Topography Modulates Effects of Nitrogen Deposition on Asymbiotic N2 Fixation in Soil but not Litter or Moss in a Secondary Karst Forest.

Evidence shows that nitrogen (N) addition may suppress or have no significant effect on asymbiotic N2 fixation (ANF) in soil, litter, or moss. However, the mechanisms underlying the differential responses of ANF to N addition are not well understood. The main objectives of the current study were to assess how ANF in soil, litter, and moss responds to N addition and whether topography modulates the responses of ANF in soil, litter, or moss to N addition. We conducted an N addition experiment at two topographic positions, that is, valley and slope of a secondary karst forest in southwest China. Three N addition treatments, that is, control (0 kg N·ha−1·year−1), moderate N addition (N50, 50 kg N·ha−1·year−1), and high N addition (N100, 100 kg N·ha−1·year−1) were included. Nitrogen 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%. Our findings suggest that topography may modulate the responses of ANF to N addition, but the modulation effects likely differ among different ecosystem compartments. Plain Language Summary: Asymbiotic N2 fixation (ANF), a process carried out by free‐living N2‐fixing microorganisms to convert atmospheric N2 to ammonia, is an important source of new nitrogen for terrestrial ecosystems. Atmospheric nitrogen deposition has been found to suppress ANF but much uncertainty still exists. We evaluated the effects of nitrogen addition on ANF in soil, litter, and moss at two topographic positions, that is, valley and slope of a subtropical secondary forest in southwest China. We find that ANF in soil was inhibited by nitrogen addition in the valley but not on the slope, whereas ANF in litter was suppressed by nitrogen addition irrespective of topographic positions. ANF in moss was not affected by nitrogen addition. Our results imply that the responses of ANF to nitrogen addition depend on both topographic position and substrate. Key Points: N2 fixation in soil was inhibited by N addition in the valley but not on the slopeN2 fixation in litter was suppressed by N addition in the valley and on the slopeN2 fixation in moss was not affected by N addition in the valley or on the slope [ABSTRACT FROM AUTHOR]