Changhui Wang, Nannan Wang, Jianxing Zhu, Yuan Liu, Xiaofeng Xu, Shuli Niu, Guirui Yu, Xingguo Han, Nianpeng He

 

Soil ammonium is a key nutrient for plant growth. The availability of soil ammonium is controlled by two primary microbial processes: nitrogen ammonification (the conversion of organic nitrogen into ammonia) and nitrification (the conversion of ammonium to nitrate). Large-scale patterns of gross ammonification and gross nitrification rates show soil microbial adaptations to different vegetative and environmental conditions.

In this study, we investigated gross ammonification and gross nitrification rates in nine forest soils along a 3500-km north-south transect in eastern China. We used 15N-labeling techniques, along with field experiments and laboratory incubations, to assess in situ and potential rates of gross ammonification and gross nitrification. We found that both rates were significantly lower in warm temperate deciduous forests than in tropical and cold-temperate forests. Soil N content, microbial biomass N, and bacterial quantity jointly affected soil N transformations along the forest transect. We also identified significant relationships between meteorological factors (temperature and precipitation) and gross ammonification and gross nitrification rates during the sampling month (August 2013).

Significant differences in gross ammonification were detected between primary and secondary forests, and soil N content, microbial biomass N pool sizes and bacterial abundance were identified as factors controlling potential and in situ rates of gross ammonification and gross nitrification rates. The mean gross ammonification rate in primary forest was significantly lower in some sites than in others, whereas (with the exception of one site), the mean gross nitrification rates in primary and secondary forests showed the same trends. Significant differences in gross ammonification rates were found between primary and secondary forests at two sites, and differences were detected in gross nitrification rate at three sites.

Structural equation modeling analysis suggested that soil N contents, microbial biomass N pool sizes and bacterial abundance are the primary determinants of in situ rates of gross ammonification and gross nitrification rates. These findings advance our understanding of the biotic and abiotic factors (substrate vs. microbe vs. climate) that drive soil nutrient supply at the large scale in forest ecosystems.

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This paper is part of  a Special Focus on Functional Traits Along A Transect. Read more papers in this Special Focus here.