Latitudinal pattern of soil lignin/cellulose content and the activity of their degrading enzymes across a temperate forest ecosystem

Abstract

Temperate mixed forests, along with other high latitudinal ecosystems, are more vulnerable to global warming in comparison with warm sites, because of the slower carbon (C) turnover and higher soil organic carbon (SOC) accumulation. Lignin and cellulose are two major components of plant litter, and usually make contributions to the recalcitrant and labile SOC pool, respectively. Because the chemical composition of SOC plays key role in regulating the bioavailability of soil C pool, understanding the relationship between soil lignin or cellulose content and temperature are of great significance in evaluating the feedbacks between SOC pool and the future scenarios of global warming. The biological degradation of soil lignin or cellulose is mainly dependent on soil enzymatic activities, and thus, the response of ligninolytic and cellulolytic enzymes to increased temperature would determine C release under future warming scenarios. However, the responses of the soil lignin/cellulose content and the activity of cellulolytic and ligninolytic enzymes to increased mean annual temperature (MAT) have rarely been studied, and the factors driving these changes are not fully understood. Latitudinal gradients are often used for monitoring global-warming-related problems, because of its natural gradients of temperature. In this study, we demonstrate the latitudinal pattern of lignin/cellulose content and the activities of cellulose- and lignin- degrading enzymes in a temperate Broad-leaved Korean pine mixed forests distributed along a latitudinal gradient (with MAT ranging from −1.9 to 5.1 °C) in northeastern China. The linear mixed model revealed that soil lignin content was negatively correlated with MAT (Slope = −7.604, t = −2.608, P = 0.011), whereas soil cellulose content showed no response to increased MAT. The activity of soil polyphenol oxidase (PPO), one of the enzymes catalyze lignin decomposition, was higher in high-latitude sites, in contrast, the activity of the cellulase (CEL) complex was higher in low-latitude plots. Structural equation model (SEM) analysis indicates that MAT can directly influence soil lignin or cellulose content, and indirectly through changing NRCB, plant litter C/N, microbial biomass, and degrading enzymatic activities. The value of soil lignin/(lignin + cellulose) ratio and soil lignocellulose index (LCI, lignin/(lignin + holocellulose) ratio), varied between 0.8–0.9, and 0.6–0.8, respectively, indicating that the SOC pool in this temperate ecosystem is dominated by recalcitrant components. The negative correlations between MAT and LCI, soil lignin/(lignin + cellulose), and log (PPO + PER)/log(CEL) (Slope = −0.008, t = −2.363, P = 0.021; Slope = −0.004, t = −3.134, P = 0.003, and Slope = −0.057, t = −4.477, P < 0.001, respectively) suggested that the recalcitrance of SOC would decrease with elevated MAT. Thus, we propose the content and the proportion of recalcitrant carbon in soil organic matter will decrease under the projected global warming, and thus, the temperature sensitivity of SOC decomposition will accordingly to be predicted to decline. This may have consequences on SOC stability in this temperate forest ecosystem.