The nitrification inhibitor 3,4,-dimethylpyrazole phosphate strongly inhibits nitrification in coarse-grained soils containing a low abundance of nitrifying microbiota

Abstract

The effectiveness of the nitrification inhibitor 3,4,-dimethylpyrazole phosphate (DMPP) on sandy soils containing low nitrifying microbial abundance has not been established. Two coarse-grained soils, representative of Western Australia’s agricultural zones, were incubated with 100mgNkg–1 soil, added as either urea, urea+DMPP or urea+nitrapyrin as an alternative nitrification inhibitor for comparative purposes. Ammonium (NH4+) and nitrate (NO3–) concentrations, potential nitrification rates (PNR) and the abundance of ammonia-oxidising bacteria (AOB) and archaea (AOA) were measured over time. Interactions between soil type and inhibitor type altered the extent of nitrification observed in these soils. When N was supplied as urea alone, NH4+-N concentrations decreased from 100mgNkg–1 soil to approximately 20mgNkg–1 soil in the high nutrient soil (Williams) and approximately 60mgNkg–1 soil in the low nutrient soil (Vasse). These differences were reflected in AOB abundance, which was higher (~105genecopiesg–1 soil) in Williams soil than in Vasse soil (<104genecopiesg–1 soil). This difference could have been attributable to differences in soil pH between Williams and Vasse (5.4 vs 4.0 respectively) and/or copper (Cu) availability (~1.5 vs ~0.5mgCukg–1 soil respectively), both of which have been demonstrated to reduce AOB abundance or limit nitrification. On the Williams soil, DMPP limited nitrification, resulting in approximately 80mgNkg–1 soil being retained as NH4+-N. Nitrapyrin was similarly effective for the first 56 days of incubation, but declined considerably in effectiveness between Days 56 and 100. Changes in soil nitrification rates were accompanied by changes in AOB abundance, which was below 103genecopiesg–1 soil when nitrification was impaired. Both DMPP and nitrapyrin inhibit nitrification via chelating Cu and, because these soils contained low Cu concentrations, it may be possible that interactions between DMPP, naturally low abundance of AOB and low Cu availability facilitated the long-term inhibition of nitrification in these soils.