High Rates of Endophytic Nitrogen Fixation and Rhizosphere Phosphatase Activity for Multiple Grass Species across Environmental Gradients in Serengeti National Park

Abstract

Nitrogen fixation in and phosphatase exudation to soil may be critical in mitigating nutrient limitation. However, little is known about these processes for different dominant herbaceous legume and grass species in natural grassland and savanna ecosystems. Here we report evidence of substantial, persistent root N 2 -fixation associated with root and soil phosphatase activity for several dominant grass species and a common legume at five sites in the Serengeti ecosystem that differ in fire frequency, rainfall and soil N and P. N 2 -fixation was measured with 15 N 2 root incorporation assays for 225 plants in two different years. For 122 of these same plants, we also measured phosphatase activity in roots and rhizosphere soils. Community root biomass and per cent cover, as well as root and rhizosphere soil 15 N for each species, were measured to allow area-specific estimates of N 2 -fixation. Four dominant grass species and the legume exhibited comparable root diazotroph abundance and mass-specific N 2 -fixation activity, but different species exhibited peak activity at different sites. Assayed rates were associated with greater root tissue 15 N, indicating persistent seasonal N 2 -fixation. Area-specific annual fixation from grass roots varied from 16 - 42 kgN . ha -1 yr -1 , with the highest values at sites with the lower P, higher rainfall and/or greater fire frequency. Phosphatase activity in both roots and rhizosphere soil of all five species was significantly associated with root N 2 -fixation. N 2 -fixation in grasses may be a major, previously overlooked source of N for grasslands and savannas that may balance high mean annual N losses from grazing and fire. Fixed N may also stimulate phosphatase synthesis and exudation to mitigate P limitation.