Soil nitrogen and phosphorus dynamics under Prosopis juliflora invasion: Nutrient availability and nitrogen transformation rates in arid ecosystem

Invasive species like Prosopis spp. can alter ecosystem functioning by modifying nitrogen (N) and phosphorus (P) cycling. Prosopis’ ability to fix N and access deep soil resources influences the concentration and turnover of inorganic N (ammonium [NH₄⁺], nitrite [NO₂⁻], and nitrate [NO₃⁻]) and biologically available P (Olsen-extractable P), essential for plant and microbial functioning. However, little is known about how Prosopis establishment affects these nutrients across the soil profile, particularly in dryland ecosystems. We investigated the effects of Prosopis juliflora establishment on soil N and P dynamics in an arid desert, comparing it to native Acacia tortilis and unvegetated soils. Soil samples were collected during two campaigns in dry summer and moist spring to a depth of 100 cm. All samples were analyzed for total N, inorganic N, biologically available P, and microbial biomass. In addition, N transformation rates were measured in the topsoil (0–24 cm) layer. Under both P. juliflora and A. tortilis , total soil N, microbial biomass, and net N mineralization and net nitrification rates in the topsoil were elevated as compared to unvegetated soils. However, in soil under A. tortilis , more NO₃⁻ and biologically available P accumulated at greater depths (60–100 cm) compared to soil under P. juliflora , particularly in spring. In soil under P. juliflora , N turnover in the topsoil was faster but inorganic N was similar to unvegetated soil. Biologically available P was higher under both tree species compared to unvegetated soil but accumulated primarily in topsoil under P. juliflora and in depth under A. tortilis . Soil inorganic N and P correlated positively with both P. juliflora and A. tortilis canopy areas. Our results indicate that irrespective of whether the legume is invasive or native, tree presence enriches topsoil in N and P compared to unvegetated soils, but with contrasting depth distributions that likely shape nutrient availability to plants and microbes.