Annual dynamics of plant litter calcium and magnesium stocks in a subtropical forest headwater stream

Forest headwater streams serve as critical interfaces between terrestrial forests and downstream aquatic ecosystems, playing essential roles in the storage and movement of carbon (C) and nutrients. However, despite their importance, our understanding of the dynamics of plant litter calcium (Ca) and magnesium (Mg) stocks within these streams remains limited. In this study, we conducted a quantitative analysis of the spatiotemporal dynamics of plant litter Ca and Mg concentrations and stocks in a subtropical forest headwater stream from March 2021 to February 2022. We found that: (1) the average concentrations of litter Ca and Mg were 9.9 and 0.7 mg/g, respectively, with mean stocks of 8 792.3 and 620.8 mg/m2, respectively; (2) significant variations in litter Ca and Mg concentrations were observed among non-woody debris (13.1 and 0.9 mg/g), fine woody debris (9.0 and 0.5 mg/g), and coarse woody debris (6.1 and 0.4 mg/g), though plant litter type did not significantly affect the stocks of Ca and Mg; and (3) the stocks of Ca and Mg were positively correlated with factors such as rainfall amount, rainfall frequency, water temperature, flow velocity, water depth, electrical conductivity, and discharge, while negatively correlated with stream water alkalinity and dissolved oxygen levels. These findings highlight the critical role of plant litter in headwater streams as a component of forest nutrient stocks and provide empirical support for incorporating headwater streams into the assessment of nutrient stocks and fluxes in forest ecosystems.