Abstract:Carbon inputs from litter and roots to soil are key processes for nutrient cycling in forest ecosystem. Changes in carbon input pathways and quantities can directly affect the carbon sources to soil microorganisms and soil carbon sink function. However, there is debate regarding the effects of the carbon inputs on soil microbial carbon metabolism. In this study, a field experiment involving the control of litter input pathways (aboveground vs. underground) and quantities (doubled or removed vs. control), implemented through the addition or removal of litter and the removal of roots, was conducted in a natural secondary oak forest (35°10'-36°00' N, 117°35'-118°20' E) in the Yimeng mountainous area, Shandong Province, China. The experiment commenced in November 2014 and had a randomized block design with five different treatments (control, litter doubling, litter removal, root removal, and no detritus inputs by removing both litter and root) and five replicates for each treatment. On August 13th, 2016, the soils were sampled at a depth of 0-10 cm to analyze soil microbial carbon metabolism. Soil microbial carbon metabolism was based on community-level physiological profiles (CLPPs) using Biolog Eco MicroPlate cultivation method. Each soil sample (10-3 soil dilution) was inoculated 3 times, and cultivated at 25℃ for 240 h. The plates were scanned at 590 nm every 12 h. The litter doubling treatment increased the soil microbial carbon metabolism function, and increased the microbial utilization of sugars and amines. In contrast, the litter removal, root removal, and no input treatments reduced microbial metabolism of carbon sources, and decreased the microbial ability to utilize sugars, amino acids, carboxylic acids, amines, and polymers. Compared with root removal treatment, litter removal treatment resulted in a greater decrease in the microbial metabolism of carbon, indicating that litter exerted a greater influence on the carbon metabolism in this study. However, when considering the effects of trenched roots, the relative effects of litter and root might change. The redundancy analysis was used to analyze the relationship between soil microbial carbon metabolism and soil physiochemical characteristics including pH, contents of moisture, organic carbon, total nitrogen, ammonium, nitrate, and available phosphorus. The redundancy analysis revealed that the soil organic carbon and ammonium nitrogen contents significantly influenced the community-level physiological profiles of soil microbes (P<0.05). Spearman correlation analysis showed that soil organic carbon and ammonium nitrogen contents were positively correlated with carbon metabolism function. The ordination diagram for the redundancy analysis of the microbial community level physiological profiles and soil physicochemical properties indicated that the litter doubling treatment was distributed in the directions of increasing ammonium nitrogen and organic carbon contents, whereas the litter removal and no input treatments were located in the opposite direction. This indicated that litter doubling treatment increased the soil microbial carbon metabolism function by increasing the contents of ammonium nitrogen and organic carbon, whereas litter removal and no input treatments reduced the carbon metabolic function by decreasing the contents of ammonium nitrogen and organic carbon. The results of the study further deepen our understanding of the influences of carbon input pathways (aboveground litter vs. underground roots) and quantities (litter doubling or litter removal vs. control) on soil carbon metabolism processes in natural secondary forests in temperate zones.