Reactive nitrogen that enters forest ecosystems through atmospheric deposition can effectively alleviate nitrogen limitation to plant growth, and thus can increase plant biomass. However, excessive nitrogen input may result in soil acidification and soil nutrient imbalance, and damaging the stability of ecosystem. Recent researches have been focused on effects of nitrogen deposition on plant growth, plant diversity and plant community structure, but how the increasing nitrogen input affects microbial diversity and nutrient use efficiency is still poorly understood. 

In August 2019, Associate Prof. WANG Chao, together with master student LI Jing from the Institute of Applied Ecology (IAE) of the Chinese Academy of Sciences (CAS), collected soil samples from a nitrogen addition experiment, which was established in 2014 in a natural Korean pine and broadleaf mixed forest in Changbai Mountain.  

The researchers estimated microbial carbon use efficiency (CUE) and nitrogen use efficiency (NUE) by ¹⁸O-labelled H₂O incubation method and found that nitrogen addition and soil horizon affected microbial CUE and NUE, however no obvious change regulation was observed.

In addition, they also found a polynomial correlation between microbial CUE and NUE, with the inflection point at CUE=0.4. Regression analysis showed that the variation in microbial community structure and the phosphorus stoichiometry imbalance were two main factors that affected microbial nutrient use efficiency.  

The results indicate that phosphorus deficiency caused by nitrogen deposition may alter functions of microorganisms in order to maintain the stability of microbial communities. So far, little is known about how does phosphorus stoichiometry imbalance caused by nitrogen addition changes microbial community structure. 

In view of this, assistant Prof. XIA Zongwei and his colleagues from Prof. WANG's group conducted a nitrogen and phosphorus addition experiment, which was established in 2015 in a natural Korean pine and broadleaf mixed forest in Changbai Mountain.

The researchers measured soil microbial community structure with high-throughput sequencing technology and found that nitrogen addition reduced microbial diversity in organic soil, while phosphorus addition could lessen this effect to a large extent. Specifically, nitrogen addition reduced carbon sequestration in organic soil through affecting metabolic potential of bacteria and fungi, while phosphorus addition alleviated or change these effects.  

The results clarified the mechanisms by which nitrogen and phosphorus regulate the structure and function of soil microbial communities in the temperate forest. 

The results were published in Soil Biology & Biochemistry entitled "Stoichiometric imbalance and microbial community regulate microbial elements use efficiencies under nitrogen addition", and the journal Microorganisms entitled "Phosphorus reduces negative effects of nitrogen addition on soil microbial communities and functions".  

The studies were supported by the National Natural Science Foundation of China and the the Youth Promotion Association of CAS.