NITRAPYRIN DELAYS DENITRIFICATION ON MANURED SOILS.

Excessive application of manure may lead to NO3- leaching to groundwater and fluxes of nitrogen oxides to the atmosphere. Nitrification inhibitors such as nitrapyrin (N-serve; 2-chloro-6-(trichloromethyl)pyridine) may help to conserve manure N in the root zone by limiting NO3- supply to denitrifiers. The objective of this study was to test the effect of nitrapyrin on the timing and amounts of denitrification and N2O fluxes in manured soils under conditions favorable to denitrification. The study consisted of a laboratory incubation of soils under aerobic conditions. Three agricultural soils and a sand were included in the study, all with high moisture and initial NO3--N content. Each soil received three treatments: 1) manure plus nitrapyrin (190 mg nitrapyrin kg-1 soil), 2) manure alone (0.15 mg manure N g^-1), and 3) soil alone controls. Nitrapyrin was mixed with the manure before addition to soil. Destructive samplings were carried out weekly for 10 weeks. At each sampling, soil-extractable mineral N, microbial biomass N, denitrified N, and N2O fluxes were measured. Nitrapyrin was effective in reducing nitrification, thus enhancing soil NH4^+-N accumulation and possibly reducing the potential for nitrate leaching. Although nitrapyrin was effective in reducing nitrification in manured soils, the effect on soil mineral N and potential N supply to plants varied across soils because of the interaction between nitrification, denitrification, and N immobilization. Neither manure nor nitrapyrin consistently affected net N mineralization in the five different soil types. Microbial N immobilization and/or denitrification were strong sinks of N that reduced net N mineralization. Nitrapyrin did not affect cumulative denitrification, but some soils had delayed denitrification when nitrapyrin was added. Manure had a strong effect on N2O fluxes and denitrified N in some soils, but the effects of nitrapyrin were inconsistent. Nitrapyrin significantly reduced microbial N immobilization in two agricultural soils. The observed reductions in microbial biomass may affect N availability beyond the time frame of the experiment because less N will be available for remineralization.