Nitrogen and carbon mineralization of potential manure components

Current estimates of plant availability of manure N are highly variable. A study was conducted to examine C and N mineralization characteristics of ma nure components from feeds (immature and mature alfalfa and orchardgrass, s oybean meal, roasted soybean meal), forage cell walls (acid detergent fiber [ADF] and neutral detergent fiber [NDF]), simple N compounds (urea, amino acids and peptides), and nondietary metabolic components (ruminal bacteria and colonic cells). Nitrogen and C mineralization were determined by aerobi c incubation of sample-amended soil. Changes in NH4+ and NO3- concentration and CO2 production were monitored over 112 d. At C to N ratios below 15, t here was a declining exponential relationship between the initial potential rate of C mineralization and the sample C to N ratio. However, at C to N r atios above 15, the initial potential rate of C mineralization was low and insensitive to C to N ratio. Percent of added N mineralized ranged from 92% for urea and 13% for mature orchardgrass to a net N immobilization for ADF - and NDF-amended soils. When the C to N ratio was below 42, there was a li near relationship between readily mineralizable N and the C to N ratio. The re were also positive relationships between readily mineralizable C and N a nd the components' total N concentration. Manure contains a range of compou nds that have rapid or intermediate N mineralization characteristics, or th at are strong immobilizers of N. These results suggest that improved estima tes of manure N mineralization may be obtained by considering both the read ily available N components and components that strongly immobilize N.