A γ-Polyglutamic Acid–Conjugated Nitrification Inhibitor with Enhanced Thermal Stability and Sustained Inhibition of Soil Nitrification

3,4-Dimethylpyrazole phosphate (DMPP) is an effective nitrification inhibitor, but its poor thermal stability limits its performance in fertilizer formulations and soil systems. In this study, DMPP was conjugated with γ-polyglutamic acid (γ-PGA) via a proposed covalent linkage to enhance its stability and nitrification inhibition efficacy in soil. The optimized DMPP-γ-PGA conjugate formed a compact three-dimensional network with a DMPP loading of 85.1%. Thermal analyses showed that γ-PGA conjugation reduced DMPP loss by 72.6% after heating at 200 °C for 30 min. Soil incubation experiments showed that the DMPP-γ-PGA conjugate (1:10) sustained nitrification inhibition (average 46.8% over 21 days) and reduced cumulative N 2 O emissions by 50.1% relative to urea alone, outperforming free DMPP at the same application rate (45.4%). The improved performance was attributed to enhanced thermal stability and controlled release of DMPP in soil. This study provides a biodegradable polymer–inhibitor conjugation strategy for improving nitrification inhibitor functionality in agricultural systems.