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Full denitirifcation of NO 3 − to N 2 is most likely when soil pH is elevated and oxygen levels are low. In the period immediately following urine deposition, soil pH and dissolved organic carbon (DOC) concentrations rapidly increase, coupled with O 2 consumption - conditions conducive to denitrification and gaseous N-loss - yet the potential for rapid denitrification during this early period post urine deposition has received little attention. In addition, recent research has shown that the amount of DOC (and N) released as pH increases is affected by the valence of the accompanying base cation, with monovalent cations releasing more. Liming has been used to positively influence N 2 O/(N 2 O + N 2 ) ratios in the later stages of N-transformations post urine deposition, yet the relative amounts of DOC (and N) released is likely to be affected in the presence of divalent Ca 2+ potentially lowering N-transformation potential. Soil pH in anaerobic microcosms was adjusted to values between 4.5 and 9.5 using either Ca(OH) 2 as a liming agent, or KOH as an analog of urine derived NH 4 OH. After 16 h incubation, up to ∼6800 μg g −1 of DOC and ∼480 μg g −1 of dissolved organic nitrogen (DON) was solubilized by KOH amendment which was ∼10-fold higher than Ca(OH) 2 ammendment, while NH 4 + and NO 3 − increased to concentrations well above native soil concentrations. In Ca(OH) 2 amended microcosms, denitrification enzyme activity (DEA) assays produced a maximum of 875 ng N 2 O-N g −1 h −1 at pH 7.2 with respiration rates <10 μg CO 2 -C g −1 h −1 . In contrast, DEA and respiration continually climbed in KOH amended microcosms with rates up to 4798 ng N 2 O-N g −1 h −1 and 47 μg CO 2 -C g −1 h −1 at pH 8.7 and 8.9, respectively. By using KOH as proxy for NH 4 OH derived from urea hydrolysis, this research has shown that rapid denitrification could occur shortly after urine deposition along with substantial DOC and DON solubilisation and N-mineralisation. However, elevated soil pH combined with excess Ca 2+ from liming might actually suppress denitrification potential. We propose that excess Ca 2+ affects how soil microbes access and process DOC and DON at elevated pH, with Ca 2+ acting to stabilize DOC and DON. Our results emphasize that although liming promotes soil conditions conducive to denitrification, DEA can be markedly affected by base type and prevailing soil chemical conditions. [ABSTRACT FROM AUTHOR]

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 < 0.05) in urine-amended, fluvial soil. We suggest some possible mechanisms that would explain such large differences in the N 2 O/(N 2 O + N 2 ) product ratio. [ABSTRACT FROM AUTHOR]

Abstract: Nitrous oxide (N2O) emissions in semi-arid regions are often greater following summer rainfall events when the soil is fallow, than in response to N fertiliser applications during crop growth. Nitrogen fertiliser management strategies are therefore likely to be ineffective at mitigating N2O emissions from these cropped agricultural soils. Here we examined the influence of raising soil pH on N2O emissions, nitrification rates, and both nitrifier and denitrifier populations following simulated summer rainfall events. The soil pH was raised by applying lime to a field site 12 months before conducting the laboratory experiment, resulting in soil of contrasting pH (4.21 or 6.34). Nitrous oxide emissions ranged from 0 when the soil was dry to 0.065 μg N2O–N g dry soil−1 h−1 following soil wetting; which was attributed to both denitrification and nitrification. Increasing soil pH only decreased N2O emissions when losses were associated with nitrification, and increased amoA gene copy numbers. We propose increasing soil pH as a strategy for decreasing soil N2O emissions from acidic soils following summer rainfall in semi-arid regions when emissions result from nitrification. [Copyright &y& Elsevier]