Differential patterns of nitrogen nutrition and growth cost of the indigenous Vachellia sieberiana and the introduced Chromolaena odorata in the savannah environment

Abstract

Vachellia sieberiana fixes atmospheric nitrogen (N) and distributes it back into ecosystems. We hypothesize that biological nitrogen fixation in this plant species is limited by competition from the invasive shrub, Chromolaena odorata. Competition would therefore result in the legume plant switching its limited nitrogen (N) sources in phosphorus-poor soils in savannah ecosystems when resources have to be shared. This study investigated the different patterns of N use and growth costs by a native and an introduced leguminous shrubby species. We propose that the two species sharing the same environment might result in competition. The competitive effect would induce in the indigenous legume to better utilize atmospheric-derived N modifying plant growth kinetics and plant mineral concentrations. Seedlings of V. sieberiana were cultivated in natural soil inoculum with low levels of phosphorus (mg L^-1 ± SE) of 3.67 ± 0.88. The experiments were divided into two treatments where (i) seedlings of V. sieberiana were subjected to competition by cultivating them together with seedlings of C. odorata, and (ii) seedlings of V. sieberiana were cultivated independently. Although V. sieberiana was subjected to competition, the N₂-fixing bacteria that occupied the nodules was Mesorhizobium species, similar to plants not subjected to competition. Total plant biomass was similar between treatments although V. sieberiana plants subjected to competition accumulated more below-ground biomass and showed higher carbon construction costs than plants growing individually. Total plant phosphorus and nitrogen decreased in seedlings of V. sieberiana under competition, whereas no differences were observed in percent N derived from the atmosphere (%NDFA) between treatments. The specific nitrogen utilization rate (SNUR) was higher in V. sieberiana plants subjected to competition while specific nitrogen absorption rate (SNAR) showed the opposite response. Vachellia sieberiana is highly adapted to nutrient-poor savannah ecosystems and can withstand competition from invasive shrubs by utilizing both atmospheric and soil nitrogen sources.

Keywords: Chromolaena odorata; N2 fixation; Vachellia sieberiana; competition; phosphorus (P) deficiency; savanna ecosystem.

Figure 1.

Growth kinetics of 120 days V. sieberiana tree legume sharing resources with C. odorata and the tree legume, V. sieberiana grown independently. Values are means ± 1 SEM. Different letters indicate significant differences among treatments.

Figure 2.

(A) Percentage N derived from the atmosphere, (B) soil N concentration and (C) atmospheric nitrogen concentration of 120 days V. sieberiana tree legume grown in the presence of C. odorata and the tree legume, V. sieberiana, grown independently. Different letters indicate significant differences among treatments.

Figure 3.

(A) SNAR and (B) SNUR of 120 days V. sieberiana tree seedlings subjected to competition with C. odorata and the tree legume, V. sieberiana grown independently without competition from C. odorata. Different letters indicate significant differences.

Figure 4.

(A) Root systems of V. sieberiana tree seedlings and C. odorata plants grown together and (B) root nodules of V. sieberiana tree seedlings grown independently without C. odorata.