Date of Award

12-11-2025

Degree Type

Thesis

Degree Name

Master of Science (M.S.)

Department

Agricultural and Environmental Sciences

First Advisor

Jianwei Li

Abstract

Understanding the effects of multiple global change drivers, e.g., 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 (Rs) in a switchgrass (Panicum virgatum) cropland located in Middle Tennessee, U.S. 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-1yr-1). Hourly measurements of Rs, soil temperature (T), and volumetric moisture (Mv), along with biweekly SOC, TN, microbial biomass carbon (MBC) and nitrogen (MBN), and EEAs were quantified consecutively for 2 years in soil samples (0-10 cm). During the first year, warming significantly increased T, reduced Mv, and enhanced Rs by 2.62℃, 32%, and 19% respectively, but significantly reduced nitrogen acquisition enzyme (Nacq), acid phosphatase (AP), peroxidase (PER), and oxidase (OX). Relative to unfertilized treatment, N fertilization significantly reduced Rs by 14%. There were no significant main or interactive effects of warming and N on MBC, MBN, SOC, TN, and C: N. A significant negative interaction of warming and N fertilization on Rs was observed such that N fertilization suppressed Rs by 24% under warming conditions compared to unfertilized warming condition. 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 resource use. This research work based on the first-year data has been published in Global Change Biology Bioenergy (Pandey et al., 2025). The dataset for the first two years was analyzed. Results showed that warming significantly increased soil T, reduced Mv, and enhanced Rs by 3.4 ℃, 36%, and 23%, respectively, but significantly reduced nitrogen-acquiring enzyme activity (NAG and Nacq), acid phosphatase (AP), and phenol oxidase (PHO). Relative to unfertilized treatment, N fertilization significantly reduced Rs and AG activity by 12% and 6% respectively. There were no significant main or interactive effects of warming and N fertilization on MBC, MBN, SOC, TN, or C:N ratio. A significant negative interaction between warming and N fertilization on Rs was observed such that N fertilization suppressed Rs by 25% under heated conditions, but slightly increased Rs by 6% under ambient conditions. These results suggest that warming consistently enhanced respiratory C losses over two years, while N fertilization reduced this effect under heated conditions, likely through soil acidification, reduced nutrient-acquiring and oxidative enzyme activities, and shifts in microbial functional resource allocation. Our analyses provided a unique opportunity to compare the warming impacts over different durations of experiment, i.e., first year vs. first two years. Both analyses revealed consistent warming-induced stimulation of Rs, accompanied by functional reallocation within microbial communities rather than changes in biomass. Random forest and PCA analyses identified T as the dominant driver of Rs in both years. While Mv, Mg, and MBC also contributed significantly to explaining variation in Rs during the first year, substrate utilization emerged as another important variable replacing MBC in explaining the variation during the first two years. These findings highlight continuously elevated soil CO2 efflux under warming during the first two years experiment, but a contrast regarding the underlying mechanisms driving the elevated respiratory losses over the two time periods. The first-year soil efflux responses are driven by soil temperature and microbial biomass, whereas over the two years, temperature remained dominant but was accompanied by shifts toward substrate utilization.

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