Free-living N₂ fixation (FNF hereafter) is an important pathway of external nitrogen input to natural terrestrial ecosystems.

Recently, researchers at the Institute of Subtropical Agriculture of Chinese Academy of Sciences investigated the FNF in soil and litter in three dominant shrublands (i.e., Pterolobium punctatum, Vitex negundo and Rhus chinensis) in a karst catchment, Southwest China. The finding was published in Ecosystems.

N₂ fixation is crucial in determining ecosystem structure, processes and functions. So far, few measurements of N₂ fixation have been conducted in shrubland.

In this study, field measurements were carried out in January and July 2016, respectively. Researchers found that N₂ fixation had distinct patterns among shrublands or between seasons.

Across the two seasons, they found that soil N₂ fixation rate in Rhus chinensis was 31% greater than that in Pterolobium punctatum, and litter N₂ fixation rates in Vitex negundo and Rhus chinensis were 13 and 16 times greater than the rate in Pterolobium punctatum, respectively.

Besides, they found that both soil and litter N₂ fixation rates were similar between the two seasons in Pterolobium punctatum, but the rates were significantly higher in July than in January in the other two shrublands.

The results showed that 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. Researchers estimated that the annual N₂ fixation rates were variedfor the three shrublands.

The finding showed that large variation in N₂ fixation may occur among shrublands within a small scale.

More measurements are needed to get a representative range of N₂ fixation rates for the shrubland biome.

Shrubland is widely distributed globally from the tropics to the polar regions. As a single biome, shrubland occupies the largest area in terrestrial ecosystems by accounting for more than 14% of the total. In the karst region of Southwest China, shrubland is also common due to widespread post-agricultural succession.

This study was supported by the National Natural Science Foundation of China, the National Key Research and Development Program of China, and the National Key Basic Research Program of China.

Schematic diagrams showing the relative locations of the three shrublands in the catchment. (Image by LI Dejun)