Changing pH shifts the microbial sourceas well as the magnitude of N2O emission from soil
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Here, we examine the effect of long-term pH differences and short-term pH change on N2O emissions from soil, and the microbial source (ammonia oxidation versus denitrification) of 15N-N2O emissions. 15N-fertiliser (20 g N m−2; 10 atom% excess 15N) was applied to (1) a silt loam soil of pH 7 held at 50% and 65% water-filled pore space (WFPS) (experiment 1) and (2) a loamy sand soil maintained at pH 4.5 and pH 7 for over 40 years (experiment 2). Soils were limed with CaCO3 or acidified with H2SO4, and comparisons were made with unadjusted soils. Ammonia oxidation was the main microbial source of 15N-N2O in soils limed to pH 7.0–8.1, unadjusted pH 7.1 (Experiment 1) and long-term pH 7 (experiment 2) soils. Eighty percent of 15N-N2O from the long-term pH 4.5 soil (experiment 2) was derived from denitrification suggesting a possible inhibition of N2O reduction. Short-term acidification to pH 5.6 or 4.3 lowered N2O emissions. Liming of the pH 4.5 soil resulted in over four times greater N2O emission (11 mg 14+15N-N2O m−2 over 41 days) than from the long-term pH 7.0 soil (experiment 2), with an associated increase in ammonia oxidiser-N2O and decrease in denitrifier-N2O production. This is the first report of a pH-induced change in microbial source of N2O. Our results highlight the importance of distinguishing between short- and long-term effects of pH management when predicting N2O emissions from soil, as they exhibit predominance of different microbial groups in N2O production, with likely adaptation of the microbial community.
KeywordsAmmonia oxidation Denitrification Liming Nitrous oxide pH Stable isotopes
This work was funded by the Biotechnology and Biological Sciences Research Council Agrifood Committee studentship for EJ Bateman, by the Natural Environment Research Council studentship for CL Smales, and by the Biotechnology and Biological Sciences Research Council Wain Research Fellowship and the Natural Environment Research Council Advanced Research Fellowship awarded to EM Baggs. We thank the Scottish Agricultural College for acces to their pH plots at Craibstone.
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