Carbon and Nitrogen Dynamics During Incubation of Manured Soil

Abstract

Denitrification N losses during manure net mineralizable N assays may lead to miscalculation of the manure's N‐supplying capacity. In this study we measured denitrification, manure properties, gas fluxes, nutrient pools, and mineralizable N during laboratory incubation of manured soil. Different dairy manures (n = 107) were added to soil at a rate of 0.1 mg N g−1 Manured and control soils were incubated and sampled weekly for soil mineral N, CO2 flux, and N2O flux. The denitrification enzyme activity (DEA) was measured at the end of the experiment. Weekly N2O and CO2 production increased in the manured soils during the first 3 wk of incubation. There was a positive correlation between added manure C and cumulative CO2 production. Nitrate content increased in all soils throughout the 6‐wk period, but the increase was more marked in the manured soils. In most manured soils, ammonium concentration was initially high then declined rapidly during the first 2 wk. This high net NH4+ decline in the manured soils suggests that N was immobilized during the incubation. Microbial biomass N should be determined during manure mineralization assays to account for all potential manure N sinks. No correlation existed between DEA and N pools or gas fluxes in the manured soils. Manures with negative N mineralization had an average C/N of 19.0, while manures with positive N mineralization had an average C/N of 16.0. On average, denitrification accounted for approximately 5% of the added manure N. Higher proportions of denitrified N were observed in some manures, supporting our hypothesis that N losses through denitrification may be significant in manure mineralizable N assays.