Reduction of N2O emissions by DMPP depends on interaction of nitrogen source (digestate vs. urea) with soil properties

Inhibition of nitrification by mixing nitrification inhibitors (NI) with fertilizers is emerging as an effective method to reduce fertilizer-induced nitrous oxide (N₂O) emissions. The 3,4-dimethylpyrazole phosphate (DMPP) apparently inhibits ammonia oxidizing bacteria (AOB) more than ammonia oxidizing archaea (AOA), which dominates nitrification in alkaline and acid soil, respectively. However, the efficacy of DMPP in terms of nitrogen sources interacting with soil properties remains unclear. We therefore conducted a microcosm experiment with three typical Chinese agricultural soils with contrasting pH (fluvo-aquic soil, black soil and red soil), which were fertilized with either digestate or urea, with a range of DMPP concentrations. In the alkaline fluvo-aquic soil, fertilization with urea and digestate both induced a peak in N₂O emission (60 ug N kg⁻¹ day⁻¹) coinciding with the rapid nitrification within 3 days following fertilization. DMPP almost eliminated this peak in N₂O emission by nearly 90%, despite that the nitrification rate was only reduced by 50%. In the acid black soil, only digestate induced N₂O emission that increased gradually, reaching its maximum (20 ug N kg⁻¹ day⁻¹) after 5-7 days. Nitrification rate and N₂O emission were both marginally reduced by DMPP in the black soil, and the N₂O yield (N₂O-N per NO₂⁻+NO₃⁻-N produced) was exceptionally high at 3.5%, suggesting that the digestates induced heterotrophic denitrification. In the acid red soil, N₂O emission spiked in digestate and urea treatments at 50 and 10 ug N kg⁻¹ day⁻¹, respectively, and DMPP reduced the rate substantially by nearly 70%. Compared with 0.5% DMPP, the higher concentrations of DMPP (1.0 to 1.5%) did not exert a significantly (P<0.05) better inhibition effect on the N₂O emissions in these soils (both with digestate and urea). Our study highlights the importance of matching the nitrogen sources, soil properties and NIs to achieve a high efficiency to reduce N₂O emissions.