Effects of mixed coniferous and broad-leaved litter on bacterial and fungal nitrogen metabolism pathway during litter decomposition

Abstract

Litter decomposition and nitrogen metabolism are the key determinants of nutrient cycling in forest ecosystems. However, how processes and functional groups are affected by mixed litter decomposition is poorly understood. The aim of this research is to determine how the microbial functional pathways involved in nitrogen cycling varied according to the changes in the microbial community composition induced by mixing of litter. The fallen leaf litters were collected on days 60, 150, 270, and 360 in Larix, Sassafras, and Larix/Sassafras plantations. The nitrogen properties, enzyme activities, microbial communities, and nitrogen metabolism pathways were evaluated during decomposition of three litter types. The pH, nitrate content, and organic nitrogen degradative enzyme activities of mixed litter were higher than those of Larix litter. Mixed litter promoted the abundances and potential nitrogen metabolism functions of Sphingomonas, and Janthinobacterium versus Larix litter during decomposition. The abundances of genes associated with microbial organic nitrogen degradation and assimilatory nitrate reduction differed significantly according to litter types. The abundance of microbial functional genes related to the production of ammonium was significantly higher in mixed litter than in Larix litter. Co-occurrence network analyses showed that mixed litter had a less complex but more stable microbial co-occurrence pattern versus monospecific litter. The difference of litter pH and nitrate content between mixed litter and Larix litter are determinants of changes in microbial functional potentials of nitrogen metabolism. Mixing Sassafras/Larix litter would selectively modulate nitrogen metabolism related bacterial groups, together with functional pathways of organic nitrogen degradation and assimilatory nitrate reduction processes. These changes were mainly driven by litter pH and nitrate content.