Soil Enzyme Activity Regulates the Response of Soil C Fluxes to N Fertilization in a Temperate Cultivated Grassland

Exogenous nitrogen (N) inputs greatly change the emission and uptake of carbon dioxide (CO2) and methane (CH4) from temperate grassland soils, thereby affecting the carbon (C) budget of regional terrestrial ecosystems. Relevant research focused on natural grassland, but the effects of N fertilization on C exchange fluxes from different forage soils and the driving mechanisms were poorly understood. Here, a three-year N addition experiment was conducted on cultivated grassland planted with alfalfa (Medicago sativa) and bromegrass (Bromus inermis) in Inner Mongolia. The fluxes of soil-atmospheric CO2 and CH4; the content of the total dissolved N (TDN); the dissolved organic N (DON); the dissolved organic C (DOC); NH4+–N and NO3−–N in soil; enzyme activity; and auxiliary variables (soil temperature and moisture) were simultaneously measured. The results showed that N fertilization (>75 kg N ha−1 year−1) caused more serious soil acidification for alfalfa planting than for brome planting. N fertilization stimulated P-acquiring hydrolase (AP) in soil for growing Bromus inermis but did not affect C- and N-acquiring hydrolases (AG, BG, CBH, BX, LAP, and NAG). The oxidase activities (PHO and PER) of soil for planting Bromus inermis were higher than soil for planting Medicago sativa, regardless of N, whether fertilization was applied or not. Forage species and N fertilization did not affect soil CO2 flux, whereas a high rate of N fertilization (150 kg N ha−1 year−1) significantly inhibited CH4 uptake in soil for planting Medicago sativa. A synergistic effect between CO2 emission and CH4 uptake in soil was found over the short term. Our findings highlight that forage species affect soil enzyme activity in response to N fertilization. Soil enzyme activity may be an important regulatory factor for C exchange from temperate artificial grassland soil in response to N fertilization.