Search

If a dry soil is wetted it can produce a large flush of nitrous oxide (N2O), which can be 10 times above the background rate, this phenomenon is called a hot moment. However, there is uncertainty in the literature regarding the mechanisms that cause hot moments. Therefore, this study aimed to induce two different hot moments on soil cores to investigate changes in nitrogen (N) cycling functional genes, and correlate these changes with subsequent N2O emissions, and transformations in soil chemistry. We induced a 'pre‐dry' treatment which wetted soils from 50% to 90% water‐filled pore space (WFPS) and 'pre‐wet' treatment which wetted soils from 70% to 90% WFPS to analyse changes in gas fluxes (N2O, nitric oxide [NO] and carbon dioxide [CO2]), soil chemistry (pH, dissolved organic carbon [DOC], total oxidised nitrogen [TON], ammonium [NH4+]) and microbiology (bacterial 16S, fungal ITS and nitrogen cycling functional genes associated with denitrification). The soil produced emissions of N2O that were significantly higher for the pre‐dry treatment compared with the pre‐wet treatment (p = <0.001; 9 times that of the pre‐wet). The pre‐dry treatments did not prime the soil any more than the pre‐wet treatments for CO2 and NO emissions, and the results showed that the soils TON and DOC concentrations were significantly higher in the pre‐wet conditions despite its lower N2O emissions. This suggests that the cause of the hot moment was not as dependant on the soil's substrate concentrations as previously thought. Moreover, the amount of denitrification genes in the pre‐dry soil was statistically similar to the pre‐wet (nirK, norB, nifH, nosZI, nxrA), apart from nirS which was marginally higher (p = 0.03). Contrary to other experiments, this study provides evidence that changes in soil substrates and functional gene abundance are not the cause of N2O hot moments. We therefore suggest that future studies should investigate changes in mRNA abundance related to N cycling genes. Highlights: Rewetting following drought caused a significant release of N2O.The pre‐dry soil produced higher N2O fluxes but contained less key substrates (TOC, NH4+ and DOC) than the pre‐wet soil.The pre‐dry and pre‐wet soil had a similar quantity of denitrification functional genes.Differences in soil chemistry and denitrification functional genes between the pre‐dry and pre‐wet soil cannot explain the differences in N2O emissions. [ABSTRACT FROM AUTHOR]

Extensively grazed grasslands are understudied in terms of their contribution to greenhouse gas (GHG) emissions from livestock production. Mountains, moorlands and heath occupy 18% of the UK land area, however, in situ studies providing high frequency N 2 O emissions from sheep urine deposited to such areas are lacking. Organic soils typical of these regions may provide substrates for denitrification-related N 2 O emissions, however, acidic and anoxic conditions may inhibit nitrification (and associated emissions from nitrification and denitrification). We hypothesised urine N 2 O-N emission factors (EFs) would be lower than the UK country-specific and IPCC default value for urine, which is based on lowland measurements. Using automated GHG sampling chambers, N 2 O emissions were determined from real sheep urine (930 kg N ha -1 ) and artificial urine (920 kg N ha -1 ) applied in summer, and from an artificial urine treatment (1120 kg N ha -1 ) and a combined NO 3 - and glucose treatment (106 kg N ha -1 ; 213 kg C ha -1 ) in autumn. The latter treatment provided an assessment of the soils capacity for denitrification under non-substrate limiting conditions. The artificial urine-N 2 O EF was 0.01 ± 0.00% of the N applied in summer and 0.00 ± 0.00% of the N applied in autumn. The N 2 O EF for real sheep urine applied in summer was 0.01 ± 0.02%. A higher flux was observed in only one replicate of the real urine treatment, relating to one chamber where an increase in soil solution NO 3 - was observed. No lag phase in N 2 O emission was evident following application of the NO 3 - and glucose treatment, which emitted 0.69 ± 0.15% of the N applied. This indicates nitrification rates are the bottle-neck for N 2 O emissions in upland organic soils. We calculated the potential impact of using hill-grazing specific urine N 2 O EFs on the UK inventory of N 2 O emissions from sheep excreta, and found a reduction of ca. 43% in comparison to the use of a country-specific excretal EF., (Copyright © 2019 The Authors. Published by Elsevier B.V. All rights reserved.)