Nitrogen Addition Enhances CO2 Mineralization Under Drying–Rewetting in Road-Disturbed Alpine Meadow Soils

Road construction can remove or redistribute organic-rich topsoil and disturb underlying subsoil, but its short-term C mineralization responses to repeated drying–rewetting remain poorly understood. We incubated topsoil and subsoil from a road-disturbed alpine meadow in Zoige for 67 days under three N-addition levels (0, 10, and 20 mg N per 60 g dry soil; 0, 167, and 333 mg N kg−1 dry soil) and four moisture regimes: constant moisture near 60% water-holding capacity (WHC) and drying–rewetting treatments targeting minimum WHC values of 40%, 30%, or 20%. Across sealed incubation intervals, cumulative CO2 release per unit dry soil was consistently higher in topsoil. After conversion to CO2-C and normalization to layer-specific mean initial SOC, however, cumulative release represented 1.59–2.03% of initial SOC in topsoil and 4.33–5.58% in subsoil. N addition generally increased cumulative CO2 release, but the response depended on moisture regime. The largest N-related responses were often observed under WHC30, although WHC30 did not consistently exceed WHC20 across N levels in topsoil. Drying–rewetting generally reduced microbial biomass, whereas β-glucosidase activity tended to be higher under lower WHC and in response to N addition. Short-term C mineralization therefore varied with soil layer, moisture regime, and N addition, and the layer contrast depended on the basis used to express C release.