Asymbiotic N₂ fixation (ANF) is an important source of new nitrogen for terrestrial ecosystems. Evidence shows that nitrogen (N) addition may suppress or have no significant effect on ANF in soil, litter or moss.

However, the mechanisms underlying differential responses of ANF to N addition are not well understood.

Recently, in order to assess how ANF in soil, litter and moss responds to N addition, and whether topography modulates responses of ANF in soil, litter or moss to N addition, a research team led by LI Dejun at the Institute of Subtropical Agriculture (ISA) of the Chinese Academy of Sciences conducted an N addition experiment at two topographic positions, i.e., valley and slope of a secondary karst forest in southwest China.

The N addition treatments included control (0 kg N ha^-1 yr^-1), moderate N addition (N50, 50 kg N ha^-1 yr^-1) and high N addition (N100, 100 kg N ha^-1 yr^-1).

The team found that N addition had no significant effect on moss ANF at both topographic positions. Soil ANF was lower by 17.1% in the N100 plots relative to the control in the valley, but was not significantly altered by N addition on the slope. In contrast, litter ANF was suppressed by N addition at both topographic positions by 77.9% to 87.4%.

It was suggested that topography may modulate the responses of ANF to N addition, but the modulation effects likely differ among different ecosystem compartments in the karst secondary forest, southwest China.

Besides, more investigations were suggested to better predict the responses of biological N₂ fixation to atmospheric N deposition.

The study, published in Journal of Geographical Researc: Biogeosciences, was supported by the National Natural Science Foundation of China, the National Key Research and Development Program of China, Guangxi Bagui Scholarship Program to Dejun Li, National High-Level Talents Special Support Program to Dejun Li, and the Strategic Priority Research Program of Chinese Academy of Sciences.