N2 fixation per unit microbial biomass increases with aridity

The aim of this study was to test the hypothesis that N(2 )fixation by free-living microorganisms is a quantitatively important process in arid and semiarid ecosystems and that N-2 fixation per unit microbial biomass increases with increasing aridity. For this purpose, we studied soils along a precipitation gradient in Chile (ranging from 10 to 1084 mm mean annual precipitation), comprising the arid, semiarid, Mediterranean, and humid climate zone. We sampled three soil depth increments (0-10, 10-20, and 20-30 cm) at four sites along the precipitation gradient, and measured the rates of N2 fixation by free-living microorganisms (using 15N-N2) and microbial respiration at two soil water contents (30% and 65% soil water holding capacity (WHC)). In addition, we determined carbon (C) and nitrogen (N) contents in roots, microbial biomass (MB) and soil as well as the soil delta C-14 and delta N-15 signature. We found that the soil total organic C (TOC) content and the total soil organic C-to-total N (TOC:TN) ratio increased with declining aridity from the arid to the humid site by a factor of 44 and 1.9, respectively. The rate of N2 fixation per unit microbial biomass (qN2) decreased with declining aridity from 51.0 ng N mg(-1 )MBC d(-1 ) at the arid site to 2.3 ng N mg( -1) MBC d(-1) at the humid site, at 30% WHC. The ratio of respiration-to-N-2 fixation increased very strongly with declining aridity along the precipitation gradient by a factor of about 1760 from the arid to the humid site. N2 fixation per m2 was higher at the humid site than at the arid site by a factor of two, while microbial respiration was 186 times higher at the humid than at the arid site. At 65% soil WHC, N-2 fixation per m(2) was highest at the Mediterranean site, which is in accordance with the low natural abundance soil delta N-15 at this site. In conclusion, the study shows that N2 fixation by free-living microorganisms is a quantitatively important process in (semi-)arid ecosystems and that qN(2) is high in (semi-)arid ecosystems. The high N-2 fixation rate relative to the soil TOC and the low C:N ratio of organic matter inputs to the soils are very likely the most important reasons why the soil TOC:TN ratio is typically low in arid and semiarid ecosystems.