N2 fixation per unit microbial biomass increases with aridity

Abstract

The aim of this study was to test the hypothesis that N2 fixation by free-living microorganisms is a quantitatively important process in arid and semiarid ecosystems and that N2 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 Δ14C and δ15N 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-N2 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, N2 fixation per m2 was highest at the Mediterranean site, which is in accordance with the low natural abundance soil δ15N 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 qN2 is high in (semi-)arid ecosystems. The high N2 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.