Admixing Fir to European Beech Forests Improves the Soil Greenhouse Gas Balance

The admixture of fir to pure European beech hardly affected soil-atmosphere CH4 and N2O fluxes but increased soil organic carbon (SOC) stocks at a site in the Black Forest, Southern Germany.  Admixing deep-rooting silver fir has been proposed as a measure to increase the resilience of beech forests towards intensified drying-wetting cycles. Hence, the goal of this study was to quantify the effect of fir admixture to beech forests on the soil-atmosphere-exchange of greenhouse gases (GHGs: CO2, CH4 and N2O) and the SOC stocks by comparing pure beech (BB) and mixed beech-fir (BF) stands in the Black Forest, Germany.  To account for the impact of drying-wetting events, we simulated prolonged summer drought periods by rainout shelters, followed by irrigation.  The admixture of fir to pure beech stands reduced soil respiration, especially during autumn and winter. This resulted in increased SOC stocks down to a 0.9 m depth by 9 t C ha−1 at BF. The mixed stand showed an insignificantly decreased sink strength for CH4 (−4.0 under BB and −3.6 kg C ha−1 year−1 under BF). With maximal emissions of 25 µg N m−2 h−1, N2O fluxes were very low and remained unchanged by the fir admixture. The total soil GHG balance of forest conversion from BB to BF was strongly dominated by changes in SOC stocks. Extended summer droughts significantly decreased the soil respiration in both BB and BF stands and increased the net CH4 uptake.  Overall, this study highlights the positive effects of fir admixture to beech stands on SOC stocks and the total soil GHG balance. In view of the positive impact of increased SOC stocks on key soil functions such as water and nutrient retention, admixing fir to beech stands appears to be a suitable measure to mitigate climate change stresses on European beech stands.

Related papers:

title: Physiological responses of forest trees to heat and drought.
abstract: The heat wave of summer 2003 was the largest and the most persistent ever experienced in Central Europe and has fuelled concern about the effects of climate change on European ecosystems. Since forests constitute the most important European ecosystems, in this review article we assess current knowledge on the effects of heat and drought on key metabolic processes for growth and productivity of forest trees. In particular, the general consequences of heat and drought on (1) photosynthesis and respiration at the cellular and community level, and (2) on nutrient uptake, partitioning and competition for nutrients are summarized. The latter are a major sink for photosynthetic energy and, therefore, are indirectly but strongly connected to the performance of photosynthesis. In addition, the interaction of heat and drought with stress compensation mechanisms and emission of biogenic volatile organic compounds (BVOC) are discussed, since these processes are directly connected to carbon metabolism. Effects on the emission of BVOC are also included because they constitute an important feedback mechanism on ozone formation and, thus, on atmospheric pollution. As far as available, data collected during the 2003 heat wave are included and discussed.

title: Measurements of biosphere-atmosphere exchange of CH₄ in terrestrial ecosystems.
abstract: This chapter focuses on methods for measuring CH(4) exchange between the biosphere and the atmosphere. In the context of the global importance of the biosphere as a source and a sink of atmospheric CH(4,) special emphasis is given to details of gas flux measurements. Due to their widespread use and suitability for targeted process studies, chamber techniques as a means for measuring CH(4) fluxes are highlighted. Besides detailed recommendations for measurements of fluxes with chambers, potential problems of the chamber technique are also discussed, such as changes in environmental conditions due to chamber installations. Further, a short overview is provided of how different pathways of CH(4) exchange, specifically plant-mediated transport, ebullition or diffusion, can be separated and quantified under field conditions. Finally, a short summary of micrometeorological CH(4) measuring techniques such as the eddy covariance method is provided. This technique relies on fast-response sonic anemometers and CH(4) analyzers (~10Hz) to make direct measurements of the vertical CH(4) flux at a point above the vegetation surface.

title: Gas pooling:A sampling technique to overcome spatial heterogeneity of soil carbon dioxide and nitrous oxide fluxes
abstract: Small-scale spatial variability in soil carbon dioxide (CO2) and nitrous oxide (N2O) fluxes poses serious challenges to the experimental design, and number of gas samples needed to provide a reliable estimate of flux usually exceeds analytical capacities. We pooled gas samples -analogously to soil pooling - toovercome this challenge. Our sample pooling technique collects a composite gas sample from several chambers instead of the conventional practise of analyzing samples from chambers individually, thusreducing numbers of gas samples. The method was verified to be reasonably accurate in forest, grassland and agricultural fields over a four week measurement campaign. Pooling technique results differed by2-8% for CO2 and by 3-4% for N2O when compared to individual chamber means. That shows pooling of gas samples across individual static chambers is an acceptable approach to integrate spatial heterogeneity.

title: Soil CO2 efflux measurements in a mixed forest: impact of chamber disturbances, spatial variability and seasonal evolution
abstract: A closed-dynamic-chamber system (CDCS) was used to measure the spatial and seasonal variability of the soil CO2 efflux (Fs) in beech and in Douglas fir patches of the Vielsalm forest (Belgium). First the difference between natural and measured soil CO2 efflux induced by the presence of the CDCS was studied. The impact on the measurements of the pressure difference between the outside (natural condition) and the inside of the chamber was found to be small (0.4%). The influence of wind disturbance in the closed chamber was tested by comparison with an open-chamber system characterized by a different wind distribution. A very good correlation between the two systems was found (r2 = 0.99) but the open system yielded slightly lower fluxes than the closed one (slope = 0.88 ± 0.05). A measurement procedure has been developed to minimize the effect of the other sources of perturbation. 
 
The spatial and seasonal evolution of the soil CO2 efflux was obtained by performing regular measurements on 29 spots in the beech patch over a period of 12 months and on 24 spots in the Douglas fir patch over 8 months. For each spot, the experimental relationship between Fs and soil temperature was compared with the fitted line for an Arrhenius relationship with a soil temperature-dependent activation energy. 
 
Soil temperature explains 73% of the seasonal variation for all the data. The spatial average of the soil CO2 efflux at 10 °C (Fs10) in the beech patch is 2.57 ± 0.41 μmol m−2 s−1, approximately twice the average in the Douglas fir patch recorded at 1.42 ± 0.22 μmol m−2 s−1. 
 
The litter fall analysis seems to indicate that soil organic matter quality and quantity may be one the reasons for this difference. Finally the annual soil CO2 efflux was calculated for the beech and Douglas fir patches (870 ± 140 and 438 ± 68 gC m−2 y−1, respectively). The beech value would represent 92 ± 15% of the annual ecosystem respiration estimated from the eddy covariance measurements.

title: Belowground carbon pools and processes in different age stands of Douglas-fir.
abstract: Forest floor material and soil organic matter may act as both a source and a sink in global CO2 cycles. Thus, the ecosystem processes controlling these pools are central to understanding the transfers of carbon (C) between the atmosphere and terrestrial systems. To examine these ecosystem processes, the effect of stand age on temporal carbon source-sink relationships was examined in 20-year-old, 40-year-old and old-growth stands of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in the Cascade Mountains of south-central Washington State.  C and nitrogen (N) storage and soil respiration were measured. In addition, nylon mesh bags containing homogenized soils from each site were buried at the respective sites to quantify root ingrowth and potential C sequestration and loss. The sites supporting the 20- and 40-year-old stands had soil C stores reflecting the C contributions from logging residue, coarse woody debris and stumps left after harvest. Because the N-fixer red alder (Alnus rubra Bong.) comprised 33% of the 40-year-old stand, this site had significantly greater concentrations and pools of N in the forest floor than sites without red alder. This N-rich site had consistently lower soil CO2 efflux rates during the growing season than the sites supporting the 20-year-old and old-growth stands. Estimated annual soil C efflux was 1367, 883 and 1194 g m-2 for the sites supporting the 20-, 40- and old-growth stands, respectively. These values are higher than previously reported values. Root ingrowth was significantly less in the 40-year-old stand than in the 20-year-old stand, and both young stands showed markedly less fine root growth than the old-growth stand. At the sites supporting the young stands, C and N were lost from the soil bags, whereas there was an increase in C and N in the soil bags at the site supporting the old-growth stand. The fine root growth and soil respiration data support the hypothesis that belowground C allocation decreases with increasing fertility. Quantification of the source-sink relationship of soil C at the three stands based on litterfall, relative root ingrowth and soil respiration measurements was compromised because of significant CO2 flux from decaying organic matter in the young stands.

title: Carbon dioxide efflux and concentrations in two soils under temperate forests
abstract: The carbon dioxide efflux to the atmosphere and the concentrations at various depths in two soils were measured, for more than a year, under pure stands of silver fir ( Abies alba Mill.) and European beech ( Fagus sylvatica L.) in central Italy. Microbial biomass and activity at the monitored depths were determined in the laboratory and the CO2 evolved from incubated samples was submitted to radiocarbon analysis to assess the mean residence time of the organic matter degraded by microorganisms. The CO2 efflux showed similar trends in the two soils, with highest values in October and lowest in January. The efflux depended more on air and soil temperatures than soil moisture, and was related to these variables better under fir than under beech. In both soils, the CO2 concentration increased with depth: in the top horizon it was low and similar to that of the atmosphere, while in the deeper horizons it often amounted to considerable values (up to more than 1% by volume in the BC horizon under fir). The subsoil of the fir stand generally showed much higher CO2 concentrations than that of the beech. The basal respiration as determined in the laboratory was at a maximum in the topsoil and decreased sharply downwards. Therefore, the high CO2 concentrations measured in the field at the bottom of the profiles—where roots were few, and microbial biomass and available C pool were at a minimum—appeared to be due more to slow diffusivity of the soil matrix rather than to heavy release of the gas by the biota. The organic matter respired by microorganisms in incubated soil samples showed positive values of Δ14C that revealed a recent synthesis. The estimated mean residence time increased with depth, suggesting a generally higher degree of stabilisation of the organic pool in the subsoil.

title: Vegetation and soil respiration: Correlations and controls
abstract: Soil respiration rates vary significantly among major plant biomes, suggesting that vegetation type influences the rate of soil respiration. However, correlations among climatic factors, vegetation distributions, and soil respiration rates make cause-effect arguments difficult. Vegetation may affect soil respiration by influencing soil microclimate and structure, the quantity of detritus supplied to the soil, the quality of that detritus, and the overall rate of root respiration. At the global scale, soil respiration rates correlate positively with litterfall rates in forests, as previously reported, and with aboveground net primary productivity in grasslands, providing evidence of the importance of detritus supply. To determine the direction and magnitude of the effect of vegetation type on soil respiration, we collated data from published studies where soil respiration rates were measured simultaneously in two or more plant communities. We found no predictable differences in soil respiration between cropped and vegetation-free soils, between forested and cropped soils, or between grassland and cropped soils, possibly due to the diversity of crops and cropping systems included. Factors such as temperature, moisture availability, and substrate properties that simultaneously influence the production and consumption of organic matter are more important in controlling the overall rate of soil respiration than is vegetation type in most cases. However, coniferous forests had ∼10% lower rates of soil respiration than did adjacent broad-leaved forests growing on the same soil type, and grasslands had, on average, ∼20% higher soil respiration rates than did comparable forest stands, demonstrating that vegetation type does in some cases significantly affect rates of soil respiration.

title: Contribution of litter layer to soil greenhouse gas emissions in a temperate beech forest
abstract: Background and aimsThe litter layer is a major source of CO2, and it also influences soil-atmosphere exchange of N2O and CH4. So far, it is not clear how much of soil greenhouse gas (GHG) emission derives from the litter layer itself or is litter-induced. The present study investigates how the litter layer controls soil GHG fluxes and microbial decomposer communities in a temperate beech forest.MethodsWe removed the litter layer in an Austrian beech forest and studied responses of soil CO2, CH4 and N2O fluxes and the microbial community via phospholipid fatty acids (PLFA). Soil GHG fluxes were determined with static chambers on 22 occasions from July 2012 to February 2013, and soil samples collected at 8 sampling events.ResultsLitter removal reduced CO2 emissions by 30 % and increased temperature sensitivity (Q10) of CO2 fluxes. Diffusion of CH4 into soil was facilitated by litter removal and CH4 uptake increased by 16 %. This effect was strongest in autumn and winter when soil moisture was high. Soils without litter turned from net N2O sources to slight N2O sinks because N2O emissions peaked after rain events in summer and autumn, which was not the case in litter-removal plots. Microbial composition was only transiently affected by litter removal but strongly influenced by seasonality.ConclusionsLitter layers must be considered in calculating forest GHG budgets, and their influence on temperature sensitivity of soil GHG fluxes taken into account for future climate scenarios.

title: Soil organic carbon and nitrogen stocks under pure and mixed stands of European beech, Douglas fir and Norway spruce
abstract: Numerous studies have addressed tree species effects on forest soil carbon (C) and nitrogen (N); however, knowledge of how and to what extent specific tree species and species mixtures impact forest soil C and N stocks is scarce and inconsistent across soil types. Therefore, we studied three forest sites in Southern Germany differing in parent material, soil properties as well as nutrient and water supply. Each site comprises adjacent groups of pure mature European beech ( Fagus sylvatica ), Douglas fir ( Pseudotsuga menziesii ) and Norway spruce ( Picea abies ) as well as single-tree mixtures of beech with Douglas fir or Norway spruce. To account for tree-species-specific spatial heterogeneity, we sampled the forest floor and mineral soil to a depth of 60 cm at different distances from the trees. Significant tree species and species mixing effects on soil organic carbon (OC) and N concentrations, C/N ratios and soil OC and N stocks were mainly found in the forest floor and in the uppermost (0–15 cm) mineral soil. Forest floor OC and N stocks and total soil OC stocks were higher under Douglas fir and Norway spruce compared with beech. While tree species effects on soil OC and N were present across sites, the influence of soil type induced variations in their magnitude. The forest floor C/N ratio under Douglas fir was low and comparable with beech in soils developed from nutrient-rich parent material, whereas it was higher and similar to spruce in the soil formed from sandstone. Tree species-specific differences in foliar nutrient concentrations between beech and conifer stands might influence litter decomposition rates among the species and thus modify soil OC and N stocks. Forest floor OC stocks were significantly higher in mixed beech–conifer stands compared with pure beech, and most often smaller than or similar to pure conifer stands. Forest floor N stocks showed the same tendency, but differences were inconsistent and not always significant across sites. Admixture of beech with Douglas fir or Norway spruce reduced the share of OC and N stored in the forest floor compared with the pure conifer stands and significantly increased mineral topsoil (0–15 cm) OC stocks compared with pure beech stands. Hence, the vertical distribution of OC and N in the soil profile varied depending on the tree species composition. Total soil (forest floor + mineral soil) OC and N stocks of mixtures were similar to pure beech, pure conifers or intermediate depending on site and soil type.

title: Separating root and soil microbial contributions to soil respiration: A review of methods and observations
abstract: Forest soil respiration is the sum of heterotrophic (microbes, soil fauna) and autotrophic (root) respiration. The contribution of each group needs to be understood to evaluate implications of environmental change on soil carbon cycling and sequestration. Three primary methods have been used to distinguish hetero- versus autotrophic soil respiration including: integration of components contributing to in situ forest soil CO2 efflux (i.e., litter, roots, soil), comparison of soils with and without root exclusion, and application of stable or radioactive isotope methods. Each approach has advantages and disadvantages, but isotope based methods provide quantitative answers with the least amount of disturbance to the soil and roots. Published data from all methods indicate that root/rhizosphere respiration can account for as little as 10 percent to greater than 90 percent of total in situ soil respiration depending on vegetation type and season of the year. Studies which have integrated percent root contribution to total soil respiration throughout an entire year or growing season show mean values of 45.8 and 60.4 percent for forest and nonforest vegetation, respectively. Such average annual values must be extrapolated with caution, however, because the root contribution to total soil respiration is commonly higher during the growing season and lower during the dormant periods of the year.

title: Methane and nitrous oxide fluxes of soils in pure and mixed stands of European beech and Norway spruce
abstract: Tree species can affect the sink and source strength of soils for atmospheric methane and nitrous oxide. Here we report soil methane (CH 4 ) and nitrous oxide (N 2 O) fluxes of adjacent pure and mixed stands of beech and spruce at Soiling, Germany. Mean CH 4 uptake rates ranged between 18 and 48 μg C m -2 hour -1 during 2.5 years and were about twice as great in both mixed and the pure beech stand as in the pure spruce stand. CH 4 uptake was negatively correlated with the dry mass of the O horizon, suggesting that this diminishes the transport of atmospheric CH 4 into the mineral soil. Mean N 2 O emission was rather small, ranging between 6 and 16 μg N m -2 hour -1 in all stands. Forest type had a significant effect on N 2 O emission only in one mixed stand during the growing season. We removed the O horizon in additional plots to study its effect on gas fluxes over 1.5 years, but N 2 O emissions were not altered by this treatment. Surprisingly, CH 4 uptake decreased in both mixed and the pure beech stands following the removal of the O horizon. The decrease in CH 4 uptake coincided with an increase in the soil moisture content of the mineral soil. Hence, O horizons may maintain the gas diffusivity within the mineral soil by storing water which cannot penetrate into the mineral soil after rainfall. Our results indicate that conversion of beech forests to beech-spruce and pure spruce forests could decrease soil CH 4 uptake, while the long-term effect on N 2 O emissions is expected to be rather small.

title: Drying and Wetting Effects on Carbon Dioxide Release from Organic Horizons
abstract: Drying and wetting cycles of O horizon in forest soils have not received much attention, partly due to methodological limitations for nondestructive monitoring of the O horizon water content. The objective of this study was to determine the importance of moisture limitations in the O horizon of a temperate forest on summertime soil respiration. We measured soil respiration in three replicated plots in a mixed deciduous forest at Harvard Forest, Massachusetts, weekly from May to October 2001. Direct Current (DC) half-bridge sensors that had been calibrated using destructive samples of the Oi and Oe/ Oa horizons were placed in the Oi and Oe/Oa horizons to record hourly changes of gravimetric water contents. Soil temperature explained 47% of the variation in soil respiration using the Arrhenius equation. The residuals of the temperature model were linearly correlated with gravimetric water content of the Oi horizon (r2 = 0.72, P < 0.0001) and Oe/Oa horizon (r2 = 0.56, P < 0.001), indicating that temporal variation in soil respiration can be partly explained by water content of the O horizon. Additionally, a laboratory study was performed to evaluate drying/wetting cycles of the O horizon at constant temperature. Even small simulated rainfall amounts of 0.5 mm significantly increase CO2 flux from dry O horizon within a few minutes. The duration of CO2 pulses increased with the amount of applied water, lasting from a few hours to days. A strong correlation between CO2 release and water content of the O horizons demonstrates a clear regulatory role of litter water content on decomposition within the O horizons.

title: The effect of forest management on trace gas exchange at the pedosphere–atmosphere interface in beech (Fagus sylvatica L.) forests stocking on calcareous soils
abstract: 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.

title: Oxidation of atmospheric methane in Northern European soils, comparison with other ecosystems, and uncertainties in the global terrestrial sink
abstract: This paper reports the range and statistical distribution of oxidation rates of atmospheric CH4 in soils found in Northern Europe in an international study, and compares them with published data for various other ecosystems. It reassesses the size, and the uncertainty in, the global terrestrial CH4 sink, and examines the effect of land-use change and other factors on the oxidation rate. 
 
Only soils with a very high water table were sources of CH4; all others were sinks. Oxidation rates varied from 1 to nearly 200 μg CH4 m−2 h−1; annual rates for sites measured for ≥1 y were 0.1–9.1 kg CH4 ha−1 y−1, with a log-normal distribution (log-mean ≈ 1.6 kg CH4 ha−1 y−1). Conversion of natural soils to agriculture reduced oxidation rates by two-thirds –- closely similar to results reported for other regions. N inputs also decreased oxidation rates. Full recovery of rates after these disturbances takes > 100 y. Soil bulk density, water content and gas diffusivity had major impacts on oxidation rates. Trends were similar to those derived from other published work. Increasing acidity reduced oxidation, partially but not wholly explained by poor diffusion through litter layers which did not themselves contribute to the oxidation. The effect of temperature was small, attributed to substrate limitation and low atmospheric concentration. 
 
Analysis of all available data for CH4 oxidation rates in situ showed similar log-normal distributions to those obtained for our results, with generally little difference between different natural ecosystems, or between short-and longer-term studies. The overall global terrestrial sink was estimated at 29 Tg CH4 y−1, close to the current IPCC assessment, but with a much wider uncertainty range (7 to > 100 Tg CH4 y−1). Little or no information is available for many major ecosystems; these should receive high priority in future research.

title: Greenhouse gas emissions from soils—A review
abstract: Soils act as sources and sinks for greenhouse gases (GHG) such as carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O). Since both storage and emission capacities may be large, precise quantifications are needed to obtain reliable global budgets that are necessary for land-use management (agriculture, forestry), global change and for climate research. This paper discusses exclusively the soil emission-related processes and their influencing parameters. It reviews soil emission studies involving the most important land-cover types and climate zones and introduces important measuring systems for soil emissions. It addresses current shortcomings and the obvious bias towards northern hemispheric data. When using a conservative average of 300 mg CO 2 e m −2  h −1 (based on our literature review), this leads to global annual net soil emissions of ≥350 Pg CO 2 e (CO 2 e = CO 2 equivalents = total effect of all GHG normalized to CO 2 ). This corresponds to roughly 21% of the global soil C and N pools. For comparison, 33.4 Pg CO 2 are being emitted annually by fossil fuel combustion and the cement industry.

title: Interactive effects of tree species and soil moisture on methane consumption
abstract: Methane consumption by temperate forest soils is a major sink for this important greenhouse gas, but little is known about how tree species influence CH 4 uptake by soils. Here, we show that six common tree species in Siberian boreal and temperate forests significantly affect potential CH 4 consumption in laboratory microcosms. Overall, soils under hardwood species (aspen and birch) consumed CH 4 at higher rates than soils under coniferous species and grassland. While NH 4 + addition often reduces CH 4 uptake, we found no effect of NH 4 + addition, possibly because of the relatively high ratio of CH 4 -to-NH 4 + in our incubations. The effects of soil moisture strongly depended on plant species. An increase in soil moisture enhanced CH 4 consumption in soils under spruce but had the opposite effect under Scots pine and larch. Under other species, soil moisture did not affect CH 4 consumption. These results could be explained by specific responses of different groups of CH 4 -oxidizing bacteria to elevated moisture.

title: Methane oxidation kinetics differ in European beech and Norway spruce soils
abstract: Coniferous forest soils often consume less of the greenhouse gas methane (CH 4 ) than deciduous forest soils. The reasons for this phenomenon have not been resolved. It might be caused by differences in the diffusive flux of CH 4 through the organic layer, pH or different concentrations of potentially inhibitory compounds. Soil samples were investigated from three adjacent European beech (Fagus sylvatica) and Norway spruce (Picea abies) stands in Germany. Maximal CH 4 oxidation velocities (V max(app) ) and Michaelis Menten constants (K M(app) ), retrieved from intact soil cores at constant CH 4 concentrations, temperature and matric potential, were twice as great in beech as in spruce soils. Also atmospheric CH 4 oxidation rates measured in homogenized soil samples displayed the same trend. Greatest atmospheric CH 4 oxidation rates were detected in the O a horizon or in the upper 5 cm of the mineral soil. In contrast to the beech soils, the O a horizon of the spruce soils consumed no CH 4 . A differential effect due to divergent diffusive flux through the litter layer was not found. pH and ammonium concentration were similar in samples from both forest soil types. Ethylene accumulation in all soils was negligible under oxic conditions. These collective results suggest that the different atmospheric CH 4 uptake by beech and spruce soils is caused by different CH 4 oxidizing capacities of methanotrophic communities in the O a horizon and top mineral soil.

title: Inhibition of atmospheric methane oxidation by monoterpenes in Norway spruce and European beech soils
abstract: Biological oxidation of atmospheric methane (CH4) in forest soils of the northern hemisphere is an important sink in the global CH4 cycle. The effect of tree species on atmospheric CH4 oxidation is not well understood. Previous studies suggest that soils under European beech (Fagus sylvatica) consume more atmospheric CH4 than soils under Norway spruce (Picea abies). A major difference in the chemical constituents of these tree species is the production of monoterpenes. The objectives of the present study were (i) to quantify monoterpenes in leaves, needles, organic horizons (Oi, Oe, Oa), and mineral soil of a Norway spruce site and an adjacent European beech site of a temperate forest (Steigerwald, Germany) and (ii) to evaluate the potential of abundant monoterpenes to inhibit atmospheric CH4 oxidation. Major compounds were α- and β-pinene, limonene, and camphene. Highest concentrations were measured in Norway spruce samples (up to 63.9 μmol g dw − 1 ). In European beech samples, monoterpene concentrations were close to or below the detection limit (≤0.00015 μmol g dw − 1 ). For limonene, α- and β-pinene, the dose-dependent inhibition on atmospheric CH4 oxidation was determined. β-Pinene had the highest inhibition efficiency, followed by limonene and α-pinene. Norway spruce roots and mineral soil samples displayed similar monoterpene profiles, suggesting that roots can be considered as a source for inhibitory monoterpenes in Norway spruce soils. Monoterpene addition was always coupled to an increased carbon dioxide production. This indicates that monoterpenes may be microbially mineralized in these soils. In summary, the study revealed that the release of monoterpenes by both roots and litter may be sources in soil and that in situ monoterpene concentrations in spruce soil are high enough to explain reduced atmospheric CH4 uptake.

title: Exchange of N-gases at the Höglwald Forest – A summary
abstract: During 4 years continuous measurements of N-trace gas exchange were carried out at the forest floor-atmosphere interface at the Höglwald Forest that is highly affected by atmospheric N-deposition. The measurements included spruce control, spruce limed and beech sites. Based on these field measurements and on intensive laboratory measurements of N2-emissions from the soils of the beech and spruce control sites, a total balance of N-gas emissions was calculated. NO2-deposition was in a range of −1.6 −2.9 kg N ha−1 yr−1 and no huge differences between the different sites could be demonstrated. In contrast to NO2-deposition, NO- and N2O-emissions showed a huge variability among the different sites. NO emissions were highest at the spruce control site (6.4–9.1 kg N ha−1 yr−1), lowest at the beech site (2.3–3.5 kg N ha−1 yr−1) and intermediate at the limed spruce site (3.4–5.4 kg N ha−1 yr−1). With regard to N2O-emissions, the following ranking between the sites was found: beech (1.6–6.6 kg N ha−1 yr−1) >> spruce limed (0.7–4.0 kg N ha−1 yr−1) > spruce control (0.4–3.1 kg N ha−1 yr−1). Average N-trace gas emissions (NO, NO2, N2O) for the years 1994–1997 were 6.8 kg N ha−1 yr−1 at the spruce control site, 3.6 kg N ha−1 yr−1 at the limed spruce site and 4.5 kg N ha−1 yr−1 at the beech site. Considering N2-losses, which were significantly higher at the beech (12.4 kg N ha−1 yr−1) than at the spruce control site (7.2 kg N ha−1 yr−1), the magnitude of total gaseous N losses, i.e. N2-N + NO-N + NO2-N + N2O-N, could be calculated for the first time for a forest ecosystem. Total gaseous N-losses were 14.0 kg N ha−1 yr−1 at the spruce control site and 15.5 kg N ha−1 yr−1 at the beech site, respectively. In view of the huge interannual variability of N-trace gas fluxes and the pronounced site differences in N-gas emissions it is concluded that more research is needed in order to fully understand patterns of microbial N-cycling and N-gas production/emission in forest ecosystems and mechanisms of reactions of forest ecosystems to the ecological stress factor of atmospheric N-input.

title: Nitrous oxide emissions from soils: how well do we understand the processes and their controls?
abstract: Although it is well established that soils are the dominating source for atmospheric nitrous oxide (N2O), we are still struggling to fully understand the complexity of the underlying microbial production and consumption processes and the links to biotic (e.g. inter- and intraspecies competition, food webs, plant–microbe interaction) and abiotic (e.g. soil climate, physics and chemistry) factors. Recent work shows that a better understanding of the composition and diversity of the microbial community across a variety of soils in different climates and under different land use, as well as plant–microbe interactions in the rhizosphere, may provide a key to better understand the variability of N2O fluxes at the soil–atmosphere interface. Moreover, recent insights into the regulation of the reduction of N2O to dinitrogen (N2) have increased our understanding of N2O exchange. This improved process understanding, building on the increased use of isotope tracing techniques and metagenomics, needs to go along with improvements in measurement techniques for N2O (and N2) emission in order to obtain robust field and laboratory datasets for different ecosystem types. Advances in both fields are currently used to improve process descriptions in biogeochemical models, which may eventually be used not only to test our current process understanding from the microsite to the field level, but also used as tools for up-scaling emissions to landscapes and regions and to explore feedbacks of soil N2O emissions to changes in environmental conditions, land management and land use.

title: The response of methane and nitrous oxide fluxes to forest change in Europe
abstract: Forests in Europe are changing due to interactions between climate change, nitrogen (N) deposition and new forest management practices. The concurrent impact on the forest greenhouse gas (GHG) balance is at present difficult to predict due to a lack of knowledge on controlling factors of GHG fluxes and response to changes in these factors. To improve the mechanistic understanding of the ongoing changes, we studied the response of soil–atmosphere exchange of nitrous oxide (N2O) and methane (CH4) at twelve experimental or natural gradient forest sites, representing anticipated future forest change. The experimental manipulations, one or more per site, included N addition (4 sites), changes of climate (temperature, 1 site; precipitation, 2 sites), soil hydrology (3 sites), harvest intensity (1 site), wood ash fertilisation (1 site), pH gradient in organic soil (1 site) and afforestation of cropland (1 site). On average, N2O emissions increased by 0.06 ± 0.03 (range 0–0.3) g N2O-N m−2 yr−1 across all treatments on mineral soils, but the increase was up to 10 times higher in an acidic organic soil. Soil moisture together with mineral soil C / N ratio and pH were found to significantly influence N2O emissions across all treatments. Emissions were increased by elevated N deposition, especially in interaction with increased soil moisture. High pH reduced the formation of N2O, even under otherwise favourable soil conditions. Oxidation (uptake) of CH4 was on average reduced from 0.16 ± 0.02 to 0.04 ± 0.05 g CH4-C m−2 yr−1 by the investigated treatments. The CH4 exchange was significantly influenced by soil moisture and soil C / N ratio across all treatments, and CH4 emissions occurred only in wet or water-saturated conditions. For most of the investigated forest manipulations or natural gradients, the response of both N2O and CH4 fluxes was towards reducing the overall GHG forest sink. The most resilient forests were dry Mediterranean forests, as well as forests with high soil C / N ratio or high soil pH. Mitigation strategies may focus on (i) sustainable management of wet forest areas and forested peatlands, (ii) continuous forest cover management, (iii) reducing atmospheric N input and, thus, N availability, and (iv) improving neutralisation capacity of acid soils (e.g. wood ash application).

title: Regional variation in canopy transpiration of Central European beech forests
abstract: Forest hydrologists have hypothesised that canopy transpiration (Ec) of European temperate forests occurs at rather similar rates in stands with different tree species and hydrologic regimes. We tested this hypothesis by synchronously measuring xylem sap flow in four mature stands of Fagus sylvatica along a precipitation gradient with the aim (1) of exploring the regional variability of annual canopy transpiration (Ec(t)) in this species, and (2) of analysing the relationship between precipitation (P) and Ec(t). Ec(t) rates of 216, 225, 272 and 303 mm year−1 corresponded to precipitation averages of 520, 710, 801 and 1,040 mm year−1 in the four stands. We explored the regional variability of Ec(t) in Central European colline to sub-montane beech stands in two meta-analyses based on (1) existing sap flow data on beech (n=5 observations), or (2) all canopy transpiration data on beech obtained by different techniques (sap flow, micrometeorological or soil water budget approaches, n=25). With a coefficient of variation (CV) of 20%, the regional variability of Ec(t) (213–421 mm year−1) was smaller than the variation in corresponding precipitation (550–1,480 mm year−1). The mean Ec(t) for beech was 289 (±58) mm year−1 (n=25). A humped-shaped relationship between Ec(t) and P, with a broad transpiration maximum in the precipitation range from ca. 700 to 1,000 mm year−1, was found which may indicate soil moisture limitation of transpiration for P 1,000 mm year−1. Thus, the precipitation level significantly influences canopy transpiration of humid temperate forests; however, the size of the P influence on Ec(t) and, in part, the direction of its effect differ from forests in semi-arid or arid climates. European beech has the capacity to maintain high Ec rates in both humid and partly dry summer climates (P<550 mm year−1).

title: Drying-rewetting events reduce C and N losses from a Norway spruce forest floor
abstract: Periods of prolonged summer drought are likely to be expected for this century, with possibly strong effects on carbon (C) and nitrogen (N) mineralization in soils. Drought generally reduces mineralization rates, but the possibility of excess mineralization pulses during rewetting raises the question about the net effect of drying–rewetting events. In this experiment, we measured C and N mineralization in undisturbed soil columns that were either kept under continuously moist conditions (control) or that were subjected to drying-rewetting. We had three treatments (D1–D3) with different drying intensity (increasing from D1 to D3) but uniform rewetting intensity (4 mm d−1). Soil columns were taken from a Norway spruce forest in Bavaria, Germany. The CO2 fluxes from control and treatment groups were identical before drying. Over the 80 d drought period, total CO2 emissions from D1, D2, and D3 were only 72, 52 and 43% of that from the control, respectively. Rewetting resulted in a fast increase of CO2 fluxes to approx. the same level as in the control. Rewetting could not restore soil moisture of the dry soil to the level of the control, presumably because of preferential flow and water repellency of soil organic matter. No significant excess C mineralization during the 40 d rewetting period was observed. Adding up total CO2 fluxes during drought and rewetting period, the treatments D1, D2, and D3 emitted only 88, 71 and 67% of the CO2 emitted by the control. Measurements of dissolved organic carbon (DOC) did only show minor differences between control and treatment columns, indicating that no significant accumulation of DOC took place during the drought period. Radiocarbon signature of emitted CO2 indicated that C mineralization was reduced with decreasing water availability and no new substrate became bioavailable. Net N mineralization over the course of the whole experiment was reduced by drought to 77, 65 or 52% of the control. Net nitrification was virtually zero during drought whereas net ammonification continued at reduced levels. In summary, we found that drying–rewetting generally reduced C and N mineralization in this soil and that the total reduction increased with drought intensity.

title: Effect of wetting intensity on soil GHG fluxes and microbial biomass under a temperate forest floor during dry season
abstract: The increasing frequency of periodic droughts followed by heavy rainfalls is expected for this current century, but little is known about the effects of wetting intensity on the in situ biogenic greenhouse gas (GHG) fluxes of forest soils and soil microbial biomass. To gain new insights into the underlying mechanisms responsible for wetting-induced GHG fluxes in situ , rain simulation field experiments during a natural prolonged drought period were done under a temperate forest in northeast China. The intensity of rainfall-induced CO 2 pulses increased from 0.84 to 2.08 g CO 2 –C m − 2  d − 1 with the intensity of wetting up to ca . 80% water-filled pore space, which coincided with an increase in soil microbial biomass and with a decrease in soil labile organic C following wetting. Methane uptake rates decreased from 1.76 to 0.87 mg CH 4 –C m − 2  d − 1 with the intensity of wetting. Wetting dry forest floor increased N 2 O fluxes from 6.2 to 25.9 μg N 2 O–N m − 2  d − 1 , but there was no significant difference between all experimental wetted plots. The rainfall-induced N 2 O pulses with increasing wetting intensity were opposite to that of the CO 2 pulses, showing a maximum response at the lowest wetting intensity. An analysis of the temperature sensitivity of GHG fluxes indicated that temperature had an increased effect on the in situ CO 2 flux and CH 4 uptake, respectively, under wetted and dry conditions. The global warming potential of GHG fluxes and Q 10 value of the temperature response of CO 2 fluxes increased linearly with wetting intensity. The results indicate that the rainfall-induced soil CO 2 pulse is mainly due to enhanced microbial consumption on substrates and highlight the complex nature of belowground C-cycling responses to climate change in northeast China forests that normally experience periodic droughts followed by heavy rainfalls over the year.

title: Microbial biomass dynamics in recently air-dried and rewetted soils compared to others stored air-dry for up to 103 years
abstract: Our aim was to compare the soil microbial biomass concentration and its activity (measured as CO 2 –C evolved) following the rewetting and aerobic incubation of soils which have previously been stored air-dry for different periods. Some of the soils have been stored in the Rothamsted sample archive for 103 years, others were comparable freshly sampled soils following air-drying and rewetting and other soils were stored air-dry for 2 years then rewetted for the work described here. Following air-drying, soil ATP concentrations were variable in recently air-dried soil, comprising about 10–35% of the initial ATP concentrations in fresh soil. Following rewetting, the percentage recovery of ATP increased in all soils by 7 days, then declined to between 73% and 87% of the original ATP concentration in the air-dried soils by day 12. Storage of air-dried soils decreased the ability of the microbial biomass to restore its ATP concentrations. For example, the ATP concentration in a soil sampled from stubbed (i.e. tree seedling, saplings and bushes cut frequently to ground level) grassland of the Broadbalk continuous wheat experiment at Rothamsted then air-dried for 2 years was only about 14% of that in the fresh soil at 2 days after rewetting. In other soils from the Hoosfield Barley Experiment, also at Rothamsted, previously given NPK or FYM since 1852, and sampled then stored air-dry for between 13 and 83 years, from 52% to 57% of the ATP in the comparable fresh soils was measured at two days after rewetting. The soil ATP concentration then changed little more up to 12 days. One of the most interesting findings was that while the microbial biomass ATP concentration in the above NPK soils only ranged from about 2 to 4 μmol ATP g −1 biomass C, in the FYM soil the microbial biomass ATP concentrations (range 11.5–13.6 μmol ATP g −1 biomass C) were the same as we repeatedly measure in fresh moist aerobic soil. We do not yet know the reasons for this. More than twice as much CO 2 –C was evolved from the long-term stored soils than from freshly sampled ones. However, the specific respiration of the microbial biomass did not change much after the first 12 years of storage, indicating that loss of viability mainly occurred in the earlier years.

title: Response of microbial biomass to alternate moist and dry conditions in a soil incubated with 14C- and 15N-labelled plant material
abstract: A red mediterranean soil was incubated for 1.5 yr, with 14C- and 15N-labelled plant material under constant temperature and moisture conditions. Then a portion of the soil was submitted to 4 rapid drying (at 40°C) and rewetting cycles. The duration of the dry periods ranged from 8 to 10 days and the wet periods from 15 to 20 days. Another portion of the soil was incubated under continuously moist conditions. At the end of each dry and moist period, biomass-C and -N were estimated, using the chloroform fumigation technique. The portion of biomass killed on drying and that restored after rewetting were calculated, by the difference between the sizes of biomass present after the dry and the moist periods. 
 
Soil drying destroyed 13 to 14 of biomass and at each cycle, after remoistening, the biomass was progressively restored to approximately the same size as before drying. 
 
The labelled-C to total-C ratio of the CO2 released from undisturbed and continuously moist soil, ranged from 6 to 7%. In biomass, which survived the drying, the values ranged from 20 to 22%, whereas in the killed biomass they ranged between 7 and 8%, i.e. the same orders of magnitude as that of CO2 evolved from undisturbed soil. 
 
A comparison of the labelled-C to total-C ratio of (1) CO2C released from undisturbed and continuously moist soil, (2) the extra CO2-C evolved as a result of alternate drying-remoistening conditions, (3) CO2C released from the soil at the end of moist periods, (4) the C of microbial biomass which survived the drying, and (5) the C of the biomass present at the end of the moist periods, revealed that the calculated portion of biomass killed on drying essentially corresponded to a still relatively “active” fraction of biomass and that the biomass surviving rapid drying, essentially corresponded to a dormant and protected fraction. 
 
In contrast to the labelled-C to total-C ratio, the labelled-N to total-N ratio, in the fraction of biomass which was destroyed on drying, was not different from that of the surviving fraction. During incubation, labelled nitrates accumulated progressively in soil; transformations of N were probably affected by a remetabolization of the nitrates and of material made decomposable on drying, including destroyed microflora and non-biomass material.

title: Microbial biomass response to a rapid increase in water potential when dry soil is wetted
abstract: The proportion of microbial biomass-C released into the soil environment following rapid water potential increase was quantified in two soils using a modified chloroform-fumigation biomass assay. Dry samples were isopiestically equilibrated to −2.8 and − 6.9 M Pa and then wetted to field capacity with either H2O or KCl solutions. The KC1 solutions wetted the soils without altering total soil water potential. The biomass-C released by water potential increase ranged from 17 to 70% of total, depending on the soil, the magnitude of the increase, and the method of calculation. In both soils, a greater proportion of biomass-C was released following a 6.9 MPa than a 2.8 M Pa increase. Biomass-C release was also demonstrated by an increase in soluble organic C in leachates of soils subjected to rapid wetting. Respiration of biomass-C mobilized by water potential increase exceeded respiration of biomass-C made available by preceding desiccation, thereby comprising a significant component of the pulse of respiration observed following wetting of dry soil. Water potential increases associated with the wetting of dry soil may be a major catalyst for soil C turnover.

title: Effects of soil rewetting and thawing on soil gas fluxes: a review of current literature and suggestions for future research
abstract: The rewetting of dry soils and the thawing of frozen soils are short-term, transitional phenomena in terms of hydrology and the thermodynamics of soil systems. The impact of these short-term phenomena on larger scale ecosystem fluxes is increasingly recognized, and a growing number of studies show that these events affect fluxes of soil gases such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ammonia (NH3) and nitric oxide (NO). Global climate models predict that future climatic change is likely to alter the frequency and intensity of drying-rewetting events and thawing of frozen soils. These future scenarios highlight the importance of understanding how rewetting and thawing will influence dynamics of these soil gases. This study summarizes findings using a new database containing 338 studies conducted from 1956 to 2011, and highlights open research questions. The database revealed conflicting results following rewetting and thawing in various terrestrial ecosystems and among soil gases, ranging from large increases in fluxes to non-significant changes. Studies reporting lower gas fluxes before rewetting tended to find higher post-rewetting fluxes for CO2, N2O and NO; in addition, increases in N2O flux following thawing were greater in warmer climate regions. We discuss possible mechanisms and controls that regulate flux responses, and recommend that a high temporal resolution of flux measurements is critical to capture rapid changes in gas fluxes after these soil perturbations. Finally, we propose that future studies should investigate the interactions between biological (i.e., microbial community and gas production) and physical (i.e., porosity, diffusivity, dissolution) changes in soil gas fluxes, apply techniques to capture rapid changes (i.e., automated measurements), and explore synergistic experimental and modelling approaches.