N2 fixation is less sensitive to changes in soil water content than carbon and net nitrogen mineralization

Biogeochemical processes catalyzed by soil microorganisms depend on the soil water content (SWC). Yet, little is known about the sensitivity of non-symbiotic N2 fixation to changes in SWC in comparison to carbon (C) and net nitrogen (N) mineralization. Therefore, we determined the rates of N2 fixation, C mineralization, and net N mineralization in soils at five SWCs that were created by water addition to the field-moist soil (rewetting), resulting in SWCs of 20, 40, 60 and 80% of the soil water holding capacity (WHC) which correspond to 14, 28, 42, and 57% water-filled pore space (WFPS). The soils originated from an arid, semiarid, Mediterranean, and humid site located in the Coastal Cordillera of Chile. The N2 fixation rate was measured immediately after the adjustment of the SWC and additionally after a five-day pre-incubation period. We found that the rates of all three processes where higher in the rewetted soils compared to the field-moist soils, but the sensitivity to changes in SWC differed between the processes. N2 fixation tended not to increase with increasing SWC beyond 20% WHC. Furthermore, the N2 fixation rate increased faster after rewetting in the soils of the semiarid and Mediterranean zone than in the soil of the humid zone. In contrast, C mineralization reached the highest rate at 80% WHC in the soil of the humid zone and at 60% and 80% WHC in the soils of the other three climate zones. The net N mineralization rate was highest at 40% and 60% WHC in the soils of the semiarid and Mediterranean zone, and was significantly decreased at 80% WHC compared to 60% WHC. Our study shows, first, that N2 fixation is less sensitive to changes in SWC than C and net N mineralization and tends to be already at its maximum in a given soil at low SWC, i.e., 20% WHC. This is likely because the diffusion of N2 in soil does not increase with the SWC, in contrast to the diffusion of organic solutes which are the substrate of C and N mineralization. Second, our results indicate that the N2 fixation rate increases more quickly in response to rewetting of soils of the more arid climate zones than of the humid zone, which might indicate a microbial adaptation to the climate conditions. The results have important implications since they suggest that the N2 fixation rate is likely less affected than the C and net N mineralization rates by decreases in the SWC that might occur more frequently in the future due to climate change.