Soil carbon and nitrogen dynamics in southern African savannas: the effect of vegetation-induced patch-scale heterogeneities and large scale rainfall gradients

Abstract

Savanna ecosystems are mixed plant communities in which trees and grasses co-exist thereby providing a heterogeneous landscape with a mosaic of tree-dominated and grass-dominated soil patches. Despite the important role that nutrient availability plays in these systems, detailed knowledge of differences in carbon and nitrogen cycling in soil patches predominantly covered by tree canopies or by grasses is still lacking. In this study, a process-based model was used to investigate the carbon and nitrogen dynamics in soil plots located in grass-dominated and tree/shrub-dominated soil patches along the Kalahari Transect (KT). The KT in southern Africa traverses a dramatic aridity gradient, across relatively homogenous soils, providing an ideal setting for global change studies. Here we show that there are distinctly different dynamics for soil moisture, decomposition and nitrogen mineralization between soil plots located under tree canopies and in open canopy areas, especially at the dryer end of the KT. Such differences diminished when approaching the wetter end of this transect. This study shows that in savanna ecosystems, water availability determines the patterns and rates of nutrient cycling at large scales, while at the local scales, vegetation patchiness plays an important role in nutrient cycling.

Ecosystem-scale spatial heterogeneity of stable isotopes of soil nitrogen in African savannas

“…The same principle but converse trend (higher rainfall and lower δ 15 N CV value) applies to the dry site, as the rainfall amount is larger than the MAP at the drier site. There was larger difference in NO3 -+NO2 -between under canopy and open canopy areas at the dry site compared with the wet site (Table 1), which is similar to the modeling study result in this region (Wang et al 2009a). The soil moisture was higher at the wet site (~3%) than the dry site (~0.7%) but the differences between microhabitats were small at both sites (Table 1).…”

“…We should stress, however, that these soil moisture measurements were taken as snapshot of the soil water content at the time of soil sampling. Even though these data capture the higher soil moisture existing at the wet site (3.25% vs. 0.71% at the wet and dry end, respectively, (Wang et al 2009a). Because 15 N is more sensitive than the total N changes, it is expected that soil moisture spatial distribution will affect the soil δ 15 N spatial patterns.…”

“…For instance, soil moisture dynamics strongly control soil N transformations in drylands (Austin et al 2004;Wang et al 2009a). Because 15 N is one component of total N, soil δ 15 N dynamics are also influenced by soil moisture variations.…”

“…Three hypotheses were tested: 1) soil δ 15 N is spatially structured at both ends of the KT (dry vs. wet); 2) the spatial pattern of soil δ 15 N is influenced by woody plant distribution at both ends of the KT. The first two hypotheses are based on the results of previous studies showing a relationship between the spatial distribution of woody plants and total soil N ) and based on the positive relationships between total N and δ 15 N in many non-N fixing dominant ecosystems (e.g., a global synthesis in Wang et al 2009b); and 3) soil moisture affects the spatial distribution of soil δ 15 N. This is based on the modeling and field observations showing the effect of soil moisture on soil N dynamics (e.g., Wang et al 2009a) as well as the positive relationships between total N and δ 15 N in non-N fixing dominant ecosystems (Wang et al 2009b). Supposedly higher soil moisture will lead to higher 6 mineralization and nitrification rates (Austin et al 2004), since mineralization/nitration fractionate 15 N, the residuals will be enriched.…”

The effects of short-term rainfall variability on leaf isotopic traits of desert plants in sand-binding ecosystems

“…The constant annual foliar δ 15 N values suggest that water availability does not play an important role in nitrogen cycles and/or plant uptake in these two years. This conclusion is different from many earlier studies which demonstrate strong rainfall/soil moisture controls on soil nitrogen cycle in water-limited systems (e.g., Wang et al, 2009). The likely reason for this discrepancy is that both years were drought years (65% of the mean annual precipitation in 2004 and 42% in 2005), and the effect of soil moisture on nitrogen cycle (e.g., plant uptake) diminished significantly.…”