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.

本研究的目的是检验这样的假设，即自由生活的微生物对 N ₂的固定是干旱和半干旱生态系统中定量重要的过程，并且每单位微生物生物量的 N ₂固定随着干旱的增加而增加。为此，我们研究了智利沿降水梯度的土壤（年平均降水量为 10 至 1084 毫米），包括干旱、半干旱、地中海和潮湿气候区。

我们沿降水梯度在四个地点采样了三个土壤深度增量（0-10、10-20 和 20-30 cm），并测量了自由生活微生物对 N ₂的固定率（使用¹⁵ N-N ₂）和两种土壤含水量（30% 和 65% 土壤持水量 (WHC)）下的微生物呼吸。此外，我们确定了根、微生物生物量 (MB) 和土壤中的碳 (C) 和氮 (N) 含量以及土壤 Δ ¹⁴ C 和 δ ¹⁵ N 特征。

我们发现土壤总有机碳（TOC）含量和土壤总有机碳与总氮（TOC：TN）的比率随着干旱程度的下降而增加，从干旱到潮湿的地方分别增加了44和1.9倍。每单位微生物生物量的 N _{2固定率 (qN 2} ) 随着干旱程度的下降从干旱地区的 51.0 ng N mg ^-1 MBC d ^-1下降到潮湿地区的 2.3 ng N mg ^-1 MBC d ^-1，在30% WHC。从干旱到潮湿，随着降水梯度的下降，呼吸与 N _{2固定的比率非常强烈地增加，大约为 1760 倍。}N ₂固定每平方米²潮湿地区的微生物呼吸比干旱地区高两倍，而潮湿地区的微生物呼吸作用是干旱地区的 186 倍。在 65% 的土壤 WHC 下，每 m ^{2的 N} ₂固定在地中海站点最高，这与该站点的低自然丰度土壤 δ ¹⁵ N 一致。

总之，该研究表明，自由生活微生物对 N ₂的固定是（半）干旱生态系统中定量重要的过程，并且 qN ₂在（半）干旱生态系统中较高。相对于土壤 TOC的高 N ₂固定率和土壤中有机质输入的低 C:N 比很可能是干旱和半干旱生态系统中土壤 TOC:TN 比通常较低的最重要原因。