representing nearoptimum conditions for primary production at these sites. The mean specific leaf area and leaf size were positively related to mean annual rainfall (MAR); species with larger leaves of lower density were more abundant in wetter sites. Almost all species at all sites showed high net light-saturated photosynthetic rates (Amax @10lmolCO2m � 2 s � 1 ) due both to high CO2 carboxylation (Vc,max) and RubP-regeneration capacity (Jmax). These high rates were associated with high values of leaf [N]. Across all sites, the temperature response of Amax showed no clear optimum, and a gradual drop from 251 Ct o 351C, without notable temperature limitation at leaf temperatures in excess of 351C. Dark respiration rate (Rday) across all species and sites increased exponentially with increasing leaf temperature. Species sampled at selected sites revealed a negative relationship between leaf d 13 C (stable carbon isotope ratio) and MAR, suggesting higher leaf-level water-use efficiency at drier sites when integrated over the life of the leaf. At wetter sites, specific leaf [N] was lower and photosynthetic nitrogen-use efficiency increased, a pattern reflected at the ecosystem level by less 15 N enrichment of leaves at these sites. Taken together, the results suggest a switch from water-use to nitrogen-use efficiency constraints with increasing moisture availability. These constraints impact leaf form and function significantly, and may emerge at the ecosystem level in aspects of water and N cycling.

Nitrogen (N) cycling was analyzed in the Kalahari region of southern Africa, where a strong precipitation gradient (from 978 to 230 mm mean annual precipitation) is the main variable affecting vegetation. The region is underlain by a homogeneous soil substrate, the Kalahari sands, and provides the opportunity to analyze climate effects on nutrient cycling. Soil and plant N pools, 15N natural abundance (δ15N), and soil NO emissions were measured to indicate patterns of N cycling along a precipitation gradient. The importance of biogenic N2 fixation associated with vascular plants was estimated with foliar δ15N and the basal area of leguminous plants. Soil and plant N was more 15N enriched in arid than in humid areas, and the relation was steeper in samples collected during wet than during dry years. This indicates a strong effect of annual precipitation variability on N cycling. Soil organic carbon and C/N decreased with aridity, and soil N was influenced by plant functional types. Biogenic N2 fixation associated with vascular plants was more important in humid areas. Nitrogen fixation associated with trees and shrubs was almost absent in arid areas, even though Mimosoideae species dominate. Soil NO emissions increased with temperature and moisture and were therefore estimated to be lower in drier areas. The isotopic pattern observed in the Kalahari (15N enrichment with aridity) agrees with the lower soil organic matter, soil C/N, and N2 fixation found in arid areas. However, the estimated NO emissions would cause an opposite pattern in δ15N, suggesting that other processes, such as internal recycling and ammonia volatilization, may also affect isotopic signatures. This study indicates that spatial, and mainly temporal, variability of precipitation play a key role on N cycling and isotopic signatures in the soil–plant system.

We examined relationships between vegetation and soil nutrients along rainfall and land use gradients in the Kalahari. Soil organic carbon (SOC) was significantly lower where reduced rainfall and grazing lowered litter inputs to the soil. Soil organic nitrogen (SON) was significantly lower at the driest site and was higher than expected at two grazed sites. Ammonium levels at Mongu (wettest) were significantly higher, while nitrate was lower than expected everywhere. Site-averaged concentrations showed no significant differences in SOC and SON beneath plant canopies, but woody plant canopies alone showed elevated SOC at three sites and SON at one site.

A distinguishing, co-dominance feature of trees and grasses in Savannas was prominently explained by Walter. He hypothesized that water is the limiting factor and grasses are superior competitors for water in the upper soil, while trees have exclusive access at deeper layers. However, a new field evidence from the Kalahari, where the water table is too low for the tree roots' tapping, revealed that both plant types invest the bulk of their roots in the surface horizons. Further investigation should consider focusing on determining the co-dominance mechanism of both plant types without the displacement of the less-efficient species.

Cyanobacterial soil crusts may be important in arid and semi-arid ecosystems because of their ability to fix atmospheric nitrogen (N2). These crusts are very sensitive to trampling by animals, and their destruction can decrease ecosystem N inputs, affecting the productivity of the region. The objective of this study was to quantify the nitrogen-fixing activity in soil crusts during the wet season in southern African ecosystems using in situ acetylene reduction assays. The average acetylene reduction rates for each site ranged from 88 to 535 nmol m−2 h−1, were highly variable, and were lower than previously reported for other arid areas. All soil samples showed acetylene reduction activity; however, soils with crusts supported higher rates than did “non-crusty” soils under litter, moss, or sand. High values of 15N natural abundance (δ15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/denitrification and ammonia volatilization or atmospheric deposition of 15N-enriched nitrate or ammonium may have caused shifts in δ15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha−1 year−1, orders of magnitude lower than values estimated in other studies. The anomalous wet conditions experienced during the year of the study may have increased the temporal availability of soil mineral N and decreased N fixation rates. However, the presence of N fixation activity in all crusts analysed and their ability to survive at high temperature and after long dry periods may provide ecosystem resilience, facilitating ecosystem recovery after severe droughts.

Fires, which are an intrinsic feature of southern African ecosystems, produce biogenic and pyrogenic losses of nitrogen (N) from plants and soils. Because of the long history of fires in these savannas, it was hypothesized that N2 fixation by legumes balances the N losses caused by fires. In this study, the N2 fixation activity of woody legumes was estimated by analyzing foliar delta15N and proportional basal area of N2 fixing species along experimental fire gradients in the Kruger National Park (South Africa). In addition, soil carbon (C) and N pools, foliar phosphorus (P) and gross N mineralization and nitrification rates were measured, to indicate the effects of fires on nutrient stocks and the possible N cycling processes modified by fires. Although observations of increased soil C/N and mineralization rates in frequently burned plots support previous reports of N losses caused by fires, soil %N did not decrease with increasing fire frequency (except in 1 plot), suggesting that N losses are replenished in burned areas. However, relative abundance and N2 fixation of woody legumes decreased with fire frequency in two of the three fire gradients analyzed, suggesting that woody legume N2 fixation is not the mechanism that balances N losses. The relatively constant %N along fire gradients suggests that these ecosystems have other mechanisms to balance the N lost by fires, which could include inputs by atmospheric deposition and N2 fixation by forbs, grasses and soil cyanobacteria. Soil isotopic signatures have been previously used to infer patterns of fire history. However, the lack of a relationship between soil delta15N and fire frequency found in this study indicates that the effects of fires on ecosystem delta15N are unpredictable. Similar soil delta15N along fire gradients may reflect the contrasting effects of increased N gaseous emissions (which increases delta15N) and N2 fixation other than that associated with woody legumes (which lowers delta15N) on isotopic signatures.