Base cations, K+ and Ca2+, have contrasting effects on soil carbon, nitrogen and denitrification dynamics as pH rises

Abstract 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 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.