Free-living N2 Fixation in Three Karst Shrublands, Southwest China

Abstract

Free-living N2 fixation is an important pathway of external nitrogen input to natural terrestrial ecosystems. However, few measurements of N2 fixation have been conducted in shrublands. Here, free-living N2 fixation in soil (or soil N2 fixation) and litter (or litter N2 fixation) in three shrublands was measured in a karst catchment, southwest China. The three shrublands were dominated by Pterolobium punctatum Hemsl., Vitex negundo Linn. and Rhus chinensis Mill., respectively. Field measurements were carried out in January and July 2016, respectively, using acetylene reduction assay. N2 fixation had distinct patterns among shrublands or between seasons. In January, no difference was found for soil N2 fixation among the three shrublands, but litter N2 fixation rate was highest in R. chinensis and lowest in P. punctatum. In July, soil N2 fixation rate was highest in R. chinensis and lowest in P. punctatum, but litter N2 fixation was significantly lower in P. punctatum than in the other two shrublands. Across the two seasons, soil N2 fixation rate in R. chinensis was 31% greater than that in P. punctatum, and litter N2 fixation rates in V. negundo and R. chinensis were 13 and 16 times greater than the rate in P. punctatum, respectively. Both soil and litter N2 fixation rates were similar between the two seasons in P. punctatum, but the rates were significantly higher in July than in January in the other two shrublands. Annual N2 fixation rates were estimated to vary from 0.63 ± 0.07 to 0.97 ± 0.08 kg N ha−1 y−1 for the three shrublands. The strongest explanatory variable for soil N2 fixation was total nitrogen in July; and that for litter N2 fixation was nitrogen in January, but was C:N ratio in July. Our findings suggest that large variation in N2 fixation may occur among shrublands within a small scale, and hence, more measurements are needed to get a representative range of N2 fixation rates for the shrubland biome.