Nutrient addition, fire and grass competition affects biological nitrogen fixation in Vachellia sieberiana, and associated soil respiration

Biological nitrogen fixation (BNF) by woody legumes is likely to vary due to factors such as nutrient availability, fire and grazing. BNF may also alter soil properties by increasing soil fertility, consequently affecting rates of soil respiration. Given the widespread increase in woody plant density in southern African savannas, we investigate the factors influencing BNF in a common encroaching woody legume, Vachellia sieberiana. We conducted a pot experiment using a matrix of grass and seedlings of V. sieberiana. We then assessed the effects of increased nutrient availability (nitrogen (N) and phosphorous (P)) through fertilizer addition, as well as simulated fire and grazing on the rate of BNF, the number and weight of root nodules, and soil respiration. We found a significant decrease in BNF with fertilizer addition, and increases in BNF after fire application. Soil respiration increased with fertilizer addition and decreased after fire application. Grazing had no independent effect on any of the response variables. However, decreased grass biomass, and therefore reduced competitive interactions between tree seedlings and grasses, resulted in increased BNF across all treatments. Furthermore, we found that larger woody seedlings achieved higher rates of BNF, with a positive correlation between the rate of BNF and both the number and weight of root nodules. We conclude that BNF in V. sieberiana is facultative and strongly influenced by grass competition, with differing responses to varying environmental factors. Fertilizer addition suppresses BNF because the presence of readily available N negates the costs of fixation. V. sieberiana seedlings compensated for N lost by fire application by increasing BNF. Soil respiration was found to increase with fertilizer addition, possibly due to higher carbon (C) inputs into the soil. Conversely, fire reduced soil respiration by removing biomass, and thus reduced C input into the soil.