Nitrogen fertilization changes the molecular composition of soil organic matter in a subtropical plantation forest

The biochemical mechanisms responsible for soil organic C (SOC) accumulation under N enrichment are not well understood. By examining two N types (NH4Cl and NaNO3) and three rates (0, 40, and 120 kg N ha(-1) yr(-1)), we investigated the concentration and distribution of SOC by using soil aggregate separation and physical fractionation of soil organic matter (SOM). The molecular composition of SOM within bulk soil and each physical fraction was determined by pyrolysis-gas chromatography-mass spectrometry. Nitrogen fertilization led to soil NO3(-)-N accumulation and intensified soil acidification. Four years of N fertilization did not change total SOC concentrations in the 0- to 20-cm depth but a high rate of NaNO3 addition increased the concentrations of fine particulate organic C and microaggregate-associated organic C by 133 and 83.4%, respectively. The change in SOC concentration was positively correlated with the change in fine particulate organic C and mineral-associated organic C concentrations. Four years of N fertilization significantly changed the molecular composition of SOM, with an increase in the relative abundance of furfural and acetic acid and a decrease in pyrrole in the mineral-associated organic matter (MAOM) fraction. Fertilization increased the furfural/pyrrole and aliphatic/aromatic ratios in the SOM and MAOM fractions, suggesting decreased stability of the passive SOM fraction. The SOC concentration was negatively correlated with the benzene/toluene ratio in both bulk soil and the MAOM fraction. Overall, exogenous N input to this subtropical plantation forest decreased the mineralization and humification of SOM, which could be detrimental to soil C accumulation.