Interactive Effects of Warming and Nitrogen Fertilization on Soil Organic Carbon, Total Nitrogen, Soil Respiration, and Microbial Activities

ABSTRACT Understanding the effects of multiple global change drivers, for example, warming (W) and nitrogen (N) fertilization, is critical for accurately predicting ecosystem responses to climate change. This study investigates the main and interactive effects of soil warming and N fertilization on soil organic carbon (SOC), total nitrogen (TN), microbial biomass, extracellular enzyme activities (EEAs), and soil respiration (R s ) in a switchgrass ( Panicum virgatum ) cropland located in Middle Tennessee. The field experiment employed a split‐plot design with two levels of warming (ambient and heated) and two levels of N fertilization (zero and 168 kg N ha −1 year −1 ). Hourly measurements of R s , soil temperature (T), and volumetric moisture (M v ), along with biweekly SOC, TN, microbial biomass carbon (MBC) and nitrogen (MBN), and EEAs in soil samples (0–10 cm) were quantified consecutively for 1 year. Warming significantly increased T by 2.92°C, reduced M v by 32%, and enhanced R s by 19%, respectively, but significantly reduced nitrogen acquisition enzyme ( N acq ), acid phosphatase ( AP ), peroxidase ( PER ), and oxidase ( OX ). Relative to unfertilized treatment, N fertilization significantly reduced R s by 14%. There were no significant main or interactive effects of warming and N fertilization on MBC, MBN, SOC, TN, and C:N. A significant negative interaction of warming and N fertilization on R s was observed, such that N fertilization suppressed R s by 24% compared to unfertilized treatments under warming conditions. In addition, a significant negative interaction on PER and a significant positive interaction on AP were observed. These results suggest that warming significantly enhanced soil respiratory C losses, while N fertilization reduced this warming impact, likely through reduced enzyme activity and shifts in microbial community function and resource use. The need for long‐term field trials is imperative to elucidate the mechanisms underlying these responses under the combined warming and N fertilization, particularly in bioenergy croplands.