Dissolved Organic Matter as Affected by Forms and Doses of Nitrogen Applied to Soils of Temperate Deciduous Forests in China

Abstract

Dissolved organic matter (DOM) is an important component in the biogeochemical cycles of elements like nitrogen (N) and carbon. The aim of this study was to elucidate the effect of long-term inorganic N addition on the quantity and quality of DOM in forest soils. A field study was conducted on three forms of inorganic N, namely (NH4)2SO4, NH4NO3, and NaNO3, applied at low (50 kg N ha−1) or high (150 kg N ha−1) annual doses from 2011 to 2019. The total dose was split into eight equal monthly doses applied during the growing season (from March to October). Both the form and the dose increased the content of dissolved organic carbon (DOC) in soil, the strongest effect being that of NaNO3. However, the higher dose had a weaker effect because of N enrichment. UV-visible (UV-vis) and excitation-emission matrix (EEM) spectroscopy showed that the addition of N made DOM more aromatic and increased the degree of humification. EEM-parallel factor analysis (PARAFAC) modelling suggested that DOM in the forest soils mainly contained a fulvic-like constituent (C1), humic-like substances (C2), and aromatic protein-like components (C3). The addition did not change the position of the DOM fluorophore in the soil but affected the proportions of the three PARAFAC-derived components (increasing those of C1 and C2 but decreasing that of C3), indicating that long-term addition of N may amplify the decrease in protein-like constituents of surface soil. Hence, N addition increased the complexity of the DOM structure.