Copiotrophic taxa in pig manure mitigate nitrogen limitation of soil microbial communities

Microbial nitrogen (N) limitation is a common problem in terrestrial ecosystems. Pig manure, a type of solid waste, is increasingly applied to improve soil N availability in agriculture through inputs of organic matter and inorganic N. Pig manure application also introduces a lot of exogenous microorganisms, which have distinctly different N requirements and metabolic properties, into the resident soil microbial community. However, the impacts of these manure-borne microorganisms on soil N cycling have not been well determined. Here, we investigated effects of manure-borne microorganisms on the N limitation of soil microorganisms using an ecoenzymatic stoichiometry analysis. We monitored microbial communities over a 90-day period in a laboratory-controlled experiment with four treatments: (1) non-sterilized soil mixed with non-sterilized manure (S-M), (2) non-sterilized soil mixed with sterilized manure (S-sM), (3) sterilized soil mixed with non-sterilized manure (sS-M), and (4) non-sterilized soil without manure addition (S, the control). The microbial N limitations were significantly mitigated in both S-M and sS-M. By contrast, the S-sM and S showed high levels of microbial N limitation, likely stemming from differences in the microbial functional composition. We found chitin-degrading bacteria were the dominant copiotrophic manure-borne bacteria associated with N mineralization, and they may improve soil N availability. We further identified several copiotrophic manure-borne bacteria in S-M and sS-M, and their abundances had significantly negative correlation with the level of N limitation and significantly positive correlation with the stoichiometric homeostasis. As these copiotrophic taxa can maintain homeostasis through regulating enzymatic activities, our results indicate that copiotrophic taxa in pig manure contribute to the mitigation of soil microbial N limitation. Our study also highlights the invasiveness capacity of manure-borne microorganisms in soil and evaluates the biotic effects of manure application on soil N cycling.