Can pH amendments in grazed pastures help reduce N2O emissions from denitrification? – The effects of liming and urine addition on the completion of denitrification in fluvial and volcanic soils

Abstract Soil pH plays a critical role in determining the overall rate of several important processes in the agricultural nitrogen cycle. During denitrification, the activity of nitrous oxide reductase (N 2 O-R) is reduced at low pH. This effect has led to suggestions that soil pH adjustment via liming to enhance the activity of this enzyme might be a viable agricultural greenhouse gas mitigation strategy by enhancing the reduction of N 2 O in the soil to climatically inert N 2 . We assessed the effect of liming on the apparent activity of N 2 O-R by measuring the denitrification end products, N 2 O and N 2 , in a series of short-term anaerobic incubations. We compared a weakly-buffered fluvial soil and a well-buffered, volcanic soil under different incubation temperatures and in the presence or absence of a ∼600 kg ha −1 cow urine-N amendment. Our results indicated that the liming effect was heavily modulated by soil type, temperature, and urine amendment. Liming (at rates of 1.5 and 3.0 t ha −1 for the volcanic soil and at rates of 5 and 10 t ha −1 for the fluvial soil) caused pH increases of between 0.43 and 1.25 pH units. The highest reductions in N 2 O in the fluvial soil occurred when the 1.5 t ha −1 rate was used in the fluvial soils under urine addition and at the higher temperature. The combined flux of N 2 O + N 2 did not change with liming. However, an interaction of soil type and urine amendment caused large differences in the partitioning of the denitrification end-products between N 2 O and N 2 – an effect that overwhelmed the relatively modest effects of liming. When the soils were amended with urine-N, the resulting denitrification gases from the volcanic soil were mostly in the form of N 2 O (60–77%), whereas in the fluvial soil the denitrification products were mostly in the form of N 2 and a much smaller portion were in the form of N 2 O (11–45%). Nevertheless, we found liming-induced enhancements of N 2 O-R of 15–20% (P  2 O/(N 2 O + N 2 ) product ratio.