Understanding how organic amendments influence nitrogen (N) mineralization and carbon (C) respiration is essential for improving soil health and nutrient management in agroecosystems. This study, conducted by the Spring 2025 PSC 5560 Soil Analytical Techniques class at Utah State University, investigated the effects of various organic materials on soil N mineralization and CO₂ respiration through a controlled laboratory incubation. Six treatments were applied to a homogenized loamy soil collected from 0–30 cm depth: Miller compost, softwood compost, hardwood compost, wheat straw, feather meal, and an unamended control. Each treatment was mixed with soil and incubated in sealed mason jars under moist conditions for 35 days. CO₂ evolution was monitored via headspace gas sampling and analyzed with gas chromatography. Inorganic N (NH₄⁺ and NO₃⁻) was quantified on days 0, 7, 14, 21, and 35 through KCl extraction and spectrophotometric analysis. Results showed that respiration rates and mineral N release varied by treatment, reflecting differences in organic matter quality. Feather meal, with the lowest C:N ratio (3.8), released the highest levels of mineral N early in the incubation, although it did not fit the first-order kinetic model. Wheat straw, with a high C:N ratio (80.0), showed high cumulative CO₂ evolution, suggesting substantial microbial activity despite limited N mineralization. Compost treatments exhibited intermediate responses. Rate constants (k) for both C and N mineralization differed significantly among treatments, with feather meal and wheat straw showing the fastest rates for N and C, respectively. First-order kinetic modeling provided estimates for potentially mineralizable N (No) and C (Co), highlighting the variability in amendment quality. While No did not differ significantly across treatments due to high variability, k values indicated differing mineralization dynamics. These findings underscore the importance of selecting organic amendments based on crop nutrient demands and decomposition behavior. This work demonstrates the utility of laboratory incubation assays for evaluating compost quality and forecasting nutrient release. Future directions include field-scale validation and microbial community profiling to better understand the mechanisms behind observed differences in decomposition and nutrient cycling.
