Characteristics of Dissolved Organic Matter and Dissolved Lignin Phenols in Tropical Forest Soil Solutions during Rainy Seasons and Their Responses to Nitrogen Deposition

Dissolved organic matter (DOM) in soils plays a crucial role in biogeochemical cycles during rainy seasons in forest systems. However, spatiotemporal variations in DOM characteristics in forest soil solutions during the rainy season and their responses to increased nitrogen deposition are unclear. Using optical spectroscopy, dissolved lignin phenol analysis, and stable carbon isotope analysis, we characterized DOM in soil solutions collected in May, July, and September at different soil depths (20 and 40 cm) from two tropical plantations with three N addition levels (0, 50, and 100 kg N ha–1 yr–1). Although the dissolved organic carbon (DOC) concentration did not significantly differ between depths, the aromaticity and δ13C values of DOM and dissolved lignin phenol levels were lower at 40 cm than at 20 cm, which could be attributed to the preferential mineral adsorption or microbial consumption of dissolved lignin and light carbon isotopes in the layer between 20 and 40 cm. The DOC concentration decreased and DOM aromaticity (indicated by specific ultraviolet absorbance at 254 nm) increased from May to September, possibly due to increasing consumption of nonaromatic DOM and dilution of DOC in the middle and late rainy seasons. However, 8 years of nitrogen addition did not significantly alter the DOM concentrations or characteristics in the soil solution. These findings enhanced our understanding of the dynamics of DOM quantity and quality in tropical plantations and their drivers under global environmental change.