Differentiation of the biogeochemical niches of legumes and non-legumes in the Cape Floristic Region of South Africa

Abstract

Many legume species in the Cape Floristic Region, recently renamed the Core Cape Subregion (CCR), occur in distinct population patches within the landscape. We hypothesized that when compared to non-legumes, legumes occupy a distinct nutrient-enriched biogeochemical niche, and that foliar nutrient concentrations would correspond with soil chemistry. Soil and leaf samples were collected at eight sites in the CCR that were selected based on soil types including limestone-, sandstone-, granite- and sand-derived soils. At each site, samples were collected along three parallel replicate line transects originating inside the legume patch and extending into the adjacent non-legume habitat. The soil and plant samples were processed and analysed for chemical elements. Within a soil type, canonical discriminant analysis of soil nutrient concentrations separated legume from non-legume habitats on limestone, sandstone and granitic soils, thereby identifying their biogeochemical niche. Multiple nutrients were associated with the legume niche, and the nutrients varied with soil type. Relative to the non-legume habitat, the legume habitat was enriched with nutrients with greater concentrations of total N, total P, K, Mg, Na and C in both limestone and sandstone soils, but only enriched with C in granitic soil. Furthermore, a strong canonical correlation in both legume and non-legume plants (e.g. along CV1 r = 0.90 and 0.83, respectively) was found between the soil and leaf nutrients. Overall, the study provided empirical evidence for the existence of a nutrient-rich biogeochemical niche for legumes in the CCR and showed that foliar nutrients reflected soil chemistry.