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.     

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.     

Comparison of soil CO2 flux between uncleared and cleared windthrow areas in Estonia and Latvia

Storms can turn a great proportion of forests’ assimilation capacity into dead organic matter because of windthrow and thus its role as a carbon sink will be diminished for some time. However, little is known about the magnitude or extent to which storms affect carbon efflux. We compared soil CO₂ fluxes in wind-thrown forest stands with different time periods since a storm event, and with different management practices (deadwood cleared or left on-site). This study examined changes in soil CO₂ efflux in two windthrow areas in north-eastern Estonia and one area in north-western Latvia, which experienced severe wind storms in the summers of 2001, 2002 and 1967, respectively. We measured soil CO₂ fluxes in stands formerly dominated by Norway spruce (Picea abies L. Karst.) with total and partial canopy destruction (all trees or roughly half of the trees in stand damaged by storm), in harvested areas (material removed after the wind storm) and in control areas (no damage by wind). Removal of wind-damaged material decreased instantaneous CO₂ flux from the soil surface. The highest instantaneous fluxes were measured in areas with total and partial canopy destruction (0.67 g CO₂ m⁻² h⁻¹ in both cases) compared with fluxes in the control areas (0.51 g CO₂ m⁻² h⁻¹), in the new storm-damaged areas where the material was removed (0.57 g CO₂ m⁻² h⁻¹) and in the old storm-damaged area where wood was left on site (0.55 g CO₂ m⁻² h⁻¹). The only factor affecting soil CO₂ flux was location of the measuring collar (plastic collar with diameter 100 mm, height 50 mm) - either on undamaged forest ground or on the uprooted tree pit, where the mineral soil was exposed after disturbance. New wind-thrown stands where residues are left on site would most likely turn to sources of CO₂ for several years until forest regeneration reaches to substantial assimilation rates. New wind-thrown stands where residues are left on site would most likely tend to have elevated CO₂ fluxes for several years until forest regeneration reaches to substantial assimilation rates. However, forest managers might be concerned about the amounts of CO₂ immediately released into the atmosphere if the harvested logs are burned.     

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     

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.     

Post-fire soil respiration in relation to burnt wood management in a Mediterranean mountain ecosystem

After a wildfire, the management of burnt wood may determine microclimatic conditions and microbiological activity with the potential to affect soil respiration. To experimentally analyze the effect on soil respiration, we manipulated a recently burned pine forest in a Mediterranean mountain (Sierra Nevada National and Natural Park, SE Spain). Three representative treatments of post-fire burnt wood management were established at two elevations: (1) “salvage logging” (SL), where all trees were cut, trunks removed, and branches chipped; (2) “non-intervention” (NI), leaving all burnt trees standing; and (3) “cut plus lopping” (CL), a treatment where burnt trees were felled, with the main branches lopped off, but left in situ partially covering the ground surface. Seasonal measurements were carried out over the course of two years. In addition, we performed continuous diurnal campaigns and an irrigation experiment to ascertain the roles of soil temperature and moisture in determining CO₂ fluxes across treatments. Soil CO₂ fluxes were highest in CL (average of 3.34 ± 0.19 μmol m⁻² s⁻¹) and the lowest in SL (2.21 ± 0.11 μmol m⁻² s⁻¹). Across seasons, basal values were registered during summer (average of 1.46 ± 0.04 μmol m⁻² s⁻¹), but increased during the humid seasons (up to 10.07 ± 1.08 μmol m⁻² s⁻¹ in spring in CL). Seasonal and treatment patterns were consistent at the two elevations (1477 and 2317 m a.s.l.), although respiration was half as high at the higher altitude.Respiration was mainly controlled by soil moisture. Watering during the summer drought boosted CO₂ effluxes (up to 37 ± 6 μmol m⁻² s⁻¹ just after water addition), which then decreased to basal values as the soil dried. About 64% of CO₂ emissions during the first 24 h could be attributed to the degasification of soil pores, with the rest likely related to biological processes. The patterns of CO₂ effluxes under experimental watering were similar to the seasonal tendencies, with the highest pulse in CL. Temperature, however, had a weak effect on soil respiration, with Q₁₀ values of ca. 1 across seasons and soil moisture conditions. These results represent a first step towards illustrating the effects of post-fire burnt wood management on soil respiration, and eventually carbon sequestration.     

Concentrations and fluxes of dissolved organic carbon and nitrogen in a Picea abies chronosequence on former arable land in Sweden

Shifting land use from agriculture to forestry induces major changes in the carbon (C) and nitrogen (N) cycles, including fluxes of dissolved organic carbon (DOC) and nitrogen (DON). This study investigated the long-term effects of afforestation on ecosystem DOC and DON dynamics using a chronosequence approach comprising four arable fields and nine differently aged (10–92 years) Norway spruce stands growing on similar former arable soils in the same area. Along the chronosequence, concentrations and fluxes of DOC and DON were determined in bulk precipitation, throughfall, O horizon leachate and mineral soil solution during a 2–3-year period. Soil water fluxes were calculated using a soil hydrological model (SWAP). Results showed that DOC concentrations and fluxes with throughfall were strongly positively correlated with tree height (r² = 0.95; P < 0.05 for both conc. and flux) and stand age, while DON showed no such trends, suggesting different origins of DOC and DON in throughfall. The highest concentrations and fluxes of DOC and DON occurred in soil leachate from the O horizon. Here, DOC flux was 250–310 kg C ha⁻¹ yr⁻¹ and DON flux 8–9 kg N ha⁻¹ yr⁻¹ in stands afforested between 65 and 92 years ago. Concentrations and fluxes of DOC and DON in the mineral subsoil were consistently low. Flux calculations suggest that there was a net loss of >90% (230–280 kg ha⁻¹ yr⁻¹) of DOC leached from the O horizon within 0–60 cm of the mineral soil. There was no significant effect of land use or forest age on DOC concentrations in solution from the lower part of the A horizon. The effect of time since afforestation was masked by soil properties that influence DOM retention in the mineral soil. Our data indicate that DOC concentrations in the A horizon of the sites studied were primarily related to the oxalate-extractable Al and Fe amounts in the same horizon. Afforestation of arable land induced a gradual qualitative change in soil organic matter (SOM) and dissolved organic matter (DOM), with significantly increasing C:N ratios in soil and soil solution over time. The development of an O horizon and the subsequent leaching of DOC and DON to the underlying mineral soil are important drivers of a changing soil C and N turnover following afforestation.     

Dissolved organic carbon and nitrogen leaching from Scots pine, Norway spruce and silver birch stands in southern Sweden

The effects of three common tree species - Scots pine, Norway spruce and silver birch - on leaching of dissolved organic carbon and dissolved nitrogen were studied in an experimental forest with podzolised soils in southern Sweden. We analyzed soil water collected with lysimeters and modeled water fluxes to estimate dissolved C and N fluxes. Specific UV absorbance (SUVA) was analyzed to get information about the quality of dissolved organic matter leached from the different stands. Under the O horizon, DOC concentrations and fluxes in the birch stands were lower than in the spruce and pine stands; annual fluxes were 21 g m⁻² y⁻¹ for birch and 38 g m⁻² y⁻¹ and 37 g C m⁻² y⁻¹ for spruce and pine, respectively. Under the B horizon, annual fluxes for all tree species ranged between 3 and 5 g C m⁻² y⁻¹, implying greater loss of DOC in the mineral soil in the coniferous stands than in the birch stands. We did not find any effect of tree species on the quality of the dissolved organic matter, as measured by SUVA, indicating that the chemical composition of the organic matter was similar in leachates from all three tree species. Substantial amounts of nitrogen was leached out of the soil profile at the bottom of the B horizon from the pine and birch stands, whereas the spruce stands seemed to retain most of the nitrogen in the soil. These differences in N leaching have implications for soil N budgets.     

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.     

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.     

Influences of forest tending works on carbon distribution and cycling in a Pinus densiflora S. et Z. stand in Korea

This study was conducted to determine carbon (C) dynamics following forest tending works (FTW) which are one of the most important forest management activities conducted by Korean forest police and managers. We measured organic C storage (above- and below-ground biomass C, forest floor C, and soil C at 50 cm depth), soil environmental factors (soil CO₂ efflux, soil temperature, soil water content, soil pH, and soil organic C concentration), and organic C input and output (litterfall and litter decomposition rates) for one year in FTW and non-FTW (control) stands of approximately 40-year-old red pine (Pinus densiflora S. et Z.) forests in the Hwangmaesan Soopkakkugi model forest in Sancheonggun, Gyeongsangnam-do, Korea. This forest was thinned in 2005 as a representative FTW practice. The total C stored in tree biomass was significantly lower (P < 0.05) in the FTW stand (40.17 Mg C ha⁻¹) than in the control stand (64.52 Mg C ha⁻¹). However, C storage of forest floor and soil layers measured at four different depths was not changed by FTW, except for that at the surface soil depth (0–10 cm). The organic C input due to litterfall and output due to needle litter decomposition were both significantly lower in the FTW stand than in the control stand (2.02 Mg C ha⁻¹ year⁻¹ vs. 2.80 Mg C ha⁻¹ year⁻¹ and 308 g C kg⁻¹ year⁻¹ vs. 364 g C kg⁻¹ year⁻¹, respectively, both P < 0.05). Soil environmental factors were significantly affected (P < 0.05) by FTW, except for soil CO₂ efflux rates and organic C concentration at soil depth of 0–20 cm. The mean annual soil CO₂ efflux rates were the same in the FTW (0.24 g CO₂ m⁻² h⁻¹) and control (0.24 g CO₂ m⁻² h⁻¹) stands despite monthly variations of soil CO₂ efflux over the one-year study period. The mean soil organic C concentration at a soil depth of 0–20 cm was lower in the FTW stand (81.3 g kg⁻¹) than in the control stand (86.4 g kg⁻¹) but the difference was not significant (P > 0.05). In contrast, the mean soil temperature was significantly higher, the mean soil water content was significantly lower, and the soil pH was significantly higher in the FTW stand than in the control stand (10.34 °C vs. 8.98 °C, 48.2% vs. 56.4%, and pH 4.83 vs. pH 4.60, respectively, all P < 0.05). These results indicated that FTW can influence tree biomass C dynamics, organic C input and output, and soil environmental factors such as soil temperature, soil water content and soil pH, while soil C dynamics such as soil CO₂ efflux rates and soil organic C concentration were little affected by FTW in a red pine stand.     

Carbon dioxide exchange of a larch forest after a typhoon disturbance

A typhoon event catastrophically destroyed a 45-year-old Japanese larch plantation in southern Hokkaido, northern Japan in September 2004, and about 90% of trees were blown down. Vegetation was measured to investigate its regeneration process and CO₂ flux, or net ecosystem production (NEP), was measured in 2006–2008 using an automated chamber system to investigate the effects of typhoon disturbance on the ecosystem carbon balance. Annual maximum aboveground biomass (AGB) increased from 2.7 Mg ha⁻¹ in 2006 to 4.0 Mg ha⁻¹ in 2007, whereas no change occurred in annual maximum leaf area index (LAI), which was 3.7 m² m⁻² in 2006 and 3.9 m² m⁻² in 2007. Red raspberry (Rubus idaeus) had become dominant within 2 years after the typhoon disturbance, and came to account for about 60% and 50% of AGB and LAI, respectively. In comparison with CO₂ fluxes measured by the eddy covariance technique in 2001–2003, for 4.5 months during the growing season, the sum of gross primary production (GPP) decreased on average by 739 gC m⁻² (64%) after the disturbance, whereas ecosystem respiration (RE) decreased by 501 gC m⁻² (51%). As a result, NEP decreased from 159 ± 57 gC m⁻² to −80 ± 30 gC m⁻², which shows that the ecosystem shifted from a carbon sink to a source. Seasonal variation in RE was strongly correlated to soil temperature. The interannual variation in the seasonal trend of RE was small. Light-saturated GPP (P_max) decreased from 30–45 μmol m⁻² s⁻¹ to 8–12 μmol m⁻² s⁻¹ during the summer season through the disturbance because of large reduction in LAI.     

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.     

Seasonal and spatial variation of nitrogen dynamics in the litter and surface soil layers on a tropical dry evergreen forest slope

Seasonal and spatial variability of litterfall and NO₃⁻ and NH₄⁺ leaching from the litter layer and 5-cm soil depth were investigated along a slope in a tropical dry evergreen forest in northeastern Thailand. Using ion exchange resin and buried bag methods, the vertical flux and transformation of inorganic nitrogen (N) were observed during four periods (dry, early wet, middle wet, and late wet seasons) at 15 subplots in a 180-m × 40-m rectangular plot on the slope. Annual N input via litterfall and inorganic N leached from the litter layer and from 5-cm depth soil were 12.5, 6.9, and 3.7 g N m⁻² year⁻¹, respectively, whereas net mineralization and the inorganic N pool in 0–5-cm soil were 7.1 g N m⁻² year⁻¹ and 1.4 g N m⁻², respectively. During the early wet season (90 days), we observed 82% and 74% of annual NO₃⁻ leaching from the litter layer and 5-cm soil depth, respectively. Higher N input via leaf litterfall in the dry season and via precipitation in the early wet season may have led to higher NO₃⁻ leaching rate from litter and surface soil layers during the early wet season. Large spatial variability in both NO₃⁻ vertical flux and litterfall was also observed within stands. Small-scale spatial patterns of total N input via litterfall were significantly correlated with NO₃⁻ leaching rate from the surface soil layer. In tropical dry evergreen forests, litterfall variability may be crucial to the remarkable seasonal changes and spatial variation in annual NO₃⁻ vertical flux in surface soil layers.     

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.     

Carbon,nitrogen and phosphorus leaching after site preparation at a boreal forest clear-cut area

Clear-cutting followed by mechanical site preparation is the major disturbance influencing nutrient and water fluxes in Fennoscandian boreal forests. The effects of soil harrowing on the fluxes of dissolved organic carbon (DOC), dissolved nitrogen compounds (organic N, NH₄⁺ and NO₃⁻) and water soluble phosphorus (PO₄³⁻) through a podzolic soil were studied in a clear-cut in eastern Finland for 5 years. The old, mixed coniferous stand was clear-cut and stem only harvested in 1996 followed by soil harrowing in 1998 and planting in June 1999. Zero-tension lysimeters were used to collect soil water from below different soil horizons in the three types of microsites that resulted from site preparation treatment: low ridges (25% of clear-cut area), shallow furrows (30%) and the undisturbed soil (45%). After soil harrowing, the leaching of DOC, N and P from below the B-horizon increased compared to pre-treatment levels. However, the increases were short-lasting; 1–2 years for inorganic N and P, and 5 years for DOC and organic N. The highest concentrations were associated with the ridges and lowest with the furrows, reflecting the differences in amount of organic matter present in each microsite type and, for N, to enhanced mineralization and nitrification. Leaching from below the B-horizon over the 5 years following soil harrowing for the whole clear-cut area was 36.5 kg ha⁻¹ for DOC, 0.88 kg ha⁻¹ for NH₄-N, 0.46 kg ha⁻¹ for NO₃-N, 1.24 kg ha⁻¹ for organic N and 0.09 kg ha⁻¹ for PO₄-P. Site preparation increased temporarily the risk for nutrient leaching into watercourses and groundwater from the clear-cut area but soil fertility was not affected since the leached amounts remained small. The main reasons for the observed low leaching values were the rapid recovery of ground vegetation and low N deposition loads.     

Effects of [CO2] and nitrogen on morphological and biomass traits of white birch (Betula papyrifera) seedlings

To investigate the interactive effects of CO₂ concentration ([CO₂]) and nitrogen supply on the growth and biomass of boreal trees, white birch seedlings (Betula papyrifera) were grown under ambient (360 μmol mol⁻¹) and elevated [CO₂] (720 μmol mol⁻¹) with five nitrogen supply regimes (10, 80, 150, 220, and 290 μmol mol⁻¹) in greenhouses. After 90 days of treatment, seedling height, root-collar diameter, biomass of different organs, leaf N concentration, and specific leaf area (SLA) were measured. Significant interactive effects of [CO₂] and N supply were found on height, root-collar diameter, leaf biomass, stem biomass and total biomass, stem mass ratio (SMR), and root mass ratio (RMR), but not on root mass, leaf mass ratio (LMR), leaf to root ratio (LRR), or leaf N concentration. The CO₂ elevation generally increased all the growth and biomass parameters and the increases were generally greater at higher levels of N supply or higher leaf N concentration. However, the CO₂ elevation significantly reduced SLA (13.4%) and mass-based leaf N concentration but did not affect area-based leaf N concentration. Increases in N supply generally increased the growth and biomass parameters, but the relationships were generally curvilinear. Based on a second order polynomial model, the optimal leaf N concentration was 1.33 g m⁻² for height growth under ambient [CO₂] and 1.52 g m⁻² under doubled [CO₂]; 1.48 g m⁻² for diameter under ambient [CO₂] and 1.64 g m⁻² under doubled [CO₂]; 1.29 g m⁻² for stem biomass under ambient [CO₂] and 1.43 g m⁻² under doubled [CO₂]. The general trend is that the optimal leaf N was higher at doubled than ambient [CO₂]. However, [CO₂] did not affect the optimal leaf N for leaf and total biomass. The CO₂ elevation significantly increased RMR and SMR but decreased LMR and LRR. LMR increased and RMR decreased with the increasing N supply. SMR increased with increase N supply up to 80 μmol mol⁻¹ and then leveled off (under elevated [CO₂]) or stated to decline (under ambient [CO₂]) with further increases in N supply. The results suggest that the CO₂ elevation increased biomass accumulation, particularly stem biomass and at higher N supply. The results also suggest that while modest N fertilization will increase seedling growth and biomass accumulation, excessive application of N may not stimulate further growth or even result in growth decline.     

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.     

Effects of soil moisture and temperature on CH4 oxidation and N2O emission of forest soil

Soil samples were taken from depth of 0–12 cm in the virgin broad-leaved/Korean pine mixed forest in Changbai Mountain in April, 2000. 20 μL·L⁻¹ and 200 μL·L⁻¹ CH₄ and N₂O concentration were supplied for analysis. Laboratory study on CH₄ oxidation and N₂O emission in forest soil showed that fresh soil sample could oxidize atmospheric methane and product N₂O. Air-dried soil sample could not oxidize atmospheric methane, but could product N₂O. However, it could oxidize the supplied methane quickly when its concentration was higher than 20 μL·L⁻¹. The oxidation rate of methane was increased with its initial concentration. An addition of water to dry soil caused large pulse of N₂O emissions within 2 hours. There were curvilinear correlations between N₂O emission and temperature (r²=0.706, p<0.05), and between N₂O emission and water content (r²=0.2968, p <0.05). These suggested temperature and water content were important factors controlling N₂O emission. The correlation between CH₄ oxidization and temperature was also found while CH₄ was supplied 200 μL·L⁻¹ (r²=0.3573, p<0.05). Temperature was an important factor controlling CH₄ oxidation. However, when 20 μL·L⁻¹ CH₄ was supplied, there was no correlation among CH₄ oxidization, N₂O emission, temperature and water content. Foundation item: This paper was supported by Chinese Academy of Sciences. Biography: ZHANG Xiu-jun (1960-), female, Ph. Doctor, lecture 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