Quantification of greenhouse gases [nitrous oxide (N2O) and methane (CH4)] and nitric oxide (NO) emissions from subtropical conventional vegetable systems through multi-site field measurements are needed to obtain accurate regional and global estimates. N2O, NO and CH4 emissions from subtropical conventional vegetable systems were simultaneously measured at two different sites with hilly topography in the Sichuan basin, southwest China by using the static chamber gas chromatography technique. Results showed that annual soil N2O and NO fluxes for the treatment receiving N fertilizer ranged from 6.34–7.71 kg N ha−1 yr−1 and 0.69–0.85 kg N ha−1 yr−1, respectively, while decreased soil CH4 uptakes by 26.4% as compared with no N fertilizer addition across our two sites of experiment. Overall, the average direct N2O and NO emission factor (EFd) were 0.71% and 0.12%, respectively, which were both lower than the available EFd for subtropical conventional vegetable systems. This finding indicates that current regional and global estimates of N2O and NO emissions from vegetable fields are likely overestimated. Background N2O emissions (3.42–3.62 kg N ha−1 yr−1) from the subtropical conventional vegetable systems were relatively high as compared with available field measurements worldwide, suggesting that background N2O emissions cannot be ignored for regional estimate of N2O emissions in subtropical region. Nevertheless, the significantly intra- and inter-annual variations in N2O, CH4 and NO emissions were also observed in the present study, which could be explained by temporal variations of environmental variables (i.e. soil temperature and moisture). The differences in N2O and NO EFd and CH4 emissions between various vegetable systems in particular under subtropical conditions should be taken into account when compiling regional or global inventories and proposing mitigation practices.

Annual nitric and nitrous oxide emissions response to biochar amendment from an intensive greenhouse vegetable system in southeast China

ABSTRACTIncreasing greenhouse gas emissions from anthropogenic activities continue to be a mounting problem worldwide. In the semi-natural Miscanthus sinensis Andersson; grasslands of Aso, Kumamoto, Japan, which have been managed for thousands of years, we measured soil methane (CH4) and nitrous oxide (N2O) emissions before and after annual controlled burns. We estimated annual soil carbon (C) accumulation, and CH4 and N2O emissions induced by biomass burning in 2009 and 2010, to determine the impacts of this ecosystem and its management on global warming. Environmental factors affecting soil CH4 and N2O fluxes were unknown, with no effect of annual burning observed on short-term soil CH4 and N2O emissions. However, deposition of charcoal during burning may have enhanced CH4 oxidation and N2O consumption at the study site, given that emissions (CH4: −4.33 kg C ha−1 yr−1, N2O: 0.17 kg N ha−1 yr−1) were relatively lower than those measured in other land-use types. Despite significant emission of CH4 and N2O during yearly burning events in early spring, the M. sinensis semi-natural grassland had a large annual soil C accumulation, which resulted in a global warming potential of −4.86 Mg CO2eq ha−1 yr−1. Consequently, our results indicate that long-term maintenance of semi-natural M. sinensis grasslands by annual burning can contribute to the mitigation of global warming.

Abstract. Besides agricultural soils, temperate forest soils have been identified as significant sources of or sinks for important atmospheric trace gases (N2O, NO, CH4, and CO2). Although the number of studies for this ecosystem type increased more than tenfold during the last decade, studies covering an entire year and spanning more than 1–2 years remained scarce. This study reports the results of continuous measurements of soil-atmosphere C- and N-gas exchange with high temporal resolution carried out since 1994 at the Höglwald Forest spruce site, an experimental field station in Southern Germany. Annual soil N2O, NO and CO2 emissions and CH4 uptake (1994–2010) varied in a range of 0.2–3.0 kg N2O-N ha−1yr−1, 6.4–11.4 kg NO-N ha−1yr−1, 7.0–9.2 t CO2-C ha−1yr−1, and 0.9–3.5 kg CH4-C ha−1yr−1, respectively. The observed high fluxes of N-trace gases are most likely a consequence of high rates of atmospheric nitrogen deposition (>20 kg N ha−1yr−1) of NH3 and NOx to our site. For N2O, cumulative annual emissions were ≥ 0.8 kg N2O-N ha−1yr−1 in years with freeze-thaw events (5 out 14 of years). This shows that long-term, multi-year measurements are needed to obtain reliable estimates of N2O fluxes for a given ecosystem. Cumulative values of soil respiratory CO2 fluxes tended to be highest in years with prolonged freezing periods, i.e. years with below average annual mean soil temperatures and high N2O emissions (e.g. the years 1996 and 2006). Furthermore, based on our unique database on trace gas fluxes we analyzed if soil temperature, soil moisture measurements can be used to approximate trace gas fluxes at daily, weekly, monthly, or annual scale. Our analysis shows that simple-to-measure environmental drivers such as soil temperature or soil moisture are suitable to approximate fluxes of NO and CO2 at weekly and monthly resolution reasonably well (accounting for up to 59 % of the variance). However, for CH4 we so far failed to find meaningful correlations, and also for N2O the predictive power is rather low. This is most likely due to the complexity of involved processes and counteracting effects of soil moisture and temperature, specifically with regard to N2O production and consumption by denitrification and microbial community dynamics. At monthly scale, including information on gross primary production (CO2, NO), and N deposition (N2O), increased significantly the explanatory power of the obtained empirical regressions (CO2: r2 =0.8; NO: r2 = 0.67; N2O, all data: r2 = 0.5; N2O, with exclusion of freeze-thaw periods: r2 = 0.65).