Upland Forest Soils Research Articles

Spatial and vertical distribution of mercury in upland forest soils across the northeastern United States

Assessing current Hg pools in forest soils of the northeastern U.S. is important for monitoring changes in Hg cycling. The forest floor, upper and lower mineral horizons were sampled at 17 long-term upland forest sites across the northeastern U.S. in 2011. Forest floor Hg concentration was similar across the study region (274 ± 13 μg kg−1) while Hg amount at northern sites (39 ± 6 g ha−1) was significantly greater than at western sites (11 ± 4 g ha−1). Forest floor Hg was correlated with soil organic matter, soil pH, latitude and mean annual precipitation and these variables explained approximately 70% of the variability when multiple regressed. Mercury concentration and amount in the lower mineral soil was correlated with Fe, soil organic matter and latitude, corresponding with Bs horizons of Spodosols (Podzols). Our analysis shows the importance of regional and soil properties on Hg accumulation in forest soils.

Sustained large stimulation of soil heterotrophic respiration rate and its temperature sensitivity by soil warming in a cool-temperate forested peatland

We conducted a soil warming experiment in a cool-temperate forested peatland in northern Japan during the snow-free seasons of 2007–2011, to determine whether the soil warming would change the heterotrophic respiration rate and its temperature sensitivity. We elevated the soil temperature by 3°C at 5-cm depth by using overhead infrared heaters and continuously measured hourly soil CO2 fluxes with a 15-channel automated chamber system. The 15 chambers were divided into three groups each with five replications for the control, unwarmed-trenched and warmed-trenched treatments. Soil warming enhanced heterotrophic respiration by 82% (mean of four seasons (2008–2011) observation±SD, 6.84±2.22 µmol C m−2 s−1) as compared to the unwarmed-trenched treatment (3.76±0.98 µmol C m−2 s−1). The sustained enhancement of heterotrophic respiration with soil warming suggests that global warming will accelerate the loss of carbon substantially more from forested peatlands than from other upland forest soils. Soil warming likewise enhanced temperature sensitivity slightly (Q10, 3.1±0.08 and 3.3±0.06 in the four-season average in unwarmed- and warmed-trenched treatments, respectively), and significant effect was observed in 2009 (p<0.001) and 2010 (p<0.01). However, there was no significant difference in the basal respiration rate at 10°C (R10, 2.2±0.52 and 2.8±1.2 µmol C m−2 s−1) between treatments, although the values tended to be high by warming throughout the study period. These results suggest that global warming will enhance not only the heterotrophic respiration rate itself but also its Q10 in forests with high substrate availability and without severe water stress, and predictions for such ecosystems obtained by using models assuming no change in Q10 are likely to underestimate the carbon release from the soil to the atmosphere in a future warmer environment.

Soil Drainage Class Influences on Soil Carbon in a New England Forested Watershed

Imperfectly drained soils under forests can occupy extensive areas of the landscape and understanding how they might differ from upland forest soils can be critical for understanding soil organic carbon (SOC) across forested landscapes. Research on forest SOC across drainage gradients has been limited. This research was a case study at the Bear Brook Watershed in Maine (BBWM), a long‐term ecological research site, that focused on SOC pools and fluxes across three soil drainage classes (moderately well drained‐MWD, somewhat poorly drained‐SWPD, poorly drained‐PD) and two forest types (coniferous‐CF, broadleaved deciduous‐BLD). Soil respiration (RS), soil temperature, soil moisture content, and extractable soluble C were measured from May through November to assess Rsas an indicator of biological activity. The MWD mineral soil had significantly higher C concentrations(0.71 g kg−1) compared to SWPD (0.30 g kg−1) and PD (0.34 g kg−1). The MWD soils in CF had significantly greater C content (433.3 Mg C ha−1) compared to SWPD (113.95 Mg C ha−1) and PD (103.3 Mg C ha−1). Soil C concentration and content was not significantly different between SWPD and PD soils. Mean mineral soil C fraction concentrations (active, stable, passive) expressed as a percent of the total soil mass were significantly different among soil drainage classes. Results from this case study suggest that pedogenesis, above and belowground plant productivity, and moisture availability across the growing season play important roles in determining SOC dynamics in these forests.

The Yasso07 soil carbon model – Testing against repeated soil carbon inventory

Forest soils store large amounts of carbon (C), and releases of C from this pool may significantly increase the CO2 concentration in the atmosphere. Organic matter decomposition in soils has been shown to strongly depend on temperature and soil moisture and is, therefore, susceptible to the climate change. Reliable methods are needed to monitor and predict the changes in soil C stocks. In this study, we tested the Yasso07 soil C model by comparing the model predictions to repeated soil C measurements of organic layer and, furthermore, to the estimates of two other C models, namely Yasso and ROMUL. In the model simulations, we used the litter input time series derived from forest biomass estimates based on the national forest inventories. Both the repeated empirical measurements and Yasso07 simulations indicated upland forest soils to be small sinks of C in Southern Finland. The Yasso07 model was able to predict both soil C stock and C accumulation within the error limits of the measured values. Yasso07 and the earlier version, Yasso, predicted very similar soil C stocks close to the measured values, but slightly underestimated C accumulation. The annual soil C changes predicted by the Yasso07 and ROMUL models were reasonably close to each other, even though the models are based on a very different basic structure. However, the differences in the model predictions were at the highest in years with the highest precipitation, indicating that there are still uncertainties in predicting the effects of soil moisture on the soil C stock changes.

A regional approach for mineral soil weathering estimation and critical load assessment in boreal Saskatchewan, Canada

In boreal regions of the province of Saskatchewan, Canada, there is concern over emerging acid precursor emission sources associated with the oil sands industry. Base cation weathering rates (BCw) and steady-state critical loads of sulfur (CLS) were identified for upland forest soil plots (n=107) in 45 ecodistricts according to a new method for approximation of BCw in the region. This method was developed by regression of simple soil and site properties with BCw calculated through application of a soil chemical model (PROFILE). PROFILE was parameterized using detailed physicochemical data for a subset (n=35) of the sites. Sand content, soil moisture and latitude emerged as important predictive variables in this empirical regression approximation. Base cation weathering varied widely (0.1–8000mmolcm−3yr−1) across the study sites, consistent with their contrasting soil properties. Several sites had lower rates than observed in other acid-sensitive regions of Canada owing to quartz dominated mineralogy and coarse-textured soils with very low surface area. Weathering was variable within ecodistricts, although rates were consistently low among ecodistricts located in the northwest of the province. Overall, half of the forest plots demonstrated CLS less than 45mmolcm−2yr−1. Historically, the acidification risk in this region has been considered low and monitoring has been limited. Given the very low CLS in many northern ecodistricts and the potential for increased acid deposition as oil sands activities expand, soil acidification in these regions warrants further study.

Evolution of organic matter in lignite-containing sediments revealed by analytical pyrolysis (Py–GC–MS)

Newly vegetated sites provide opportunities to enlighten organic matter (OM) transformation mechanisms in soils and sediments at very early stages of development which, in turn, is relevant to better understand general ecosystem functioning. Mine acid soils and sediments in the Lusatian open cast lignite mining district (Germany) contains a high concentration of fossil carbon (lignite) in ad mixture with recent OM from the local vegetation, both contributing to the humified OM pool. In this study, analytical pyrolysis (Py–GC–MS) was used to monitor the different C sources (lignite or plant derived) in developing mine tailing soils and sediments and their degree of degradation in contrasting environments. Representative vegetation and the organic carbon (OC) rich soil/sediment fraction (humus fraction) were sampled at two depths (0–5 and 5–10cm) in three plots along a transect covering an upland forest soil, a partially submerged sediment at the land–water interface and a constantly submerged sediment. The analysis of plant (lipds, isoprenoids, methoxyphenols and carbohydrates) and possible lignite (alkyl napththalenes, alkyl benzenes and PAHs) biomarkers released after pyrolysis supports previous findings in the area using other proxies. It was possible to discern OM sources in soil/sediment humus fractions, both from the substrate (lignite) as well as from the prevailing vegetation of the area. Environmental conditions in the submerged sediment seem to favour OM protection and the accumulation of decomposing plant material, whereas more intense OM degradation seems to prevail in the land–water interface areas characterized by fluctuating water level. In addition, a well resolved series of organic sulfur compounds (OSCS) found in the submerged sediments of rehabilitated acid lakes, indicates the possible occurrence of particular mechanisms of C preservation in this extreme anoxic S rich environment, i.e. via sulphur “quenching” with plant derived lipids during early diagenesis.

The distribution coefficient for cesium in different clay fractions in soils developed from granite and Paleozoic shales in Japan

The distribution coefficient of cesium (Cs) (KdCs) was investigated for clays which were collected from upland forest soils developed from granites and Paleozoic shales in Japan. They were divided into three size fractions, coarse (2.0–1.0 µm), medium (1.0–0.2 µm), and fine-clay fractions (<0.2 µm), abbreviated CC, MC, and FC, respectively. The KdCs value increased in the fractions as follows: FC < MC < CC in all the horizon samples. The amount of potassium (K) in the clay fractions also followed this same pattern. As the amount of K in the clay fraction can be a quantitative indicator of illitic minerals, the positive relationship between the KdCs and the amount of K indicated that the coarser clay fraction had a higher 137Cs adsorption potential due to containing higher amounts of illitic minerals. The soil clay fraction from the Paleozoic shales both contained a large amount of K and showed greater KdCs values than that of the granite when comparing the same particle sizes. Hence, the type of parent material can be a significant factor in determining the 137Cs adsorption potential of the soils. In all particle fractions, those collected from deeper in the profile had smaller KdCs values than those from closer to the soil surface in the same profile, although amount of K present was similar. The X-ray diffraction (XRD) analysis showed that the hydroxy-aluminum (Al) polymer was present in the 2:1 interlayer sites to a greater extent in the clay fractions from deeper in the soil profile than in those from upper in the same soil profile. This result suggests that the hydroxy-Al polymers may be fixed at the expanded weathering front of the illitic minerals, blocking Cs adsorption on the frayed edge sites of these minerals. Thus, the KdCs value of the soil clays is determined by the degree of illitic minerals and hydroxy-Al interlayering present, with the larger the amount of illitic minerals the greater the Cs adsorption potential (with any hydroxy-Al interlayering reducing this effect).

Assimilation of acetate by the putative atmospheric methane oxidizers belonging to the USCα clade

Forest soils are a major biological sink for atmospheric methane, yet the identity and physiology of the microorganisms responsible for this process remain unclear. Although members of the upland soil cluster α (USCα) are assumed to represent methanotrophic bacteria adapted to the oxidation of the trace level of methane in the atmosphere and to be an important sink of this greenhouse gas, so far they have resisted isolation. In particular, the question of whether the atmospheric methane oxidizers are able to obtain all their energy and carbon solely from atmospheric methane still waits to be answered. In this study, we performed stable-isotope probing (SIP) of RNA and DNA to investigate the assimilation of (13) C-methane and (13) C-acetate by USCα in an acidic forest soil. RNA-SIP showed that pmoA mRNA of USCα was not labelled by (13) C of supplemented (13) C methane, although catalysed reporter deposition - fluorescence in situ hybridization (CARD-FISH) targeting pmoA mRNA of USCα detected its expression in the incubated soil. In contrast, incorporation of (13) C-acetate into USCαpmoA mRNA was observed. USCαpmoA genes were not labelled, indicating that they had not grown during the incubation. Our results indicate that the contribution of alternative carbon sources, such as acetate, to the metabolism of the putative atmospheric methane oxidizers in upland forest soils might be substantial.

Temperature sensitivity of soil respiration is affected by nitrogen fertilization and land use

Wetlands are an important component of the terrestrial ecosystem, and play a crucial role in sequestering carbon. However, to date, there is little information about the land-use and nitrogen-fertilization effects on temperature sensitivity of soil respiration in wetland. In this investigation, effects of land use and nitrogen fertilization on temperature sensitivity of soil respiration (Q 10) in a freshwater marsh of northeast China were studied. The results showed that change of land use significantly affected Q 10-value, which followed the order: Intact Deyeuxia angustifolia wetland soil >upland forest soil >abandoned cultivated soil >cultivated soil. Our data confirmed that soil temperature and moisture were important factors affecting Q 10-values. Besides temperature and soil moisture, availability of C and N and microbial activity in soil were important factors affecting Q 10-values. Nitrogen fertilization resulted in an increase in Q 10-value not only in the intact wetland, but also in the cultivated soil. Although availability of N could stimulate temperature sensitivity of soil respiration, high nitrogen fertilization (i.e., 240 kg N ha−1 in this study) inhibited temperature sensitivity. Further studies are indicated as a means of answering these questions and providing additional information on the effects of nitrogen fertilization on Q 10-value.

Temperature sensitivity of organic matter decomposition in two boreal forest soil profiles

Controversial conclusions from different studies suggest that the decomposition of old soil organic matter (SOM) is either more, less, or equally temperature sensitive compared to the younger SOM. Based on chemical kinetic theory, the decomposition of more recalcitrant materials should be more temperature sensitive, unless environmental factors limit decomposition. Here, we show results for boreal upland forest soils supporting this hypothesis. We detected differences in the temperature sensitivity 1) between soil layers varying in their decomposition stage and SOM quality, and 2) inside the layers during a 495 day laboratory incubation. Temperature sensitivity increased with increasing soil depth and decreasing SOM quality. In the organic layers, temperature sensitivity of decomposition increased during the early part of a 495 day laboratory incubation, after respiration rate and SOM quality had notably decreased. This indicates that decomposition of recalcitrant compounds was more temperature sensitive than that of the labile ones. Our results imply that Q 10 values for total heterotrophic soil respiration determined from short-term laboratory incubations can either underestimate or overestimate the temperature sensitivity of SOM decomposition, depending on soil layer, initial labile carbon content and temperature range used for the measurements. Using Q 10 values that ignore these factors in global climate models provides erroneous estimates on the effects of climate change on soil carbon storage.

Carbon Mineralization and Labile Organic Carbon Pools in the Sandy Soils of a North Florida Watershed

The large pool of actively cycling carbon (C) held in soils is susceptible to release due to changes in landuse, management, or climate. Yet, the amount and distribution of potentially mineralizable C present in soils of various types and the method by which this soil C fraction can best be quantified, are not well established. The distribution of total organic C (TOC), extractable C pools (hot-water-extractable and acid-hydrolyzable), and in vitro mineralizable C in 138 surface soils across a north Florida watershed was found to be quite heterogeneous. Thus, these C quality parameters could not statistically distinguish the eight landuses or four major soil orders represented. Only wetland and upland forest soils, with the largest and smallest C pool size, respectively, were consistently different from the soils of other landuse types. Variations in potential C mineralization were best explained by TOC (62%) and hot-water-extractable C (59%), whereas acid-hydrolyzable C (32%) and clay content (35%) were generally not adequate indicators of C bioavailability. Within certain landuse and soil orders (Alfisol, Wetland and Rangeland, all with >3% clay content), however, C mineralization and clay content were directly linearly correlated, indicating a possible stimulatory effect of clay on microbial processing of C. Generally, the sandy nature of these surface soils imparted a lack of protection against C mineralization and likely resulted in the lack of landuse/soil order differences in the soil C pools. If a single parameter is to be chosen to quantify the potential for soil C mineralization in southeastern U.S. coastal plain soils, we recommend TOC as the most efficient soil variable to measure.

Positive relationships between phenol oxidase activity and extractable phenolics in estuarine soils

Decomposition of recalcitrant materials such as phenolics is known to play a pivotal role in organic matter decomposition and nutrient cycling in estuaries. The specific goals of this study were to determine temporal and spatial variations of phenol oxidase and phenolics in estuarine soils, and to elucidate controlling factors for phenol oxidase activity. To achieve these goals, phenol oxidase activity and phenolic content were measured in soils developed along the side of an estuary in the Han River, Korea. Soil samples were collected in three locations with different vegetation: mud flats, Zizania-dominated soils, and Salix-dominated soils. Monthly measurements were also made in a Zizania-dominated site over a year period. Phenol oxidase activity varied between 0.00 and 0.28 diqc min−1 g−1, whilst phenolic content ranged from 0.0–10.5 μg g−1. A correlation analysis revealed that phenol oxidase activity exhibited positive correlations with phenolic content in both seasonal and spatial data. The same relationship was found when the data were analysed separately for each site. Unlike peatlands or upland forest soils where negative correlations were often found between phenol oxidase activity and phenolics, substrate induction appears to account for the positive correlation in the present study.

Temperature effects on ethylene and methane production from temperate forest soils

Change in temperature affects the activity of soil microorganisms. However, there is limited knowledge about temperature effects on ethylene (C2H4) and methane (CH4) production from forest soils. Topsoil samples (0–5 cm) collected from different temperate forest stands (e.g., Pinus sylvestris L., Cryptomeria japonica, and Quercus serrata) were used to compare C2H4 and CH4 production from soils at temperature from 5 to 35°C under oxic and anoxic conditions. The rates of C2H4 and CH4 production from soils under oxic conditions were measured by using inhibition of acetylene (C2H2) and carbon monoxide (CO). The consumption of C2H2 by soils at an initial concentration of c. 250 Pa C2H2 was negligible at 5 and 15°C, but it was significantly increased at 25 and 35°C. The presence of 2 kPa CO in the headspace gases tended to decrease the consumption of C2H2 by soils at high temperature. The Q10 values for the soil C2H2 consumption ranged from 2.3 to 3.8, and there were no significant differences in Q10 values between these topsoil samples. The rate of CH4 production from each sample under oxic conditions was larger than the soil C2H4 production at 5–35°C, particularly at low temperature, and presented a smaller Q10 value. Ethylene production from soil after 1 week of anoxic incubation at 5–35 °C was larger than the soil CH4 production, and presented a larger Q10 value. However, CH4 production from Quercus serrata forest soil and its response to temperature increased significantly with incubation time. Long-term anoxic conditions of in situ upland forest soils are normally not prevalent, so it can be reasonably concluded that there is a larger C2H4 production rather than CH4 production under temperate forest stands due to heavy rainfall in summer.

Modeling of Sea‐Level Rise and Deforestation in Submerging Coastal Ultisols of Chesapeake Bay

Global warming and the predicted adverse environmental implications in coastal areas have prompted the use of a quantitative pedological approach to model the history of relative sea‐level rise and the subsequent lateral migration of marshes (deforestation) during the pedogenic transformation of Ultisols (forest soils) to Histosols (marsh soils). The modeling was based on the transgressing soil‐landscape concept and data collected from sampling units along two marsh transects in Dorchester County, Maryland. In low‐relief submerging environments, coastal marshes accrete vertically and migrate laterally over adjacent Ultisols to keep pace with sea‐level rise. The marsh substrate is organic in nature overlying what once were upland forest soils. The history of relative sea‐level rise was a two‐step linear function illustrating acceleration in the last two centuries. During the last150 yr, the 210Pb‐based rate of relative sea‐level rise averaged 2.4 ± 0.7 mm yr −1 Before the last few hundred years, the long‐term 14C‐based rate of relative sea‐level rise was 0.82 ± 0.15 mm yr −1 The rate of lateral migration of coastal marshes (deforestation) showed similar acceleration following the history of relative sea‐level rise. During the last 150 yr, the average rate of lateral migration of marshes (deforestation) ranged from 3.51 ± 2.0 to 6.78 ± 7.4 m yr−1 Before the last150 yr, the average long‐term rate of lateral migration of marshes (deforestation) ranged between 0.18 ± 0.07 and 1.27 ± 1.2 m yr−1 Within a given region where sea‐level rise and tidal range remain constant, the rate of lateral migration of marshes is a function of site characteristics and the slope of the adjacent uplands. The predicted rate of lateral migration of marshes in the next century ranges from 3.2 to 33 m yr−1, averaging 14 ± 11 m yr−1 Assuming that the present rate of relative sea‐level rise remains constant, the time needed to complete the pedogenic transformation of Ultisols to Histosols is 180 ± 35 yr. The most conservative future scenario of sea‐level rise has reduced this time by 63%. The projected sequestration of C, total N, and pyrite in the newly formed coastal marshes during the next 100 yr average 231 ± 124, 11.7 ± 6.30, and 6.63 ± 3.57 Mg ha−1, respectively. These projections are half the nutrient sequestration potential of existing coastal marshes.

Soil organic carbon quality in forested mineral wetlands at different mean annual temperature

Forested mineral soil wetlands (FMSW) store large stocks of soil organic carbon (SOC), but little is known on: (i) whether the quality of SOC stored in these soils (proportion of active versus more resistant SOC compounds) differs from SOC in upland soils; (ii) how the quality of SOC in FMSW varies with mean annual temperature (MAT); and (iii) whether SOC decomposition rates in these environments respond to warming and drying more strongly than those observed in upland soils. To address this substantial knowledge gap, we identified nine FMSW and fifteen paired upland forest sites across three bioregions in North America (sub-alpine in Colorado; north-temperate in Minnesota; and south-temperate in South Carolina) to test the following three hypotheses. First, FMSW store a higher proportion of active SOC compared with upland systems because long anaerobic periods favor the accumulation of labile substrates. Second, in FMSW, SOC quality decreases from cold to warm bioregions because high quality detritus accumulates preferentially at cool sites where decomposition is slow. Finally, decomposition of SOC in FMSW will respond more strongly to warming under aerobic conditions than SOC from upland forest soils because of higher accumulation of active SOC in FMSW. To test these hypotheses, we incubated FMSW and upland forest soils at two constant temperatures (10 and 30 °C) for 525-d under aerobic conditions and constant moisture. In contrast to our first hypothesis, we observed similarly rapid depletion of active SOC compounds at initial stages of incubation across FMSW and upland sites, and across the 525-d incubations we observed overall lower SOC decomposition rates in our FMSW soils. In line with our second hypothesis, and across FMWS and upland soils, we found greater SOC loss in the sub-alpine bioregion than both temperate regions. In contrast to our last hypothesis, we found no difference in the temperature sensitivity ( Q 10) of SOC decomposition in FMSW and upland forest soils. Critically, total SOC loss (g SOC per g soil) was larger in FMSW because of the large amount of SOC stored in these ecosystems, indicating that despite a lack of difference between FMSW and upland responses, the total release of C from FMSW that could result from global warming may be large.

Methane emissions from upland forest soils and vegetation

Most work on methane (CH(4)) emissions from natural ecosystems has focused on wetlands because they are hotspots of CH(4) production. Less attention has been directed toward upland ecosystems that cover far larger areas, but are assumed to be too dry to emit CH(4). Here we review CH(4) production and emissions in upland ecosystems, with attention to the influence of plant physiology on these processes in forests. Upland ecosystems are normally net sinks for atmospheric CH(4) because rates of CH(4) consumption exceed CH(4) production. Production of CH(4) in upland soils occurs in microsites and may be common in upland forest soils. Some forests switch from being CH(4) sinks to CH(4) sources depending on soil water content. Plant physiology influences CH(4) cycling by modifying the availability of electron donors and acceptors in forest soils. Plants are the ultimate source of organic carbon (electron donor) that microbes process into CH(4). The availability of O(2) (electron acceptor) is sensitive to changes in soil water content, and therefore, to transpiration rates. Recently, abiotic production of CH(4) from aerobic plant tissue was proposed, but has not yet been verified with independent data. If confirmed, this new source is likely to be a minor term in the global CH(4) budget, but important to quantify for purposes of greenhouse gas accounting. A variety of observations suggest that our understanding of CH(4) sources in upland systems is incomplete, particularly in tropical forests which are stronger sources then expected.

Soil carbon modelling as a tool for carbon balance studies in forestry

Soils represent a remarkable stock of carbon, and forest soils are estimated to hold half of the global stock of soil carbon. Topical concern about the effects of climate change and forest management on soil carbon as well as practical reporting requirements set by climate conventions have created a need to assess soil carbon stock changes reliably and transparently. The large spatial variability of soil carbon commensurate with relatively slow changes in stocks hinders the assessment of soil carbon stocks and their changes by direct measurements. Due to these difficulties in measuring soil carbon, models widely serve to estimate carbon stocks and stock changes in soils. This dissertation aimed to develop the soil carbon model YASSO for upland forest soils. The model was aimed to take into account the most important processes controlling the decomposition in soils, yet remain simple enough to ensure its practical applicability in different applications. The model structure and assumptions were presented and the model parameters were defined with empirical measurements. The model was evaluated by studying the sensitivities of the model results to parameter values, by estimating the precision of the results with an uncertainty analysis, and by assessing the accuracy of the model by comparing the predictions against measured data and by comparing the model results to the results of an alternative model. The model was applied at the stand level to study the effects of intensified biomass extraction on the forest carbon balance. In another application, the effects of energy use of forest residues on soil carbon were quantified with the model. The model calculated soil carbon deficit was presented as an indirect CO2 emission. This emission was then compared to other emissions from the forest residue production chain and burning. Finally, the model was applied in an inventory based method to assess the national scale forest carbon balance for Finland’s forests from 1922 to 2004. According to the results of the uncertainty and sensitivity analyses, the soil carbon stock estimates of the model are uncertain, because those parameters that most strongly affect these estimates are poorly known. Carbon stock change estimates, on the other hand, are rather reliable, because the parameters determining these estimates are known better. According to a test conducted with a Canadian litterbag experiment, YASSO managed to describe sufficiently the effects of both the variable litter and climatic conditions on decomposition. When combined with the stand models or other systems providing litter information, the dynamic approach of the model proved to be powerful for estimating changes in soil carbon stocks on different scales. The climate dependency of the model, the effects of nitrogen on decomposition and forest growth as well as the effects of soil texture on soil carbon stock dynamics are areas for development when considering the applicability of the model to different research questions, different land use types and wider geographic regions. Intensified biomass extraction affects soil carbon stocks, and these changes in stocks should be taken into account when considering the net effects of forest residue utilisation as energy. On a national scale, soil carbon stocks play an important role in forest carbon balances.

Seed transfers of silver birch (Betula pendula) from the Baltic to Finland – effect on growth and stem quality

Silver birch (Betula pendula Roth) seed origins from the Baltic countries, Finland and Russia were compared for survival, growth and stem quality, and the effect of latitudinal seed transfer distance examined in two provenance trials. The trials were located on moist upland forest soils at Tuusula (60°21’N) in southern Finland and at Viitasaari (63°11’N) in central Finland. The material consisted of 21 stand and single tree origins ranging from latitudes 54° to 63°N. Survival, height, dbh, relative stem taper, stem volume/ha and the proportion of trees with a stem defect (vertical branch or forked stem), were assessed when the trees were 22 years old. Significant differences were detected among the origins regarding all the measured traits in both trials. Southern Finnish origins produced the highest volume per unit area in central Finland, whereas Estonian and north Latvian stand seed origins, as well as the southern Finnish plus tree origins, were the most productive ones in southern Finland. The more southern the origin, the higher was the proportion of trees with a stem defect in both trials. The latitudinal seed transfer distance had a significant but relatively small effect on survival, stem volume/ha and proportion of trees with a stem defect. The proportion of trees with a stem defect increased linearly in relation to the seed transfer distance from the south. The relationship of both survival and stem volume/ha to the seed transfer distance was curvilinear. Volume/ha was increased by transferring seed from ca. 2 degrees of latitude from the south. A longer transfer from the south, as well as transfer from the north, decreased the yield.

Effect of temperature on composition of the methanotrophic community in rice field and forest soil

Temperature change affects methane consumption in soil. However, there is no information on possible temperature control of methanotrophic bacterial populations. Therefore, we studied CH(4) consumption and populations of methanotrophs in an upland forest soil and a rice field soil incubated at different temperatures between 5 and 45 degrees C for up to 40 days. Potential methane consumption was measured at 4% CH(4). The temporal progress of CH(4) consumption indicated growth of methanotrophs. Both soils showed maximum CH(4) consumption at 25-35 degrees C, but no activity at >40 degrees C. In forest soil CH(4) was also consumed at 5 degrees C, but in rice soil only at 15 degrees C. Methanotroph populations were assessed by terminal restriction fragment length polymorphism (T-RFLP) targeting particulate methane monooxygenase (pmoA) genes. Eight T-RFs with relative abundance >1% were retrieved from both forest and rice soil. The individual T-RFs were tentatively assigned to different methanotrophic populations (e.g. Methylococcus/Methylocaldum, Methylomicrobium, Methylobacter, Methylocystis/Methylosinus) according to published sequence data. Two T-RFs were assigned to ammonium monooxygenase (amoA) gene sequences. Statistical tests showed that temperature affected the relative abundance of most T-RFs. Furthermore, the relative abundance of individual T-RFs differed between the two soils, and also exhibited different temperature dependence. We conclude that temperature can be an important factor regulating the community composition of methanotrophs in soil.

Nitrous oxide emissions from selected natural and managed northern ecosystems

Microbial activity in soils is the main source of nitrous oxide (N2O) to the atmosphere. Nitrous oxide is a strong greenhouse gas in the troposphere and participates in ozone destructive reactions in the stratosphere. The constant increase in the atmospheric concentration, as well as uncertainties in the known sources and sinks of N2O underline the need to better understand the processes and pathways of N2O in terrestrial ecosystems. This study aimed at quantifying N2O emissions from soils in northern Europe and at investigating the processes and pathways of N2O from agricultural and forest ecosystems. Emissions were measured in forest ecosystems, agricultural soils and a landfill, using the soil gradient, chamber and eddy covariance methods. Processes responsible for N2O production, and the pathways of N2O from the soil to the atmosphere, were studied in the laboratory and in the field. These ecosystems were chosen for their potential importance to the national and global budget of N2O. Laboratory experiments with boreal agricultural soils revealed that N2O production increases drastically with soil moisture content, and that the contribution of the nitrification and denitrification processes to N2O emissions depends on soil type. Laboratory study with beech (Fagus sylvatica) seedlings demonstrated that trees can serve as conduits for N2O from the soil to the atmosphere. If this mechanism is important in forest ecosystems, the current emission estimates from forest soils may underestimate the total N2O emissions from forest ecosystems. Further field and laboratory studies are needed to evaluate the importance of this mechanism in forest ecosystems. The emissions of N2O from northern forest ecosystems and a municipal landfill were highly variable in time and space. The emissions of N2O from boreal upland forest soil were among the smallest reported in the world. Despite the low emission rates, the soil gradient method revealed a clear seasonal variation in N2O production. The organic topsoil was responsible for most of the N2O production and consumption in this forest soil. Emissions from the municipal landfill were one to two orders of magnitude higher than those from agricultural soils, which are the most important source of N2O to the atmosphere. Due to their small areal coverage, landfills only contribute minimally to national N2O emissions in Finland. The eddy covariance technique was demonstrated to be useful for measuring ecosystem-scale emissions of N2O in forest and landfill ecosystems. Overall, more measurements and integration between different measurement techniques are needed to capture the large variability in N2O emissions from natural and managed northern ecosystems.