Responses of soil chemical and biological properties to nitrogen addition in a Dahurian larch plantation in Northeast China

Soil microbial properties play a key role in belowground ecosystem functioning, but are not well understood in forest ecosystems under nitrogen (N) enrichment. In this study, soil samples from 0–10 cm and 10–20 cm layers were collected from a Dahurian larch (Larix gmelinii Rupr.) plantation in Northeast China after six consecutive years of N addition to examine changes in soil pH, nutrient concentrations, and microbial biomass and activities. Nitrogen addition significantly decreased soil pH and total phosphorus, but had little effect on soil total organic carbon (TOC) and total N (TN) concentrations. The NO ₃ ^? -N concentrations in the two soil layers under N addition were significantly higher than that in the control, while NH ₄ ⁺ -N concentrations were not different. After six years of N addition, potential net N mineralization and nitrification rates were dramatically increased. Nitrogen addition decreased microbial biomass C (MBC) and N (MBN), and MBC/TOC and MBN/TN in the 0–10 cm soil layer, but MBC/MBN was increased by 67% in the 0–10 cm soil layer. Soil basal respiration, microbial metabolic quotient (qCO₂), and β-glucosidase, urease, acid phosphomonoesterase and nitrate reductase activities in the two soil layers showed little change after six years of N addition. However, soil protease and dehydrogenase activities in the 0–10 cm layer were 41% and 54% lower in the N addition treatment than in the control, respectively. Collectively, our results suggest that in the mid-term N addition leads to a decline in soil quality in larch plantations, and that different soil enzymes show differentiated responses to N addition.