Root exudation patterns of Chinese fir after thinning relating to root characteristics and soil conditions

Abstract

Root exudates are crucial to belowground carbon allocation and nutrient cycling in forest ecosystems. Few studies have focused on the effects of forest management activities, such as thinning, on root exudation rates of mature woody species in subtropical regions. In this study, we aimed to determine three kinds of root exudation rates (i.e., per mass, length, and area) in situ under three thinning intensities: Control (unthinned), light thinning intensity (LIT, felled 30% of individual trees), and heavy thinning intensity (HIT, felled 70% of individual trees), in a 29-year-old Chinese fir plantation. We found that root exudation rates increased after thinning and showed significant seasonal dynamics, with the highest and lowest rates being in summer and winter, respectively. The root exudation rates increased with microbial biomass C and N. Positive relationships between the root exudation rates, number of root tips, and root vitalities were observed. The root exudation rates increased with root diameter and decreasing specific root area, suggesting that resource-conservative roots of Chinese fir were biased toward accelerating root exudation rate for nutrient acquisition, instead of optimizing root morphology. In addition, thinning reduced overall soil total carbon content in a Chinese fir plantation. The soil total C content was higher in HIT than in LIT, indicating that HIT could reduce soil C loss by root exudation rates. These findings elucidate how thinning affects fine root exudation rates by altering soil conditions and root characteristics while contributing to our ability to predict belowground C allocation and nutrient cycling in response to silvicultural treatments.