Effect of Litter Removal and Addition on Root Exudation and Associated Microbial N Transformation in a Pinus massoniana Plantation

In forest ecosystems, variations in aboveground litter input caused by global changes, substantially alter soil N cycling. In field-grown plants, few studies have directly measured root exudation rates and quantified their effects on N transformations under litter manipulation. We quantified soil N transformation rate responses to litter manipulation in a Pinus massoniana plantation, and unravelled the effect of root exudation on soil N transformations. We measured in situ P. massoniana root exudation rates as well as soil microbial biomass, soil C and N concentrations, the activities of four soil enzymes involved in soil N transformations, and net N mineralization and net nitrification rates after experimental litter removal and litter addition treatments. Litter removal and litter addition treatments had little impact on soil C and N concentrations, microbial biomass, soil enzyme (urease, hydroxylamine reductase, nitrate reductase, and nitrite reductase) activity, and net N mineralization rates. However, both litter removal and addition increased net N nitrification rates. Additionally, litter removal significantly decreased root C exudation rates (in April 2021 and annually), whereas litter addition had no significant effects on root C exudation rates across all seasons. Furthermore, root C exudation rates were positively associated with urease and nitrate reductase activities, but negatively associated with hydroxylamine reductase and nitrite reductase activities, as well as net N nitrification rate. Overall, we demonstrated that root exudates may be an important physiological adjustment by which trees respond to changes in litter input caused by global environmental changes, regulating underground N biochemical processes. Furthermore, we provide new evidence from root exudates for understanding the potential influence of litter inputs on soil N cycling. A strong correlation exists between root exudates and N transformation, shedding new light on the dynamics of rhizosphere nutrient cycling crucial for maintaining forest ecosystem stability and productivity under changing environmental conditions.