Soil nitrification response to dairy digestate and inorganic ammonium sources depends on soil pH and nitrifier abundances

Abstract

AbstractTo reduce CH4 emissions, some dairy operations use manure as feedstock for anaerobic digesters, after which the nonvolatilized portion forms a digestate effluent with high NH4+ content that is used as fertilizer. When applied to soil, N fertilizers are subject to nitrification, where NH3–oxidizing archaea (AOA) and bacteria (AOB) oxidize NH3 to NO2–, NO2––oxidizing bacteria (NOB) oxidize NO2– to NO3−, and N oxides are produced from biotic and abiotic reactions. This study used soil microcosms to evaluate the properties of nitrification in response to the addition of N as digestate vs. NH4Cl in soils from the different climatic regions of the western Oregon coastal plain (CP) and eastern Oregon Columbia River basin (CRB). In each soil, properties of NH3 oxidation or N oxide accumulation did not differ when N was provided via digestate or NH4Cl over the normal temperature range; however, nitrification rates, communities, and products differed between soils. The average nitrification rate in CP soil was almost twofold higher than in CRB soil (P ≤ .001), despite no significant differences in AOB activity or AOB amoA copy numbers. With N addition, AOA activity also increased in CP soil, where AOA amoA copy numbers were fivefold higher than CRB soil (P < .002). Accumulation of NO2– only in CRB soil was likely due to NOB inhibition by high NH3 concentrations (∼560 μM) present at the high pH of CRB soil (8.5). Overall, nitrification was dictated by differences in edaphic properties, temperature, and nitrifier communities more than N source.