Changes in N and C Mineralization with Compost Maturity.
Timothy Griffin, USDA-ARS-NEPSWL, University of Maine, Orono, ME 04469-5753 and Mark L. Hutchinson, University of Maine, 377 Manktown Road, Waldoboro, ME 04572.
Utilization of composted manure must balance improvements in soil properties with nutrient availability. Improved predictive relationships between compost maturity and nitrogen (N) availability would be helpful in establishing this balance. We collected eleven separate compost samples from a single windrow over a 100 d period, beginning when the compost met minimal National Organic Standards. Compost maturity was assessed using both standard analyses (total C and N, mineral N, total volatile solids) and other methods (CO2 evolution, commercial maturity kits, and neutral and acid detergent fiber). Compost N transformations were evaluated during an aerobic incubation in a sandy loam soil, sampled at 0, 7, 14, 21, 28, 56, 84, 112, and 130 after each compost was applied at 200 mg total kg-1 soil. Subsamples collected at each date were extracted in 2 M KCl, to estimate NO3 and NH4 concentration in the soil. Net mineralization of compost N was assessed for each compost, using linear and exponential models as appropriate. Compost C mineralization was assessed by measuring CO2 evolution from each compost+soil mixture in the 24 hr period prior to each soil N sampling. Regression analysis was used to evaluate the relationship between compost maturity parameters, the rate and extent of net N mineralization, and C mineralization. Commonly used maturity parameters like total C, total N, and C:N ratio were poorly correlated with the rate and extent of mineralization. The N mineralization rate during the first 70 d of incubation was strongly correlated (r= -0.70 to -0.80) to compost Maturity Index and to compost fiber concentration. Trends in C mineralization were similar. There were few differences between composts after 70 d. Methods of characterizing compost maturity that more realistically reflect the composting process are better predictors of N release after incorporation into soil.