The humus layer promotes needle litter decomposition but not carbon release or phosphorus accumulation in a Pinus sylvestris var. mongolica plantation

Abstract Background The forest floor humus layer is an important carbon pool and serves as a key interface that influences forest soil carbon and nutrient cycling, especially in temperate and boreal forests. Over the past decades, China has implemented numerous forestry ecological programs, leading to an increasing quantity of forest floor litter and the formation of humus layers, which has altered the interface between aboveground litter and surface soil. Our previous study revealed that these alterations affect the litter decomposition rate; however, it is still unclear how the litter decomposition process changes, how nutrients are released or imported and the extent to which these changes depend on the humus layer. Results In this study, we used a 535-day in situ litterbag experiment to monitor the litter decomposition process and nutrient variations under forest floor humus layer removal in a Pinus sylvestris var. mongolica plantation in Northeast China. The results revealed that the litter decomposed quickly when a forest floor humus layer was present, with the decomposition rate constant ( k value) increasing from 0.122 to 0.328. Accordingly, during decomposition, the litter C, N and P concentrations increased, whereas their contents varied only slightly (with the exception of the litter P content, which decreased significantly) compared with those in the treatment where the humus layer was removed. However, both the litter C and N contents decreased, whereas the litter P content increased significantly compared with the initial litter content. Moreover, the litter C:N, C:P and N:P ratios decreased significantly during decomposition. In addition, the microbial community diversity of the litter showed no significant change, whereas the relative abundances of several major fungal and bacterial taxa at the phylum and genus levels varied significantly. Furthermore, redundancy analysis revealed effective relationships among the k values, chemical traits and microbial communities, and the least squares method suggested that the C, P and C:P ratios of the litter were significantly correlated with the litter decomposition rate. Conclusions These results enhance our understanding of the role of the humus layer in forest soil‒plant carbon and nutrient cycling and should be considered in carbon cycle models in the future.