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

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 (N2O-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 N2O in the soil to climatically inert N2. We assessed the effect of liming on the apparent activity of N2O-R by measuring the denitrification end products, N2O and N2, 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 N2O 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 N2O + N2 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 N2O and N2 - 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 N2O (60-77%), whereas in the fluvial soil the denitrification products were mostly in the form of N2 and a much smaller portion were in the form of N2O (11-45%). Nevertheless, we found liming-induced enhancements of N2O-R of 15-20% (P < 0.05) in urine-amended, fluvial soil. We suggest some possible mechanisms that would explain such large differences in the N2O/(N2O + N2) product ratio.