Impacts of climate and land use on N(2) O and CH(4) fluxes from tropical ecosystems in the Mt. Kilimanjaro region, Tanzania.

In this study, we quantify the impacts of climate and land use on soil N2 O and CH4 fluxes from tropical forest, agroforest, arable and savanna ecosystems in Africa. To do so, we measured greenhouse gases (GHG) fluxes from 12 different ecosystems along climate and land-use gradients at Mt. Kilimanjaro, combining long-term in situ chamber and laboratory soil core incubation techniques. Both methods showed similar patterns of GHG exchange. Although there were distinct differences from ecosystem to ecosystem, soils generally functioned as net sources and sinks for N2 O and CH4 respectively. N2 O emissions correlated positively with soil moisture and total soil nitrogen content. CH4 uptake rates correlated negatively with soil moisture and clay content and positively with SOC. Due to moderate soil moisture contents and the dominance of nitrification in soil N turnover, N2 O emissions of tropical montane forests were generally low (<1.2 kg N ha-1  year-1 ), and it is likely that ecosystem N losses are driven instead by nitrate leaching (~10 kg N ha-1  year-1 ). Forest soils with well-aerated litter layers were a significant sink for atmospheric CH4 (up to 4 kg C ha-1  year-1 ) regardless of low mean annual temperatures at higher elevations. Land-use intensification significantly increased the soil N2 O source strength and significantly decreased the soil CH4 sink. Compared to decreases in aboveground and belowground carbon stocks enhanced soil non-CO2 GHG emissions following land-use conversion from tropical forests to homegardens and coffee plantations were only a small factor in the total GHG budget. However, due to lower ecosystem carbon stock changes, enhanced N2 O emissions significantly contributed to total GHG emissions following conversion of savanna into grassland and particularly maize. Overall, we found that the protection and sustainable management of aboveground and belowground carbon and nitrogen stocks of agroforestry and arable systems is most crucial for mitigating GHG emissions from land-use change.

Gütlein A ，Gerschlauer F ，Kikoti I ，Kiese R ... -  《-》

Effect of urbanization on soil methane and nitrous oxide fluxes in subtropical Australia.

Increasing population densities and urban sprawl are causing rapid land use change from natural and agricultural ecosystems into smaller, urban residential properties. However, there is still great uncertainty about the effect that urbanization will have on biogeochemical C and N cycles and associated greenhouse gas (GHG) budgets. We aimed to evaluate how typical urbanization related land use change in subtropical Australia affects soil GHG exchange (N2 O and CH4 ) and the associated global warming potential (GWP). Fluxes were measured from three land uses: native forest, a long-term pasture, and a turf grass lawn continuously over two years using a high-resolution automated chamber system. The fertilized turf grass had the highest N2 O emissions, dominated by high fluxes >100 g N2 O-N day-1 immediately following establishment though decreased to just 0.6 kg N2 O-N ha-1 in the second year. Only minor fluxes occurred in the forest and pasture, with the high aeration of the sandy topsoil limiting N2 O emissions while promoting substantial CH4 uptake. Native forest was consistently the strongest CH4 sink (-2.9 kg CH4 -C ha-1  year-1 ), while the pasture became a short-term CH4 source after heavy rainfall when the soil reached saturation. On a two-year average, land use change from native forest to turf grass increased the non-CO2 GWP from a net annual GHG sink of -83 CO2 -e ha-1  year-1 to a source of 245 kg CO2 -e ha-1  year-1 . This study highlights that urbanization can substantially alter soil GHG exchange by altering plant soil water use and by increasing bulk density and inorganic N availability. However, on well-drained subtropical soils, the impact of urbanization on inter-annual non-CO2 GWP of turf grass was low compared to urbanized ecosystems in temperate climates.

van Delden L ，Rowlings DW ，Scheer C ，De Rosa D ，Grace PR ... -  《-》

Forest and grassland cover types reduce net greenhouse gas emissions from agricultural soils.

Western Canada's prairie region is extensively cultivated for agricultural production, which is a large source of greenhouse gas emissions. Agroforestry systems are common land uses across Canada, which integrate trees into the agricultural landscape and could play a substantial role in sequestering carbon and mitigating increases in atmospheric GHG concentrations. We measured soil CO2, CH4 and N2O fluxes and the global warming potential of microbe-mediated net greenhouse gas emissions (GWPm) in forest and herbland (areas without trees) soils of three agroforestry systems (hedgerow, shelterbelt and silvopasture) over two growing seasons (May through September in 2013 and 2014). We measured greenhouse gas fluxes and environmental conditions at 36 agroforestry sites (12 sites for each system) located along a south-north oriented soil/climate gradient of increasing moisture availability in central Alberta, Canada. The temperature sensitivity of soil CO2 emissions was greater in herbland (4.4) than in forest (3.1), but was not different among agroforestry systems. Over the two seasons, forest soils had 3.4% greater CO2 emission, 36% higher CH4 uptake, and 66% lower N2O emission than adjacent herbland soils. Combining the CO2 equivalents of soil CH4 and N2O fluxes with the CO2 emitted via heterotrophic (microbial) respiration, forest soils had a smaller GWPm than herbland soils (68 and 89kgCO2ha(-1), respectively). While emissions of total CO2 were silvopasture>hedgerow>shelterbelt, soils under silvopasture had 5% lower heterotrophic respiration, 15% greater CH4 uptake, and 44% lower N2O emission as compared with the other two agroforestry systems. Overall, the GWPm of greenhouse gas emissions was greater in hedgerow (88) and shelterbelt (85) than in the silvopasture system (76kgCO2ha(-1)). High GWPm in the hedgerow and shelterbelt systems reflects the greater contribution from the monoculture annual crops within these systems. Opportunities exist for reducing soil greenhouse gas emissions and mitigating climate change by promoting the establishment of perennial vegetation in the agricultural landscape.

Baah-Acheamfour M ，Carlyle CN ，Lim SS ，Bork EW ，Chang SX ... -  《-》

Urbanization can accelerate climate change by increasing soil N(2) O emission while reducing CH(4) uptake.

Urban land-use change has the potential to affect local to global biogeochemical carbon (C) and nitrogen (N) cycles and associated greenhouse gas (GHG) fluxes. We conducted a meta-analysis to (1) assess the effects of urbanization-induced land-use conversion on soil nitrous oxide (N2 O) and methane (CH4 ) fluxes, (2) quantify direct N2 O emission factors (EFd ) of fertilized urban soils used, for example, as lawns or forests, and (3) identify the key drivers leading to flux changes associated with urbanization. On average, urbanization increases soil N2 O emissions by 153%, to 3.0 kg N ha-1  year-1 , while rates of soil CH4 uptake are reduced by 50%, to 2.0 kg C ha-1  year-1 . The global mean annual N2 O EFd of fertilized lawns and urban forests is 1.4%, suggesting that urban soils can be regional hotspots of N2 O emissions. On a global basis, conversion of land to urban greenspaces has increased soil N2 O emission by 0.46 Tg N2 O-N year-1 and decreased soil CH4 uptake by 0.58 Tg CH4 -C year-1 . Urbanization driven changes in soil N2 O emission and CH4 uptake are associated with changes in soil properties (bulk density, pH, total N content, and C/N ratio), increased temperature, and management practices, especially fertilizer use. Overall, our meta-analysis shows that urbanization increases soil N2 O emissions and reduces the role of soils as a sink for atmospheric CH4 . These effects can be mitigated by avoiding soil compaction, reducing fertilization of lawns, and by restoring native ecosystems in urban landscapes.

Zhan Y ，Yao Z ，Groffman PM ，Xie J ，Wang Y ，Li G ，Zheng X ，Butterbach-Bahl K ... -  《-》

Greenhouse gas fluxes over managed grasslands in Central Europe.

Central European grasslands are characterized by a wide range of different management practices in close geographical proximity. Site-specific management strategies strongly affect the biosphere-atmosphere exchange of the three greenhouse gases (GHG) carbon dioxide (CO2 ), nitrous oxide (N2 O), and methane (CH4 ). The evaluation of environmental impacts at site level is challenging, because most in situ measurements focus on the quantification of CO2 exchange, while long-term N2 O and CH4 flux measurements at ecosystem scale remain scarce. Here, we synthesized ecosystem CO2 , N2 O, and CH4 fluxes from 14 managed grassland sites, quantified by eddy covariance or chamber techniques. We found that grasslands were on average a CO2 sink (-1,783 to -91 g CO2  m-2  year-1 ), but a N2 O source (18-638 g CO2 -eq. m-2  year-1 ), and either a CH4 sink or source (-9 to 488 g CO2 -eq. m-2  year-1 ). The net GHG balance (NGB) of nine sites where measurements of all three GHGs were available was found between -2,761 and -58 g CO2 -eq. m-2  year-1 , with N2 O and CH4 emissions offsetting concurrent CO2 uptake by on average 21 ± 6% across sites. The only positive NGB was found for one site during a restoration year with ploughing. The predictive power of soil parameters for N2 O and CH4 fluxes was generally low and varied considerably within years. However, after site-specific data normalization, we identified environmental conditions that indicated enhanced GHG source/sink activity ("sweet spots") and gave a good prediction of normalized overall fluxes across sites. The application of animal slurry to grasslands increased N2 O and CH4 emissions. The N2 O-N emission factor across sites was 1.8 ± 0.5%, but varied considerably at site level among the years (0.1%-8.6%). Although grassland management led to increased N2 O and CH4 emissions, the CO2 sink strength was generally the most dominant component of the annual GHG budget.

Hörtnagl L ，Barthel M ，Buchmann N ，Eugster W ，Butterbach-Bahl K ，Díaz-Pinés E ，Zeeman M ，Klumpp K ，Kiese R ，Bahn M ，Hammerle A ，Lu H ，Ladreiter-Knauss T ，Burri S ，Merbold L ... -  《-》