[Effects of Different Nitrogen Addition on Soil Labile Organic Carbon and Environmental Factors in Temperate Forests...

The labile components （LOC） of soil total organic carbon （TOC）， easily oxidizable carbon （EOC）， and dissolved organic carbon （DOC） are extremely sensitive to the changes of soil environment. Numerous studies have demonstrated that the increase of atmospheric nitrogen （N） deposition significantly affects the contents of soil LOC. As the difference in the chemical properties of various types of N fertilizers， however， their effects on LOC may be different. In this work， a typical temperate forest soil was selected， while sodium nitrate （NaNO₃）， ammonium nitrate （NH₄NO₃）， ammonium chloride （NH₄Cl）， and urea [CO（NH₂）₂] were added for four years at the rate of 10 g·（m²·a）^-1. Then， soil LOC， physicochemical properties， and extracellular enzymatic activity were assayed to compare the effects of different N fertilizers on soils. The results showed that the contents of EOC and DOC and their proportion in TOC decreased significantly after N addition （P<0.05）. Among them， the effect of NH₄Cl was the most significant （the reductions of EOC/TOC and DOC/TOC were -80.0% and -69.8%， respectively）， followed by NaNO₃， while LOC changed less after the addition of NH₄NO₃ and CO（NH₂）₂. The random forest model revealed that soil physicochemical properties， specifically pH and total nitrogen （TN）， along with extracellular enzymes involved in LOC decomposition （invertase and β-glucosidase）， constituted the crucial determinants driving the alterations in the proportion of LOC allocation. After the addition of NH₄Cl， the pH decreased by 0.8 units. The activities of soil invertase and β-glucosidase increased by 22.3% and 69.1%， respectively. These accelerated the decomposition of LOC components， resulting in a decrease in the ratios of EOC/TOC and DOC/TOC. Nitrate was easily leached， and its effects on total inorganic N （TIN） content were minimal. Furthermore， the degree of soil acidification was weak after the addition of CO（NH₂）₂， resulting in limited effects on soil extracellular enzymatic activities. In summary， nitrogen addition could affect the allocation proportion of LOC， yet the responses to different nitrogen sources were not entirely consistent. Therefore， when accurately assessing， predicting， and analyzing the effects of N deposition on soil LOC， the effects of N types should be considered.