Soil nitrogen is a primary limiting factor of terrestrial ecosystem productivity. AOA and AOB drive nitrification, i.e., microbial conversion of NH4+ into NO3-, and therefore critically modulate plant N utilization and ecosystem N retention. Over the last ten years, great attention has therefore been directed towards identifying the factors that modulate AOA and AOB abundances and distribution. Various soil physical and chemical factors such as soil pH and ammonia availability have been proposed to be the primary drivers but results were highly variable. Consequently, there is still lack of a unifying framework that describes major controls over the abundances of AOA and AOB. We collected the data of 467 sites reported by 56 publications and included 455 and 452 estimates of AOA and AOB abundances, respectively. The dataset was also divided into three subsets, representing three types of ecosystems, i.e., forests (98 for AOA, 96 for AOB), grasslands (127 each for AOA and AOB) and croplands (230 for AOA, 229 for AOB). We performed a meta-analysis to explore global patterns of AOA and AOB abundances and conducted a SEM analysis to identify the potential drivers that dominate these patterns.
Results/Conclusions
Results obtained showed that soil carbon to nitrogen (C:N) ratios explained the most of variance in AOA and AOB abundances. Also, although soil pH had a positive relationship with AOA and AOB, it only explained a few percent of variances. In addition, NH4+was negatively related to AOA but not AOB abundance. These findings indicate that the relative N availability rather than the absolute N content was most related to the abundance of AOA and AOB, suggesting that biotic interactions may function as a primary driver. We propose a new conceptual model in which biotic competitions for substrate N predominantly control AOA and AOB abundances. Our model provides new insights into the dynamic, interactive controls of soil abiotic and biotic factors over AOA and AOB in terrestrial ecosystems.