Laboratory methods for estimating available nitrogen in manures and sludges

A soil incubation experiment was conducted to determine nitrate N(NO₃−N) accumulation rates and available N (AAN_t) values for stored liquid manures from dairy cattle and swine, various poultry manures and a municipal sewage sludge applied to Norfolk and Cecil soils. Potentially available N (PAN_t) in the manures and sludge was also estimated by chemical tests which included an acid permanganate extraction, weak acid extraction, strong acid extraction and pepsin digestion. The resulting PAN_t values were compared to the AAN_t values from the soil incubation study.

The rates in which NO_3-N accumulated in soils treated with waste were described using first-order kinetics. The rate constants were highest for soils treated with poultry manure which contained high initial concentrations of ammonia N (NH₃-N) and rapidly mineralizable ORG-N. The lowest rate constants were for soils treated with municipal sludge which contained low concentrations of NH₃-N.

Incubation tests showed significant reductions in inorganic N (IN-N) in the first 2 weeks, which suggested immobilization and/or possibly denitrification or NH₃-N volatilization. After 26 weeks' incubation, AAN_t varied with waste type and soil type, being from 40% to 67% of total Kjeldahl N (TKN) applied to a Norfolk sandy soil and from 17% to 38% for a Cecil sandy loam.