Main content area

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

Li, Dejun, Zhang, Qingshan, Wang, Zhenchuan
Ecosystems 2019 v.22 no.4 pp. 818-826
Pterolobium, Rhus, Vitex negundo, acetylene reduction, carbon nitrogen ratio, karsts, nitrogen, nitrogen fixation, shrublands, soil, terrestrial ecosystems, total nitrogen, watersheds, China
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.