Nutrient equilibrium and distribution along soil profile of three legumes on highland Loess Plateau

In order to understand the short-term impact of legumes on soil nutrient condition, three types of legumes:Astragelus adsurgens (A), Medicago sativa (B) and Lespedeza davurica (C) were seeded at different densities either alone or mixed, and the distribution and equilibrium of soil organic matter (SOM), total nitrogen (STN) and total phosphorus (STP) were qualitatively and quantitatively analyzed along soil profile and treatment types. SOM and STN of all treatments exhibit an S-shaped curve within 2 m depth soil profile while STP follows a parabolic curve. N fixation ability of the three legumes seeded alone improves with increasing seeding density, and the Apparent Biological Nitrogen Fixation (ABNF) of M. sativa is 507.5 kg·hm^-2—the highest for the monoculture treatment. After a year of growth, A. adsurgens SOM averagely increases by 3.51%. Average STP output ratios of A. adsurgens and M. sativa are 43.14% and 40.24%, significantly higher than that of L. davurica (23.74%), and those of mixure of A. adsurgens and L. davurica, mixture of M. sativa and L. davurica, and mixture of A. adsurgens, M. sativa and L. davurica are 20.73%, 26.33% and 25.83% respectively. All the three legumes significantly enhance SOM. Though A. adsurgens and M. sativa consume much more STN and STP than L. davurica, they have a stronger nitrogen fixation ability. Utilization of soil resources by mixed legume community can be optimized by appropriate seeding density due to increased competition and symbiosis among the different species of the community.