Converting conventional agriculture to poplar bioenergy crops: soil greenhouse gas flux

Conversion of agricultural fields to bioenergy crops can affect greenhouse gases (GHG) such as carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Soil GHG emissions were measured seasonally in poplar bioenergy and agricultural fields at three Northwestern US locations. A forest stand was also used at one location for comparison. A portable gas analyzer was used to measure CO₂ efflux and CH₄ and N₂O fluxes were first measured with chambers and later with gradients. Agricultural soil had 17% larger CO₂ efflux rates than poplar soil. Chamber fluxes showed no differences in CH₄ uptake but did show higher N₂O fluxes in poplar than agricultural soil. Gradient CH₄ uptake rates were highest in agricultural soil in the summer but showed no N₂O flux differences. Forest soils had smaller quarterly CO₂ efflux rates than agricultural soils and greater CH₄ uptake rates than poplar soils. The largest GHG contributor to soil GHG flux was CO₂, with those being ～1000 times larger than CH₄ flux rates and ～500 times larger than N₂O flux rates based on CO₂ equivalences. Converting conventional agricultural cropland to poplar bioenergy production does not have adverse effects on soil greenhouse gas flux and these results could be useful for modeling or life cycle analysis of land use conversion.     

Soil CO<Subscript>2</Subscript>, CH<Subscript>4</Subscript> and N<Subscript>2</Subscript>O emissions from production fields with planted and remnant hedgerows in the Fraser River Delta of British Columbia

Planting hedgerows on farm field edges can help mitigate greenhouse gas (GHG) emissions from agricultural landscapes by sequestering carbon (C) in woody biomass and in soil. Sequestration rates however, must be assessed in terms of their overall global warming potential (GWP) which must also consider GHG emissions. The objectives of this study were to (1) compare carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) emissions from two types of hedgerows and adjacent annual agricultural production fields, and 2) better understand how climate, soil properties and plant species configurations affect hedgerow GHG emissions. At eight study sites in the lower Fraser River delta of British Columbia, we measured emissions from soil in both planted (P-Hedgerow) and remnant hedgerows (R-Hedgerow), as well as in adjacent annual crop production fields over 1 year using a closed-static chamber method. CO₂ emissions were 59 % higher in P-Hedgerow than R-Hedgerow, yet there were no significant differences of relative emissions of CH₄ and N₂O. The environmental variables that explained the variation in emissions differed for the three GHGs. CO₂ emissions were significantly correlated with soil temperature. CH₄ and N₂O and emissions were marginally significantly correlated with soil organic carbon (SOC) and soil water-filled pore space (WFPS), respectively. Emissions were not significantly correlated with hedgerow plant species diversity. While hedgerows sequester carbon in their woody biomass, we demonstrated that it is critical to measure hedgerow emissions to accurately ascertain their overall GHG mitigation potential. Our results show that there are no CO₂e emission differences between the management options that plant new diverse hedgerows or conserve existing hedgerows.     

Seasonal change in N<Subscript>2</Subscript>O flux from forest soils in a forest catchment in Japan

Temperate forest soils are one source of nitrous oxide (N₂O), which is an important greenhouse gas and the most important ozone-depleting substance. To clarify N₂O flux mechanisms in relation to soil temperature, moisture, and nitrification activity, we measured N₂O fluxes and net nitrification rates over 3 years at the lower (Japanese cedar) and upper (deciduous broad-leaved trees) parts of a hill slope in a small forest catchment in the northern Kanto region of Japan. The N₂O flux was measured by the closed-chamber technique every month, along with soil temperature and water-filled pore space (WFPS). At the lower slope, the N₂O flux increased with increasing soil temperature (r ² = 0.383, P < 0.01) owing to an increase in the nitrification rate. At the upper slope, no positive linear correlation of N₂O flux with soil temperature, WFPS, or nitrification rate was observed. The low N₂O flux at the upper slope during summer was caused by the low summertime WFPS there. We attributed the higher mean N₂O fluxes observed at the lower slope (median 2.36 μg N m⁻² h⁻¹) than at the upper slope (median 1.10 μg N m⁻² h⁻¹) to a high soil moisture during summer season in the surface soil of the lower slope.     

Crop residue effect on crop performance,soil N<Subscript>2</Subscript>O and CO<Subscript>2</Subscript> emissions in alley cropping systems in subtropical China

Land management practices that simultaneously improve soil properties are crucial to high crop production and minimize detrimental impact on the environment. We examined the effects of crop residues on crop performance, the fluxes of soil N₂O and CO₂ under wheat-maize (WM) and/or faba bean-maize (FM) rotations in Amorpha fruticosa (A) and Vetiveria zizanioides (V) intercropping systems on a loamy clay soil, in subtropical China. Crop performance, soil N₂O and CO₂ as well as some potential factors such as soil water content, soil carbon, soil nitrogen, microbial biomass and N mineralization were recorded during 2006 maize crop cultivation. Soil N₂O and CO₂ fluxes are determined using a closed-based chamber. Maize yield was greater after faba bean than after wheat may be due to differences in supply of N from residues. The presence of hedgerow significantly improved maize grain yields. N₂O emissions from soils with maize were considerably greater after faba bean (345 g N₂O–N ha⁻¹) than after wheat (289 g N₂O–N ha⁻¹). However, the cumulated N₂O emissions did not differ significantly between WM and FM. The difference in N₂O emissions between WM and FM was mostly due to the amounts of crop residues. Hedgerow alley cropping tended to emit more N₂O than WM and FM, in particular A. fruticosa intercropping systems. Over the entire 118 days of measurement, the N₂O fluxes represented 534 g N₂O–N ha⁻¹ (AWM) and 512 g N₂O–N ha⁻¹ (AFM) under A. fruticosa species, 403 g N₂O–N ha⁻¹ (VWM) and 423 g N₂O–N ha⁻¹ (VFM) under Vetiver grass. We observed significantly higher CO₂ emission in AFM (5,335 kg CO₂–C ha⁻¹) from June to October, whereas no significant difference was observed among WM (3,480 kg CO₂–C ha⁻¹), FM (3,302 kg CO₂–C ha⁻¹), AWM (3,877 kg CO₂–C ha⁻¹), VWM (3,124 kg CO₂–C ha⁻¹) and VFM (3,309 kg CO₂–C ha⁻¹), indicating the importance of A. fruticosa along with faba bean residue on CO₂ fluxes. As a result, crop residues and land conversion from agricultural to agroforestry can, in turn, influence microbial biomass, N mineralization, soil C and N content, which can further alter the magnitude of crop growth, soil N₂O and CO₂ emissions in the present environmental conditions.     

Soil greenhouse gas emissions from agroforestry and other land uses under different moisture regimes in lower Missouri River Floodplain soils: a laboratory approach

Changes in land use management practices may have multiple effects on microclimate and soil properties that affect soil greenhouse gas (GHG) emissions. Soil surface GHG emissions need to be better quantified in order to assess the total environmental costs of current and possible alternative land uses in the Missouri River Floodplain (MRF). The objective of this study was to evaluate soil GHG emissions (CO₂, CH₄, N₂O) in MRF soils under long-term agroforestry (AF), row-crop agriculture (AG) and riparian forest (FOR) systems in response to differences in soil water content, land use, and N fertilizer inputs. Intact soil cores were obtained from all three land use systems and incubated under constant temperature conditions for a period of 94 days using randomized complete block design with three replications. Cores were subjected to three different water regimes: flooded (FLD), optimal for CO₂ efflux (OPT), and fluctuating. Additional N fertilizer treatments for the AG and AF land uses were included during the incubation and designated as AG-N and AF-N, respectively. Soil CO₂ and N₂O emissions were affected by the land use systems and soil moisture regimes. The AF land use resulted in significantly lower cumulative soil CO₂ and N₂O emissions than FOR soils under the OPT water regime. Nitrogen application to AG and AF did not increase cumulative soil CO₂ emissions. FLD resulted in the highest soil N₂O and CH₄ emissions, but did not cause any increases in soil cumulative CO₂ emissions compared to OPT water regime conditions. Cumulative soil CO₂ and N₂O emissions were positively correlated with soil pH. Soil cumulative soil CH₄ emissions were only affected by water regimes and strongly correlated with soil redox potential.     

The effect of forest management on trace gas exchange at the pedosphere–atmosphere interface in beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) forests stocking on calcareous soils

The effect of forest management (thinning) on in situ carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) trace gas exchange between soil and atmosphere was studied in three consecutive years at three beech forest sites, which differ in aspect [southwest (SW), northeast (NE), northwest (NW)]. At all sites adjacent thinning plots (“T”) and untreated control plots (“C”) were established. Measurements at the SW and NE sites covered the years 4–6 after thinning while at the NW site measurements covered the year before and the first 2 years after thinning. Mean N₂O fluxes were <3 μg N₂O–N m⁻² h⁻¹ at all plots except for the newly thinned NWT plot. CH₄ uptake was rather low, too. Very low CH₄ oxidation rates during dry periods are explained by physiological drought stress for CH₄ oxidizers. Heterotrophic litter decomposition constitutes the largest part of total soil respiration. On the whole, no significant positive or negative effects of the silvicultural treatment on the magnitude of CO₂-, CH₄- and N₂O-trace gas exchange could be observed at the SW site 4–6 years after thinning. Also at the NE site, no effects of thinning on CO₂ and N₂O fluxes could be demonstrated. However, at this site a significant moisture-induced lower CH₄ uptake could be shown. At the NW site forest management led to a dramatic increase in N₂O emissions in the first two summers after thinning and to distinct effects on CO₂ emissions and CH₄ uptake in the first year after the felling. The unambiguous effects of thinning at the NW site are mainly related to higher C input by dead residues leading to enhanced mineralization activity, to a shift in the competition for nutrients favoring microorganisms as compared to trees and to changes in the soil water availability at the thinned plot. Considering the data obtained from the NE and SW site we expect that with the development of an understorey vegetation at the NW site the observed effects on the magnitude of trace gas exchange due to thinning will continue to decline in the following years. Our results implicate that it is indispensable to take account of the effects of forest management in order to accurately calculate trace gas emission inventories for the investigated forest ecosystem in case thinning took place immediately before.     

Dynamics of soil-derived greenhouse gas emissions from shelterbelts under elevated soil moisture conditions in a semi-arid prairie environment

Soil moisture is known to be a major control of greenhouse gas (GHG) emissions from agricultural soils. However, there is little data regarding GHG exchange from the organic matter-rich soils characteristic of shelterbelts—especially under elevated soil moisture conditions. In the present study, we quantified CO₂, CH₄ and N₂O fluxes from shelterbelts under elevated soil moisture (irrigated) and semi-arid (rainfed) conditions. Studies were carried out at the Canada-Saskatchewan Irrigation Diversification Centre (CSIDC) near Outlook, Saskatchewan. Non-steady state vented chambers were used to monitor soil GHG fluxes from three shelterbelts in 2013 and 2014. The shelterbelts consisted of a single row of caragana with a north–south orientation and a single row of Scots pine with either a north–south or east–west orientation. Each shelterbelt was divided into two areas based on whether or not it received irrigation. During the 2-year study period, N₂O emissions from the irrigated shelterbelts (IR-SB) (0.93 kg N₂O-N ha^?1) were significantly greater than those from the rainfed shelterbelts (RF-SB) (0.49 kg N₂O-N ha^?1). Soil CH₄ oxidation was significantly lower in the IR-SB compared to the RF-SB (?0.85 and ?1.20 kg CH₄-C ha^?1, respectively). Irrigation activities stimulated CO₂ production/emission in 2014, but had no effect on CO₂ emissions during the much drier 2013 season. Correlation analyses indicate a strong dependence of CO₂ and CH₄ fluxes on soil moisture in both IR-SB and RF-SB sites. There was a significant relationship between N₂O emissions and soil moisture for the IR-SB sites in 2013; however, no such relationship was observed in either the IR-SB or RF-SB sites in 2014. Our study suggests that changes in precipitation patterns and soil moisture regime due to climate change could affect soil-atmosphere exchange of GHGs in shelterbelts; however, elevated soil moisture effect on GHG emissions will depend on the availability of N and C in the shelterbelts.     

阔叶红松林土壤CO2,N2O排放和CH4吸收的研究

为研究凋落物对CO2,N2O排放和CH4吸收的影响,从2002年9月3日到2003年10月30日,采用静态密闭箱技术对长白山阔叶红松林两种类型土壤生态系统的CO2,N2O和CH4的通量进行测定。两种土壤类型分别为表层有凋落物覆盖和没有凋落物覆盖。研究结果表明,凋落物对CO2,N2O和CH4通量有显著性影响(P<0.05)。有凋落物样地的CO2,N2O和CH4通量的日变化趋势和无凋落物样地中三种气体的日变化趋势相似,且CO2,N2O和CH4的日通量峰值都出现在18:00。有凋落物样地的CO2,N2O和CH4通量的季节变化趋势和无凋落物样地中三种气体的季节变化趋势也相似,但在一年之中,CO2和CH4的峰值出现在六月,N2O的峰值却出现在八月。研究结果还表明有凋落物样地CO2,N2O的日排放通量和年均排放通量明显大于无凋落物样地中两种气体的排放通量,但有凋落物样地的CH4日吸收通量和年均排放通量却小于无凋落物样地的CH4吸收通量。     

剔除林下灌草及添加固氮植物对华南地区4种人工林土壤甲烷(CH4)通量的影响

CH4是重要的温室气体之一,其主要排放源是森林土壤。本研究采用静态箱法对华南地区尾叶桉林（Eucalyptusurophylla）（B1）,厚荚相思林（Acacia crassicarpa）（B2）,10个树种的混交林（B3）和30个树种的混交林（B4）4种林型土壤CH4通量进行了原位测定,研究剔除林下灌草和添加翅荚决明（Cassia alata）对土壤CH4通量的影响。4个处理包括：（1）剔除林下灌草并添加翅荚决明（UR＋CA）;（2）仅剔除林下灌草（UR）;（3）仅添加翅荚决明（CA）;（4）对照（CK）。研究结果表明：林型变化对土壤CH4通量有重要影响,B1和B2表现为CH4的汇,而B3和B4为CH4的源,剔除林下灌草能改善土壤微生物活性,加快土壤矿化速度,促进CH4的吸收;而林下添加翅荚决明,由于翅荚决明根系的固氮作用,能加快土壤CH4的排放,表层土壤温度和湿度与土壤CH4通量具有强相关性;土壤有机碳（SOC）和可溶性N也是影响CH4通量的重要因子。本研究对探寻人工林管理措施对土壤CH4捧放影响机制具有重要的意义。     

Nitrification and denitrification as sources of gaseous nitrogen emission from different forest soils in Changbai Mountain

The contributions of nitrification and denitrification to N₂O and N₂ emissions from four forest soils on northern slop of Changbai Mountain were measured with acetylene inhibition methods. In incubation experiments, 0.06% and 3% C₂H₂ were used to inhibit nitrification and denitrification in these soils, respectively. Both nitrification and denitification existed in these soils except tundra soil, where only denitrification was found. The annually averaged rates of nitrification and denitrification in mountain dark brown forest soil were much higher than that in other three soils. In mountain brown coniferous soil, contributions of different processes to gaseous nitrogen emissions were Denitrification N₂O>nitrification N₂O>Denitrification N₂. The same sequence exists in mountain soddy soil as that in the mountain brown coniferous soil. The sequence in mountain tundra soil was Denitrification N₂O>Denitrification N₂. Foundation item: This paper was supported by the National Natural Science Foundation of China (No.49701016) and the “Hundred Scientists” Project of Chinese Academy of Sciences. Biography: XU Hui (1967-), male, Ph. Doctor, associate research fellow in Laboratory of Ecological Process of Trace Substance in Terrestrial Ecosystem, Institute of Applied Ecology, Chinese Academy of sciences, Shenyang 110015, P. R. China. Responsible editor: Song Funan     

Biogeochemical cycling in forest soils of the eastern Sierra Nevada Mountains,USA

We review some of the unique features of biogeochemical cycling in forests of the eastern Sierra Nevada Mountains, USA. As is the case for most arid and semi-arid ecosystems, spatial and temporal variability in nutrient contents and fluxes are quite high. “Islands of fertility” are common in these forests, a result of spatial variations in both litterfall and decomposition rates. Dry summer conditions greatly inhibit biological activity in the O horizon, and thus most annual litter decomposition takes place beneath the snowpack when moisture is available. Snowmelt duration is shortened near tree boles because of local warming, resulting in earlier drying of the O horizon, significantly lower decomposition rates, and increased O horizon mass. Water and nutrient fluxes vary spatially because of snowdrift in winter and surface runoff over hydrophobic soils in summer and fall. Moisture variability in the vertical as well as the horizontal dimension has significant consequences for nutrient fluxes. Because of the very dry summers, rooting in the O horizons is absent in these forests, and thus competition between microbes and trees for nutrients in that horizon is non-existent. Nutrients mineralized from the O horizon and not taken up by plants enrich runoff through the O horizons over hydrophobic mineral soils, resulting in very high concentrations of inorganic N and P in runoff waters. Substantial temporal variations in water and nutrient fluxes occur on a seasonal (with snowmelt being the dominant hydrologic event of the year), annual, and decadal basis. The most significant temporal variation is due to periodic fire, which we estimate causes annualized N losses that are two orders of magnitude greater than those associated with leaching and runoff. We hypothesize that fire suppression during the 20th century may have contributed to the deterioration of nearby Lake Tahoe by allowing buildups of N and P in O horizons which could subsequently leach from the terrestrial ecosystem to the Lake in runoff. In general, we conclude that biogeochemical cycling in these forests is characterized by greater spatial and temporal variability than in more mesic forest ecosystems.     

Soil–atmosphere CO2, CH4 and N2O fluxes in boreal forestry-drained peatlands

Greenhouse gas emissions from managed peatlands are annually reported to the UNFCCC. For the estimation of greenhouse gas (GHG) balances on a country-wide basis, it is necessary to know how soil–atmosphere fluxes are associated with variables that are available for spatial upscaling. We measured momentary soil–atmosphere CO₂ (heterotrophic and total soil respiration), CH₄ and N₂O fluxes at 68 forestry-drained peatland sites in Finland over two growing seasons. We estimated annual CO₂ effluxes for the sites using site-specific temperature regressions and simulations in half-hourly time steps. Annual CH₄ and N₂O fluxes were interpolated from the measurements. We then tested how well climate and site variables derived from forest inventory results and weather statistics could be used to explain between-site variation in the annual fluxes. The estimated annual CO₂ effluxes ranged from 1165 to 4437 g m⁻² year⁻¹ (total soil respiration) and from 534 to 2455 g m⁻² year⁻¹ (heterotrophic soil respiration). Means of 95% confidence intervals were ±12% of total and ±22% of heterotrophic soil respiration. Estimated annual CO₂ efflux was strongly correlated with soil respiration at the reference temperature (10 °C) and with summer mean air temperature. Temperature sensitivity had little effect on the estimated annual fluxes. Models with tree stand stem volume, site type and summer mean air temperature as independent variables explained 56% of total and 57% of heterotrophic annual CO₂ effluxes. Adding summer mean water table depth to the models raised the explanatory power to 66% and 64% respectively. Most of the sites were small CH₄ sinks and N₂O sources. The interpolated annual CH₄ flux (range: −0.97 to 12.50 g m⁻² year⁻¹) was best explained by summer mean water table depth (r² = 64%) and rather weakly by tree stand stem volume (r² = 22%) and mire vegetation cover (r² = 15%). N₂O flux (range: −0.03 to 0.92 g m⁻² year⁻¹) was best explained by peat CN ratio (r² = 35%). Site type explained 13% of annual N₂O flux. We suggest that water table depth should be measured in national land-use inventories for improving the estimation of country-level GHG fluxes for peatlands.     

Greenhouse gas emissions after a prescribed fire in white birch-dwarf bamboo stands in northern Japan,focusing on the role of charcoal

Forest fires affect both carbon (C) and nitrogen (N) cycling in forest ecosystems, and thereby influence the soil–atmosphere exchange of major greenhouse gases (GHGs): carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). To determine changes in the soil GHG fluxes following a forest fire, we arranged a low-intensity surface fire in a white birch forest in northern Japan. We established three treatments, having four replications each: a control plot (CON), a burned plot (BURN), and a plot burned with removal of the resulting charcoal (BURN-CHA). Soil GHG fluxes and various properties of the soil were determined on four or five occasions during a period that spanned two growing seasons. We observed increased concentrations of ammonium-N (NH₄-N) in BURN and BURN-CHA after the fire, while nitrate–N (NO₃-N) concentration was only increased in BURN-CHA after the fire. The soil CO₂ flux was significantly higher in CON than in BURN or BURN-CHA, but there was no difference in soil CH₄ uptake between the three treatments. Moreover, the N₂O flux from BURN-CHA soil was slightly greater than in CON or BURN. In BURN-CHA, the soil N₂O flux peaked in August, but there was no peak in BURN. We found temporal correlations between soil GHG fluxes and soil variables, e.g. soil temperature or NO₃-N. Our results suggest that environmental changes following fire, including the increased availability of N and the disappearance of the litter layer, have the potential to change soil GHG fluxes. Fire-produced charcoal could be significant in reducing soil N₂O flux in temperate forests.     

Can distribution of trees explain variation in nitrous oxide fluxes?

Abstract

The impact of distance to tree stems on nitrous oxide (N₂O) fluxes was examined to determine whether it is possible to improve the accuracy of flux estimates from boreal forest soils. Dark static chambers were placed along transects between pairs of trees within a Norway spruce stand and fluxes of N₂O and carbon dioxide (CO₂) were measured during the period 1999–2003. The groundwater table was measured on every sampling occasion along the transects. In addition, radiation transmission, potential diffusion rate and biomass of forest floor vegetation were measured once at each chamber site along one of the transects and soil samples were collected at three depths, from which pH, denitrification enzyme activity, soil moisture, organic matter, and carbon and nitrogen content were determined. There was a high level of variation in the N₂O fluxes, both spatially and temporally. However, the spatial variation in the N₂O fluxes within the transect could not be explained by differences in any of the measured variables. Sometimes, mainly when no major peaks occurred, N₂O fluxes were significantly correlated with CO₂ release. It is concluded that distance to stems cannot be used to improve the design of sampling schemes or for extrapolating flux levels to larger scales.     

Reductions in greenhouse gas emissions by using wood to protect against soil liquefaction

We compared the greenhouse gas (GHG) emissions from a log pile (LP) to those from a sand compaction pile (SCP) and from cement deep mixing (CDM) as measures against soil liquefaction, assuming that forest and waste management scenarios influence the GHG (CO₂, CH₄, and N₂O) balance of wood. We found little difference between the LP and SCP methods with respect to GHG emissions from fossil fuel and limestone consumption. However, GHG emissions from the CDM method were seven times higher than emissions from the LP method. In the GHG balance of wood, when the percentage of CH₄ emissions from carbon in underground wood was lower than 3.3%, permanent storage in the log achieved greater reductions in GHG emissions than using the waste log as fuel in place of coal or heavy oil. In order to obtain reductions in GHG emissions by replacing SCPs or CDM with LPs, sustainable forest management with reforestation and prevention of CH₄ emissions from the underground log are essential. Using reforestation, permanent storage of the log, no CH₄ emission from the log, and using logging residues instead of coal, the LP can achieve reductions in GHG emissions of 121 tonnes of CO₂ per 100 m² of improvement area by replacing CDM.     

Factors controlling N2O and CH4 fluxes in mixed broad-leaved/Korean pine forest of Changbai Mountain, China

Using the closed chamber technique, thein situ measurements of N₂O and CH₄ fluxes was conducted in a broad-leaved Korean pine mixed forest ecosystem in Changbai Mountain, China, from June 1994 to October 1995. The relationships between fluxes (N₂O and CH₄) and some major environmental factors (temperature, soil water content and soil available nitrogen) were studied. A significant positive correlation between N₂O emission and air/soil temperature was observed, but no significant correlation was found between N₂O emission and soil water content (SWC). This result showed that temperature was an important controlling factor of N₂O flux. There was a significant correlation between CH₄ uptake and SWC, but no significant correlation was found between CH₄ uptake and temperature. This suggested SWC was an important factor controlling CH₄ uptake. The very significant negative correlation between logarithmic N₂O flux and soil nitrate concentration, significant negative correlation between CH₄ flux and soil ammonium content were also found. This project is supported by Chinese Academy of Sciences Responsible editor: Chai Ruihai     

A geospatial and temporal framework for modeling gaseous N and other N losses from forest soils and basins,with application to the Turkey Lakes Watershed Project,in Ontario,Canada

This article quantifies pre- to post-harvest gaseous N emissions and other N losses from forest soils and basins geospatially and temporally via digital elevation and hydrological modeling, using daily rain, snow and air temperature records, annual atmospheric N deposition rates, and basin-specific soil and forest specifications as input. The approach relates gaseous N losses from soils to soil temperature and water-filled pore space (WFPS) as affected by the depth-to-water (DTW) below the soil surface. The approach is applied to the Turkey Lakes Watershed Project (TLW) in Ontario, 60 km north of Sault St. Marie, where basin-wide N losses due to denitrification would mostly be restricted to the wetland portions of the basin. Basin-wide N losses via denitrification and stream export (mineral N and dissolved organic N) were empirically related to upland N mineralization and soil leaching as controlling processes. The calibrated model calculations, set to conform to the field-monitored N concentrations in TLW streams, suggest that the harvest-induced nitrification and denitrification pulses would be strongest near the end of the first post-harvest year, dropping to background levels within about 4–5 years later. The article concludes with assessing basin-specific denitrification efficiencies in relation to atmospheric N deposition and basin-to-basin wetland coverage.     

Selective logging of lowland evergreen rainforests in Chiloé Island, Chile: Effects of changing tree species composition on soil nitrogen transformations

Lowland evergreen rainforests in southern Chile growing on highly productive soils and accessible sites have been subjected to traditional and industrial logging of valuable timber trees. Old-growth rain forests in this area are characterized by highly conservative N cycles, which results in an efficient N use of ecosystems. We hypothesize that different logging practices, by changing forest structure and species composition, can alter the quantity and quality (i.e. C/N ratio) of litterfall and soil organic matter and soil microbial processes that determine N storage and availability. To test this hypothesis we investigated chemical properties, microbial N transformations, N fluxes and N storage in soils of lowland evergreen rainforests of Chiloé Island after 10 years since industrial selective logging (ISL) and in stands subjected to traditional selective logging (TSL) by landowners in small properties. We compared them to reference unlogged old-growth stands (OG) in the same area. Tree basal area was more reduced in the stands subjected to ISL than to TSL. Litterfall inputs were similar in both logging treatments as in OG stands. This was due to greater biomass of understory species after logging. In TSL understory tree species determined a higher litterfall C/N ratio than ISL. We found higher soil N availability and content of base cations in surface soils of logged forests than in OG. The litter horizon of OG forest had significantly higher rates of non-symbiotic N fixation than logged forests. In the ISL treatment there was a trend toward increasing soil denitrification and significantly higher NO₃–N/N_t ratio in spring waters, which led to a stronger δ¹⁵N signal in surface and deep soils. We conclude that massive understory occupation by the shade-intolerant native bamboo Chusquea quila in ISL led to enhanced litter quality (lower C/N ratios) relaxing the tightness of the N cycle, which increased soil N availability leading to a higher proportion of nitrate in spring waters and higher gaseous N losses. In contrast, under TSL a higher litterfall C/N ratio slowed decomposition and net N mineralization rates thus reducing the chances for N losses, and enhancing C and N storage in soil. We suggest that sustainable logging practices in these rain forests should be based on lower rates of canopy removal to enhance colonization of the understory by shade-tolerant trees, which are associated with a more efficient N cycle.     

Influence of plant communities and soil properties on trace gas fluxes in riparian northern hardwood forests

Wetlands contribute significant amounts of greenhouse gases to the atmosphere, yet little is known about what variables control gas emissions from these ecosystems. There is particular uncertainty about forested riparian wetlands, which have high variation in plant and soil properties due to their location at the interface between land and water. We investigated the fluxes of carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) and associated understory vegetation and soil parameters at five northern hardwood riparian sites in the Adirondack Park, NY, USA. Gas fluxes were measured in field chambers 4 times throughout the summer of 2008. CO₂ flux rates ranged from 0.01 to 0.10 g C m⁻² h⁻¹, N₂O fluxes ranged from −0.27 to 0.65 ng N cm⁻² h⁻¹ and CH₄ flux rates ranged from −1.44 to 3.64 mg CH₄ m⁻² d⁻¹. Because we observed both production and consumption of N₂O and CH₄, it was difficult to discern relationships between flux and environmental parameters such as soil moisture and pH. However, there were strong relationships between ecosystem-scale variables and flux. For example, CO₂ and N₂O flux rates were most strongly related to percent plant cover, i.e., the site with the lowest vegetation cover had the lowest CO₂ and highest N₂O emissions. These ecosystem-scale predictive relationships suggest that there may be prospects for scaling information on trace gas fluxes up to landscape and regional scales using information on the distribution of ecosystem or soil types from remote sensing or geographic information system data.     

亚热带典型丘陵坡地马尾松林土壤N2O的年通量特征

由人类活动所造成的大气中温室气体浓度急剧增加而引起的全球气候变暖和环境变化已引起全世界的广泛关注。氧化亚氮(N2O)是仅次于二氧化碳(CO2)和甲烷(CH4)的一种温室气体,在大气中含量较低却十分稳定,具有较大的增温潜能(其单分子的增温潜能是CO2的310倍)和较快的浓度增加速率(以每年0.25%的速率增加)(IPCC,2007)。N2O可吸收红外线,减少地球表面通过大气向外层空间的热辐射,导致地球表面温度增加。N2O能参与大气中许多光化学反应,破坏臭氧层(Crutzen,1970),导致到达地球表面的紫外线明显增加,给人类健康和生态环境带来多方面的危害。