Topography Modulates Effects of Nitrogen Deposition on Asymbiotic N 2 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 N 2 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 (N 50 , 50 kg N·ha −1 ·year −1 ), and high N addition (N 100 , 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 N 100 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.