An article, “Ecosystem-scale spatial heterogeneity of stable isotopes of soil nitrogen in African savannas”, written by Lixin Wang has been published in Landscape Ecology. The article explores how spatial patterns of soil d15N vary at two sites across the Kalahari Transect in southern Africa. The work explores (1) whether there are spatial patterns in soil d15N at a large spatial scale (e.g., sub-kilometer scale); and (2) how soil moisture and woody vegetation—particularly potential N-fixing species—interactively control the spatial distribution of soil d15N at this scale (in case soil d15N spatial patterns are observed).
Specifically, the manuscript is organized around three hypotheses:
- soil d15N is spatially structured at both ends of the KT (dry vs. wet);
- the spatial pattern of soil d15N is influenced by woody plant distribution at both ends of the KT
- soil moisture affects the spatial distribution of soil d15N
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 (Okin et al. 2008) and based on the positive relation- ships between total N and d15N in many non-N fixing dominant ecosystems (e.g., a global synthesis in Wang et al. 2009b), while the third is based on modeling and field observations that show the effect of soil moisture on soil N dynamics (e.g., Wang et al. 2009a) as well as positive relationships between total N and d15N in non-N fixing dominant ecosystems (Wang et al. 2009b).
Contrary to our first hypothesis, spatial patterns of soil d15N only exist at the dry site (Tshane), but not at the wet site (Mongu), which may result from less spatially structured woody plant distribution at the wet site. Consistent with our second hypothesis, based on the similar spatial distribution of woody vegetation and soil d15N, it is found that woody cover distribution affects soil d15N at the dry site; however, the two potential N-fixing species do not contribute the spatial pattern for soil d15N. This is explained by the very low N input from N-fixation under dry conditions sug- gested by the high foliar and soil d15N values. This result also indicates that woody vegetation affects the spatial distribution of soil d15N through N inputs and other transformation processes, not through N-fixation. Grass cover has negative correlations with the spatial pattern of soil d15N spatial pattern both at the dry and at the wet site because of the lower foliar d15N values of grasses compared with woody species. Contrary to our third hypothesis, soil moisture did not exhibit any control on the spatial pattern of soil d15N both at the dry and wet site, presumably because soil moisture measurement were available only as ‘‘snapshots’’ and the time scale of soil moisture dynamics are different from those of d15N. Whereas previous studies showed a strong rainfall control on vegetation distribution and tree-grass interactions along the KT (Caylor et al. 2003; D’Odorico et al. 2007; Aranibar et al. 2008; Wang et al. 2010a), this study shows that both tree and grass distributions affect the spatial patterns of soil d15N at the dry end of the KT. The combined results suggest that water availability indirectly affects the spatial distribution of soil d15N through its effects on the distribution of woody vegetation and grass vegetation at the dry site.