Werner Borken | University Of Bayreuth, Germany (original) (raw)

Papers by Werner Borken

Global Change Biology, 2012

Climate change may considerably impact the carbon (C) dynamics and C stocks of forest soils. To a... more Climate change may considerably impact the carbon (C) dynamics and C stocks of forest soils. To assess the combined effects of warming and reduced precipitation on soil CO 2 efflux, we conducted a two-way factorial manipulation experiment (4°C soil warming + throughfall exclusion) in a temperate spruce forest from 2008 until 2010. Soil was warmed by heating cables throughout the growing seasons. Soil drought was simulated by throughfall exclusions with three 100 m 2 roofs during 25 days in July/August 2008 and 2009. Soil warming permanently increased the CO 2 efflux from soil, whereas throughfall exclusion led to a sharp decrease in soil CO 2 efflux (45% and 50% reduction during roof installation in 2008 and 2009, respectively). In 2008, CO 2 efflux did not recover after natural rewetting and remained lowered until autumn. In 2009, CO 2 efflux recovered shortly after rewetting, but relapsed again for several weeks. Drought offset the increase in soil CO 2 efflux by warming in 2008 (growing season CO 2 efflux in t C ha À1 : control: 7.1 ± 1.0; warmed: 9.5 ± 1.7; warmed + roof: 7.4 ± 0.3; roof: 5.9 ± 0.4) and in 2009 (control: 7.6 ± 0.8; warmed + roof: 8.3 ± 1.0). Throughfall exclusion mainly affected the organic layer and the top 5 cm of the mineral soil. Radiocarbon data suggest that heterotrophic and autotrophic respiration were affected to the same extent by soil warming and drying. Microbial biomass in the mineral soil (0-5 cm) was not affected by the treatments. Our results suggest that warming causes significant C losses from the soil as long as precipitation patterns remain steady at our site. If summer droughts become more severe in the future, warming induced C losses will likely be offset by reduced soil CO 2 efflux during and after summer drought.

Biogeosciences, 2014

As a key component of the carbon cycle, soil CO 2 efflux (SCE) is being increasingly studied to i... more As a key component of the carbon cycle, soil CO 2 efflux (SCE) is being increasingly studied to improve our mechanistic understanding of this important carbon flux. Predicting ecosystem responses to climate change often depends on extrapolation of current relationships between ecosystem processes and their climatic drivers to conditions not yet experienced by the ecosystem. This raises the question of to what extent these relationships remain unaltered beyond the current climatic window for which observations are available to constrain the relationships. Here, we evaluate whether current responses of SCE to fluctuations in soil temperature and soil water content can be used to predict SCE under altered rainfall patterns. Of the 58 experiments for which we gathered SCE data, 20 were discarded because either too few data were available or inconsistencies precluded their incorporation in the analyses. The 38 remaining experiments were used to test the hypothesis that a model parameterized with data from the control plots (using soil temperature and water content as predictor variables) could adequately predict SCE measured in the manipulated treatment. Only for 7 of these 38 experiments was this hypothesis rejected. Importantly, these were the experiments with the most reliable data sets, i.e., those providing high-frequency measurements of SCE. Regression tree analysis demonstrated that our hypothesis could be rejected only for experiments with measurement intervals of less than 11 days, and was not rejected for any of the 24 experiments with larger measurement intervals. This highlights the importance of high-frequency measurements when studying effects of altered precipitation on SCE, probably because infrequent measurement schemes have insufficient capacity to detect shifts in the climate dependencies of SCE. Hence, the most justified answer to the question of whether current moisture responses of SCE can be extrapolated to predict SCE under altered precipitation regimes is "no" -as based on the most reliable data sets available. We strongly recommend that future experiments focus more strongly on establishing response functions across a broader range of precipitation regimes and soil moisture conditions. Such experiments should make accurate measurements of water availability, should conduct high-frequency SCE measurements, and should consider both instantaneous responses and the potential legacy effects of climate extremes. This is important, because with the novel approach presented here, we demonstrated that, at least for some ecosystems, current moisture responses could not be extrapolated to predict SCE under altered rainfall conditions.

ABSTRACT Weathering of carbonate bedrock (limestone, dolomite) contributes to the CO2 efflux from... more ABSTRACT Weathering of carbonate bedrock (limestone, dolomite) contributes to the CO2 efflux from the soil. To understand the C-cycling of forests on carbonate bedrock, it is necessary to account for this inorganic C flux. Generally, the inorganic contribution to the soil CO2 efflux is thought to be small or neglectable, but attempts to quantify the inorganic CO2 efflux are rare and sometimes complicated because of methodological problems. We used the distinctive isotopic composition of CO2 from the different sources (dolomite weathering, respiration by decomposing microbes) to quantify the inorganic flux. Mesocosms filled with undisturbed soil and underlying dolomite gravel were incubated under constant temperature and different moisture levels. The whole laboratory experiment was executed under exclusion of atmospheric CO2 in order to avoid interference with its isotopic signal. First data indicate a < 2% contribution of inorganic C to the total CO2 efflux from our soil. Further results of this long-term incubation experiment will be presented at the conference.

Soil Biology and Biochemistry, 2016

Soil Use and Management, Nov 30, 1997

AbstractAbstract. The effect of superficial liming of acidic forest soils on CO2 and N2O emission... more AbstractAbstract. The effect of superficial liming of acidic forest soils on CO2 and N2O emissions and CH4 uptake was investigated with closed chambers in two deciduous and two spruce forests, by weekly to biweekly measurements over at least one year. The flux rates of untreated areas varied between 1.94 and 4.38 t CO2-C/ha per y, 0.28 and 2.15 kg/N2O-N/ha per y and between 0.15 and 1.06 kg CH4-C/ha per y. Liming had no clear effect on CO2 emissions which may change in the long-term with decreasing root turnover and increasing C-mineralization. Apart from one exception, liming resulted in a reduction of N2O emissions by 9 to 62% and in an increase of CH4 uptake by 26 to 580%. The variability in N2O emissions between the forest sites could not be explained. In contrast, the variability of annual CH4 uptake rates could be explained by N content (r2= 0.82), C content (r2= 0.77), bulk density (r2= 60), pore space (r2= 0.59) and pH (r2= 0.40) of mineral soil at a depth of 0 to 10 cm, and by the quantity of material in the organic layer (r2= 0.66). Experiments with undisturbed columns of the same soils showed that between 1 and 73% of the total N2O emissions came from the organic layer. However, atmospheric CH4 was not oxidized in this layer, which represents a diffusion barrier for atmospheric CH4. When this barrier was removed, CH4 uptake by the mineral soil increased by 25 to 171%. These results suggest that liming of acidic forest soils causes a reduction of the greenhouse gases N2O and CH4 in the atmosphere, due to changes in the chemical, biological, and physical condition of the soils.

The flux and origin of respired carbon is of importance for determining the sink or source streng... more The flux and origin of respired carbon is of importance for determining the sink or source strength of soils for atmospheric CO2. The objective of this study was to assess the importance of respiration of recent carbon substrates and decomposition of older soil organic matter during severe summer drought, using radiocarbon measured in CO2 of soil respiration and soil air.

Peatlands in the Northern Hemisphere play a significant role in CO2 sequestration and storage but... more Peatlands in the Northern Hemisphere play a significant role in CO2 sequestration and storage but are also potential sources in the changing climate scenario. Regulation of CO2 exchange processes and the relative contribution of the respective ecosystem components in peatlands are, however, poorly understood. Intensive measurements were carried out to understand how ecosystem CO2 exchange processes are regulated and how

ABSTRACT An increase in soil temperatures substantially increases soil respiration rates as both ... more ABSTRACT An increase in soil temperatures substantially increases soil respiration rates as both processes are positively correlated. This has been demonstrated in a wide range of manipulative field and laboratory studies. However, the response of gross and net nitrogen (N) mineralization rates to enhanced soil temperatures has rarely been investigated. Previous studies ascribed the increased carbon (C) mineralization rates under elevated soil temperatures to enhanced microbial metabolic activity. We conduct a laboratory incubation experiment to determine gross and net N mineralization rates as well as CO2 production rates to test the hypothesis that elevated soil temperatures lead to an increase in gross and net N mineralization rates likewise as observed in C mineralization. We expect that gross N mineralization has a different temperature sensitivity than C mineralization. Soil samples were taken from the Achenkirch (Austria) soil warming experiment. Warmed and control plots were established in a mixed spruce-beech forest stand each with three replications in 2004. Soil temperature was enhanced in the warmed plots by 4°C above ambient temperature during the vegetation period. Due to anthropogenic deposition this site is highly N-saturated. Soil samples were taken in two depths (Oi/Oe layer and 0-10 cm). Gross ammonification and nitrification rates were measured by the 15N pool dilution technique, net N and CO2 production rates were determined in lab incubations over 12 weeks. First results of this study will be presented.

Agu Fall Meeting Abstracts, Dec 1, 2003

Low rates of soil respiration during droughts have been related to above-average rates of net eco... more Low rates of soil respiration during droughts have been related to above-average rates of net ecosystem exchange of carbon in forested ecosystems. Lower soil respiration during drought could cause a transitory soil C sink, due to a temporary decline in decomposition, or it could result from reduced root respiration. A throughfall exclusion experiment was established at the Harvard Forest in central Massachusetts to study the effects of experimental drought on soil respiration. Weekly measurements of soil respiration began in the spring of 2001, with 4 manual CO2 flux chambers installed in each of 6 plots (5 x 5 m). In July 2001, sub-canopy roofs with translucent plastic panels were installed in 3 of the plots, while the other 3 control plots were left open. Temperature probes, TDR probes, and gas tubes were buried at 4 depths in each plot. Roofs were removed in autumn 2001 to allow leaf-fall and snowfall, reinstalled in spring 2002, removed again in autumn 2002, and then left off for the entire summer of 2003. The radiocarbon contents of CO2 emissions and concentrations within the soil were measured periodically. Soil respiration was nearly identical in treatment and control plots (140 and 142 g C m-2, respectively) during the 54-d pre-treatment period. During 85 days in 2001 with roofs in place, 168 mm of throughfall were diverted, and the cumulative soil respiration was 30% lower in exclusion versus control plots (237 and 338 g C m-2, respectively). During 127 days of throughfall exclusion (344 mm diverted) in 2002, cumulative soil respiration was 40% lower in exclusion versus control plots (280 and 490 g C m-2, respectively). Clearly, soil water content is an important factor influencing soil respiration. When natural throughfall was allowed in all plots from September 2002 through August 2003, the cumulative soil respiration was only 6% higher in exclusion versus control plots (702 and 658 g C m-2, respectively), indicating only a modest increase in decomposition after previously dried soils were wetted. The Δ 14CO2 in the soil atmosphere was negatively correlated with CO2 concentrations, indicating a larger contribution of decomposition of old, radiocarbon-rich substrates in the mineral soil during drought. In dry mineral soil, root respiration produced less CO2, while gradual decomposition of old, radiocarbon-rich substrates continued at low rates, resulting in low CO2 concentrations with high Δ 14C values. In contrast, microbial decomposition of old substrates declined more rapidly than did decomposition of young substrates and root respiration in the O horizon, causing the Δ 14CO2 of surfaces fluxes to decline with drought. A depth-dependent differential response of root and microbial respiration to drought is indicated. If decreased root respiration is responsible, it may reflect altered belowground C allocation, but larger treatment plots that include entire trees would be needed to address this question. In any case, a transient soil C sink explains only a small fraction of the drought-induced reduction in soil respiration in this experiment.

Journal of Geophysical Research Biogeosciences, 2009

1] This experiment investigated the effects of prolonged summer drought on soil respiration (SR) ... more 1] This experiment investigated the effects of prolonged summer drought on soil respiration (SR) in a mountainous Norway spruce forest in south Germany. On three manipulation plots we excluded summer throughfall in the years of 2006/2007 and measured SR fluxes in comparison to three control plots. Using radiocarbon measurements we quantified the contribution of rhizosphere (RR) and heterotrophic respiration (HR) to total SR. In both manipulation years, mean CO 2 emissions (±SE) from the throughfall exclusion (TE) plots were smaller than from the control plots with 5.7 t C ha À1 (±0.3) compared to 6.7 t C ha À1 (±0.2) in 2006 and 5.9 t C ha À1 (±0.3) compared to 7.0 t C ha À1 (±0.4) in 2007. Under control conditions, CO 2 originated mainly from HR (60-95% of SR). Prolonged drought reduced HR, whereas RR was not affected or even increased slightly. Reduction of CO 2 emissions on the TE plots was found up to 6 weeks after differences in matric potential conditions disappeared, possibly either because water repellency inhibited homogeneous rewetting of the organic horizons or because of severe damage to the microbial population. No evidence was found for the release of new, formerly protected substrates by preceding drought. Continuous measurements in 2008 (no manipulation) did not reveal increased CO 2 emissions on the TE plots that could compensate for the reduction during the years 2006/2007. Based on our results, we postulate a negative feedback between increased frequency and magnitude of summer droughts and SR in Norway spruce stands.

Proceedings of the National Academy of Sciences of the United States of America, Jan 9, 2015

The desiccation of upper soil horizons is a common phenomenon, leading to a decrease in soil micr... more The desiccation of upper soil horizons is a common phenomenon, leading to a decrease in soil microbial activity and mineralization. Recent studies have shown that fungal communities and fungal-based food webs are less sensitive and better adapted to soil desiccation than bacterial-based food webs. One reason for a better fungal adaptation to soil desiccation may be hydraulic redistribution of water by mycelia networks. Here we show that a saprotrophic fungus (Agaricus bisporus) redistributes water from moist (-0.03 MPa) into dry (-9.5 MPa) soil at about 0.3 cm⋅min(-1) in single hyphae, resulting in an increase in soil water potential after 72 h. The increase in soil moisture by hydraulic redistribution significantly enhanced carbon mineralization by 2,800% and enzymatic activity by 250-350% in the previously dry soil compartment within 168 h. Our results demonstrate that hydraulic redistribution can partly compensate water deficiency if water is available in other zones of the mycel...

Journal of Plant Nutrition and Soil Science, 2014

ABSTRACT In temperate forest soils, N net mineralization has been extensively investigated during... more ABSTRACT In temperate forest soils, N net mineralization has been extensively investigated during the growing season, whereas N cycling during winter was barely addressed. Here, we quantified net ammonification and nitrification during the dormant season by in situ and laboratory incubations in soils of a temperate European beech and a Norway spruce forest. Further, we compared temperature dependency of N net mineralization in in situ field incubations with those from laboratory incubations at controlled temperatures. From November to April, in situ N net mineralization of the organic and upper mineral horizons amounted to 10.9 kg N (ha · 6 months)–1 in the spruce soil and to 44.3 kg N (ha · 6 months)–1 in the beech soil, representing 65% (beech) and 26% (spruce) of the annual above ground litterfall. N net mineralization was largest in the Oi/Oe horizon and lowest in the A and EA horizons. Net nitrification in the beech soil [1.5 kg N (ha · 6 months)–1] was less than in the spruce soil [5.9 kg N (ha · 6 months)–1]. In the range of soil temperatures observed in the field (0–8°C), the temperature dependency of N net mineralization was generally high for both soils and more pronounced in the laboratory incubations than in the in situ incubations. We suggest that homogenization of laboratory samples increased substrate availability and, thus, enhanced the temperature response of N net mineralization. In temperate forest soils, N net mineralization during the dormant season contributes substantially to the annual N cycling, especially in deciduous sites with large amounts of litterfall immediately before the dormant season. High Q10 values of N net mineralization at low temperatures suggest a huge effect of future increasing winter temperature on the N cycle in temperate forests.

Biogeochemistry, 2015

Temperate forests provide favorable conditions for carbonate bedrock weathering as the soil CO 2 ... more Temperate forests provide favorable conditions for carbonate bedrock weathering as the soil CO 2 partial pressure is high and soil water is regularly available. As a result of weathering, abiotic CO 2 can be released and contribute to the soil CO 2 efflux. We used the distinct isotopic signature of the abiotic CO 2 to estimate its contribution to the total soil CO 2 efflux. Soil cores were sampled from forests on dolomite and limestone and were incubated under the exclusion of atmospheric CO 2 . Efflux and isotopic signatures of CO 2 were repeatedly measured of cores containing the whole mineral soil and bedrock material (heterotrophic respiration ? CO 2 from weathering) and of cores containing only the mineral top-soil layer (A-horizon; heterotrophic respiration). An aliquot of the cores were let dry out during incubation to assess effects of soil moisture. Although the d 13 C values of the CO 2 efflux from the dolomite soil cores were within a narrow range (A-horizon -26.2 ± 0.1 %; whole soil profile wet -25.8 ± 0.1 %; whole soil profile dry -25.5 ± 0.1 %) the CO 2 efflux from the separated A-horizons was significantly depleted in 13 C when compared to the whole soil profiles (p = 0.015). The abiotic contribution to the total CO 2 efflux from the dolomite soil cores was 2.0 ± 0.5 % under wet and 3.4 ± 0.5 % under dry conditions. No abiotic CO 2 efflux was traceable from the limestone soil cores. An overall low contribution of CO 2 from weathering was affirmed by the amount and 13 C signature of the leached dissolved inorganic carbon (DIC) and the radiocarbon signature of the soil CO 2 efflux in the field. Together, our data point towards no more than 1-2 % contribution of abiotic CO 2 to the growing season soil CO 2 efflux in the field.

Ecological Studies, 2009

ABSTRACT Net uptake of atmospheric methane by methanotrophic bacteria in forest soils is generall... more ABSTRACT Net uptake of atmospheric methane by methanotrophic bacteria in forest soils is generally less than 20 kg CH4 ha−1 per year and therefore has almost no impact on the carbon budget of forest ecosystems. However, terrestrial soils are the most important biological sink for atmospheric methane, consuming between 20 and 45 Tg CH4 per year (Smith et al. 2000; Dutaur and Verchot 2007). The methane concentration in the atmosphere increased from about 700 to 1,774 ppb (by volume) during the last 150 years, though the concentration has possibly stabilised during the past decade (IPCC 2007). Methane currently contributes approximately 18% to the anthropogenic greenhouse effect and has a global warming potential 25 times higher than that of CO2 based on a time horizon of 100 years (IPCC 2007). Human activities have reduced the soil sink for atmospheric methane by converting natural forests and grasslands to coniferous plantations, agricultural and urban land (Ojima et al. 1993; Dobbie et al. 1996; Smith et al. 2000; Borken et al. 2003; Borken and Beese 2006) and by causing acid deposition in temperate forests (Brumme and Borken 1999). Undisturbed forest soils generally consume much higher rates of atmospheric methane than agricultural and urban soils, although the methane uptake rates may vary considerably within and among forests over all climatic regions. Comparisons of adjacent forest and agricultural soils showed that converting forest land to agricultural use reduced methane uptake rates by two-thirds in both temperate and tropical regions (Smith et al. 2000). The mechanisms for long-term reduction in methane uptake are not completely understood. Disturbances of the soil structure as well as changes in vegetation, micro-climate, and nutrient status may have diminished the population of methane-utilising bacteria in several ways after land conversion to agricultural use. It is believed that these still unidentified bacteria have an extremely slow growth rate due to the small atmospheric methane concentration. Methanotrophs have a high affinity for methane as indicated by low K m values and low threshold concentrations (Bender and Conrad 1993). Smith et al. (2000) pointed out that it probably takes more than 200 years for methane utilising bacteria to recover after reversion of agricultural land to forest or grassland.

Journal of Geophysical Research, 2009

1] This experiment investigated the effects of prolonged summer drought on soil respiration (SR) ... more 1] This experiment investigated the effects of prolonged summer drought on soil respiration (SR) in a mountainous Norway spruce forest in south Germany. On three manipulation plots we excluded summer throughfall in the years of 2006/2007 and measured SR fluxes in comparison to three control plots. Using radiocarbon measurements we quantified the contribution of rhizosphere (RR) and heterotrophic respiration (HR) to total SR. In both manipulation years, mean CO 2 emissions (±SE) from the throughfall exclusion (TE) plots were smaller than from the control plots with 5.7 t C ha À1 (±0.3) compared to 6.7 t C ha À1 (±0.2) in 2006 and 5.9 t C ha À1 (±0.3) compared to 7.0 t C ha À1 (±0.4) in 2007. Under control conditions, CO 2 originated mainly from HR (60-95% of SR). Prolonged drought reduced HR, whereas RR was not affected or even increased slightly. Reduction of CO 2 emissions on the TE plots was found up to 6 weeks after differences in matric potential conditions disappeared, possibly either because water repellency inhibited homogeneous rewetting of the organic horizons or because of severe damage to the microbial population. No evidence was found for the release of new, formerly protected substrates by preceding drought. Continuous measurements in 2008 (no manipulation) did not reveal increased CO 2 emissions on the TE plots that could compensate for the reduction during the years 2006/2007. Based on our results, we postulate a negative feedback between increased frequency and magnitude of summer droughts and SR in Norway spruce stands.

Global change biology, Jan 26, 2014

Nitrogen (N) nutrition in pristine peatlands relies on the natural input of inorganic N through a... more Nitrogen (N) nutrition in pristine peatlands relies on the natural input of inorganic N through atmospheric deposition or biological dinitrogen (N2 ) fixation. However, N2 fixation and its significance for N cycling, plant productivity, and peat buildup are mostly associated with the presence of Sphagnum mosses. Here, we report high nonsymbiotic N2 -fixation rates in two pristine Patagonian bogs with diversified vegetation and natural N deposition. Nonsymbiotic N2 fixation was measured in samples from 0 to 10, 10 to 20, and 40 to 50 cm depth using the (15) N2 assay as well as the acetylene reduction assay (ARA). The ARA considerably underestimated N2 fixation and can thus not be recommended for peatland studies. Based on the (15) N2 assay, high nonsymbiotic N2 -fixation rates of 0.3-1.4 μmol N2 g(-1) day(-1) were found down to 50 cm under micro-oxic conditions (2 vol.%) in samples from plots covered by Sphagnum magellanicum or by vascular cushion plants, latter characterized by de...

Global Change Biology, 2012

Climate change may considerably impact the carbon (C) dynamics and C stocks of forest soils. To a... more Climate change may considerably impact the carbon (C) dynamics and C stocks of forest soils. To assess the combined effects of warming and reduced precipitation on soil CO 2 efflux, we conducted a two-way factorial manipulation experiment (4°C soil warming + throughfall exclusion) in a temperate spruce forest from 2008 until 2010. Soil was warmed by heating cables throughout the growing seasons. Soil drought was simulated by throughfall exclusions with three 100 m 2 roofs during 25 days in July/August 2008 and 2009. Soil warming permanently increased the CO 2 efflux from soil, whereas throughfall exclusion led to a sharp decrease in soil CO 2 efflux (45% and 50% reduction during roof installation in 2008 and 2009, respectively). In 2008, CO 2 efflux did not recover after natural rewetting and remained lowered until autumn. In 2009, CO 2 efflux recovered shortly after rewetting, but relapsed again for several weeks. Drought offset the increase in soil CO 2 efflux by warming in 2008 (growing season CO 2 efflux in t C ha À1 : control: 7.1 ± 1.0; warmed: 9.5 ± 1.7; warmed + roof: 7.4 ± 0.3; roof: 5.9 ± 0.4) and in 2009 (control: 7.6 ± 0.8; warmed + roof: 8.3 ± 1.0). Throughfall exclusion mainly affected the organic layer and the top 5 cm of the mineral soil. Radiocarbon data suggest that heterotrophic and autotrophic respiration were affected to the same extent by soil warming and drying. Microbial biomass in the mineral soil (0-5 cm) was not affected by the treatments. Our results suggest that warming causes significant C losses from the soil as long as precipitation patterns remain steady at our site. If summer droughts become more severe in the future, warming induced C losses will likely be offset by reduced soil CO 2 efflux during and after summer drought.

Biogeosciences, 2014

As a key component of the carbon cycle, soil CO 2 efflux (SCE) is being increasingly studied to i... more As a key component of the carbon cycle, soil CO 2 efflux (SCE) is being increasingly studied to improve our mechanistic understanding of this important carbon flux. Predicting ecosystem responses to climate change often depends on extrapolation of current relationships between ecosystem processes and their climatic drivers to conditions not yet experienced by the ecosystem. This raises the question of to what extent these relationships remain unaltered beyond the current climatic window for which observations are available to constrain the relationships. Here, we evaluate whether current responses of SCE to fluctuations in soil temperature and soil water content can be used to predict SCE under altered rainfall patterns. Of the 58 experiments for which we gathered SCE data, 20 were discarded because either too few data were available or inconsistencies precluded their incorporation in the analyses. The 38 remaining experiments were used to test the hypothesis that a model parameterized with data from the control plots (using soil temperature and water content as predictor variables) could adequately predict SCE measured in the manipulated treatment. Only for 7 of these 38 experiments was this hypothesis rejected. Importantly, these were the experiments with the most reliable data sets, i.e., those providing high-frequency measurements of SCE. Regression tree analysis demonstrated that our hypothesis could be rejected only for experiments with measurement intervals of less than 11 days, and was not rejected for any of the 24 experiments with larger measurement intervals. This highlights the importance of high-frequency measurements when studying effects of altered precipitation on SCE, probably because infrequent measurement schemes have insufficient capacity to detect shifts in the climate dependencies of SCE. Hence, the most justified answer to the question of whether current moisture responses of SCE can be extrapolated to predict SCE under altered precipitation regimes is "no" -as based on the most reliable data sets available. We strongly recommend that future experiments focus more strongly on establishing response functions across a broader range of precipitation regimes and soil moisture conditions. Such experiments should make accurate measurements of water availability, should conduct high-frequency SCE measurements, and should consider both instantaneous responses and the potential legacy effects of climate extremes. This is important, because with the novel approach presented here, we demonstrated that, at least for some ecosystems, current moisture responses could not be extrapolated to predict SCE under altered rainfall conditions.

ABSTRACT Weathering of carbonate bedrock (limestone, dolomite) contributes to the CO2 efflux from... more ABSTRACT Weathering of carbonate bedrock (limestone, dolomite) contributes to the CO2 efflux from the soil. To understand the C-cycling of forests on carbonate bedrock, it is necessary to account for this inorganic C flux. Generally, the inorganic contribution to the soil CO2 efflux is thought to be small or neglectable, but attempts to quantify the inorganic CO2 efflux are rare and sometimes complicated because of methodological problems. We used the distinctive isotopic composition of CO2 from the different sources (dolomite weathering, respiration by decomposing microbes) to quantify the inorganic flux. Mesocosms filled with undisturbed soil and underlying dolomite gravel were incubated under constant temperature and different moisture levels. The whole laboratory experiment was executed under exclusion of atmospheric CO2 in order to avoid interference with its isotopic signal. First data indicate a < 2% contribution of inorganic C to the total CO2 efflux from our soil. Further results of this long-term incubation experiment will be presented at the conference.

Soil Biology and Biochemistry, 2016

Soil Use and Management, Nov 30, 1997

AbstractAbstract. The effect of superficial liming of acidic forest soils on CO2 and N2O emission... more AbstractAbstract. The effect of superficial liming of acidic forest soils on CO2 and N2O emissions and CH4 uptake was investigated with closed chambers in two deciduous and two spruce forests, by weekly to biweekly measurements over at least one year. The flux rates of untreated areas varied between 1.94 and 4.38 t CO2-C/ha per y, 0.28 and 2.15 kg/N2O-N/ha per y and between 0.15 and 1.06 kg CH4-C/ha per y. Liming had no clear effect on CO2 emissions which may change in the long-term with decreasing root turnover and increasing C-mineralization. Apart from one exception, liming resulted in a reduction of N2O emissions by 9 to 62% and in an increase of CH4 uptake by 26 to 580%. The variability in N2O emissions between the forest sites could not be explained. In contrast, the variability of annual CH4 uptake rates could be explained by N content (r2= 0.82), C content (r2= 0.77), bulk density (r2= 60), pore space (r2= 0.59) and pH (r2= 0.40) of mineral soil at a depth of 0 to 10 cm, and by the quantity of material in the organic layer (r2= 0.66). Experiments with undisturbed columns of the same soils showed that between 1 and 73% of the total N2O emissions came from the organic layer. However, atmospheric CH4 was not oxidized in this layer, which represents a diffusion barrier for atmospheric CH4. When this barrier was removed, CH4 uptake by the mineral soil increased by 25 to 171%. These results suggest that liming of acidic forest soils causes a reduction of the greenhouse gases N2O and CH4 in the atmosphere, due to changes in the chemical, biological, and physical condition of the soils.

The flux and origin of respired carbon is of importance for determining the sink or source streng... more The flux and origin of respired carbon is of importance for determining the sink or source strength of soils for atmospheric CO2. The objective of this study was to assess the importance of respiration of recent carbon substrates and decomposition of older soil organic matter during severe summer drought, using radiocarbon measured in CO2 of soil respiration and soil air.

Peatlands in the Northern Hemisphere play a significant role in CO2 sequestration and storage but... more Peatlands in the Northern Hemisphere play a significant role in CO2 sequestration and storage but are also potential sources in the changing climate scenario. Regulation of CO2 exchange processes and the relative contribution of the respective ecosystem components in peatlands are, however, poorly understood. Intensive measurements were carried out to understand how ecosystem CO2 exchange processes are regulated and how

ABSTRACT An increase in soil temperatures substantially increases soil respiration rates as both ... more ABSTRACT An increase in soil temperatures substantially increases soil respiration rates as both processes are positively correlated. This has been demonstrated in a wide range of manipulative field and laboratory studies. However, the response of gross and net nitrogen (N) mineralization rates to enhanced soil temperatures has rarely been investigated. Previous studies ascribed the increased carbon (C) mineralization rates under elevated soil temperatures to enhanced microbial metabolic activity. We conduct a laboratory incubation experiment to determine gross and net N mineralization rates as well as CO2 production rates to test the hypothesis that elevated soil temperatures lead to an increase in gross and net N mineralization rates likewise as observed in C mineralization. We expect that gross N mineralization has a different temperature sensitivity than C mineralization. Soil samples were taken from the Achenkirch (Austria) soil warming experiment. Warmed and control plots were established in a mixed spruce-beech forest stand each with three replications in 2004. Soil temperature was enhanced in the warmed plots by 4°C above ambient temperature during the vegetation period. Due to anthropogenic deposition this site is highly N-saturated. Soil samples were taken in two depths (Oi/Oe layer and 0-10 cm). Gross ammonification and nitrification rates were measured by the 15N pool dilution technique, net N and CO2 production rates were determined in lab incubations over 12 weeks. First results of this study will be presented.

Agu Fall Meeting Abstracts, Dec 1, 2003

Low rates of soil respiration during droughts have been related to above-average rates of net eco... more Low rates of soil respiration during droughts have been related to above-average rates of net ecosystem exchange of carbon in forested ecosystems. Lower soil respiration during drought could cause a transitory soil C sink, due to a temporary decline in decomposition, or it could result from reduced root respiration. A throughfall exclusion experiment was established at the Harvard Forest in central Massachusetts to study the effects of experimental drought on soil respiration. Weekly measurements of soil respiration began in the spring of 2001, with 4 manual CO2 flux chambers installed in each of 6 plots (5 x 5 m). In July 2001, sub-canopy roofs with translucent plastic panels were installed in 3 of the plots, while the other 3 control plots were left open. Temperature probes, TDR probes, and gas tubes were buried at 4 depths in each plot. Roofs were removed in autumn 2001 to allow leaf-fall and snowfall, reinstalled in spring 2002, removed again in autumn 2002, and then left off for the entire summer of 2003. The radiocarbon contents of CO2 emissions and concentrations within the soil were measured periodically. Soil respiration was nearly identical in treatment and control plots (140 and 142 g C m-2, respectively) during the 54-d pre-treatment period. During 85 days in 2001 with roofs in place, 168 mm of throughfall were diverted, and the cumulative soil respiration was 30% lower in exclusion versus control plots (237 and 338 g C m-2, respectively). During 127 days of throughfall exclusion (344 mm diverted) in 2002, cumulative soil respiration was 40% lower in exclusion versus control plots (280 and 490 g C m-2, respectively). Clearly, soil water content is an important factor influencing soil respiration. When natural throughfall was allowed in all plots from September 2002 through August 2003, the cumulative soil respiration was only 6% higher in exclusion versus control plots (702 and 658 g C m-2, respectively), indicating only a modest increase in decomposition after previously dried soils were wetted. The Δ 14CO2 in the soil atmosphere was negatively correlated with CO2 concentrations, indicating a larger contribution of decomposition of old, radiocarbon-rich substrates in the mineral soil during drought. In dry mineral soil, root respiration produced less CO2, while gradual decomposition of old, radiocarbon-rich substrates continued at low rates, resulting in low CO2 concentrations with high Δ 14C values. In contrast, microbial decomposition of old substrates declined more rapidly than did decomposition of young substrates and root respiration in the O horizon, causing the Δ 14CO2 of surfaces fluxes to decline with drought. A depth-dependent differential response of root and microbial respiration to drought is indicated. If decreased root respiration is responsible, it may reflect altered belowground C allocation, but larger treatment plots that include entire trees would be needed to address this question. In any case, a transient soil C sink explains only a small fraction of the drought-induced reduction in soil respiration in this experiment.

Journal of Geophysical Research Biogeosciences, 2009

1] This experiment investigated the effects of prolonged summer drought on soil respiration (SR) ... more 1] This experiment investigated the effects of prolonged summer drought on soil respiration (SR) in a mountainous Norway spruce forest in south Germany. On three manipulation plots we excluded summer throughfall in the years of 2006/2007 and measured SR fluxes in comparison to three control plots. Using radiocarbon measurements we quantified the contribution of rhizosphere (RR) and heterotrophic respiration (HR) to total SR. In both manipulation years, mean CO 2 emissions (±SE) from the throughfall exclusion (TE) plots were smaller than from the control plots with 5.7 t C ha À1 (±0.3) compared to 6.7 t C ha À1 (±0.2) in 2006 and 5.9 t C ha À1 (±0.3) compared to 7.0 t C ha À1 (±0.4) in 2007. Under control conditions, CO 2 originated mainly from HR (60-95% of SR). Prolonged drought reduced HR, whereas RR was not affected or even increased slightly. Reduction of CO 2 emissions on the TE plots was found up to 6 weeks after differences in matric potential conditions disappeared, possibly either because water repellency inhibited homogeneous rewetting of the organic horizons or because of severe damage to the microbial population. No evidence was found for the release of new, formerly protected substrates by preceding drought. Continuous measurements in 2008 (no manipulation) did not reveal increased CO 2 emissions on the TE plots that could compensate for the reduction during the years 2006/2007. Based on our results, we postulate a negative feedback between increased frequency and magnitude of summer droughts and SR in Norway spruce stands.

Proceedings of the National Academy of Sciences of the United States of America, Jan 9, 2015

The desiccation of upper soil horizons is a common phenomenon, leading to a decrease in soil micr... more The desiccation of upper soil horizons is a common phenomenon, leading to a decrease in soil microbial activity and mineralization. Recent studies have shown that fungal communities and fungal-based food webs are less sensitive and better adapted to soil desiccation than bacterial-based food webs. One reason for a better fungal adaptation to soil desiccation may be hydraulic redistribution of water by mycelia networks. Here we show that a saprotrophic fungus (Agaricus bisporus) redistributes water from moist (-0.03 MPa) into dry (-9.5 MPa) soil at about 0.3 cm⋅min(-1) in single hyphae, resulting in an increase in soil water potential after 72 h. The increase in soil moisture by hydraulic redistribution significantly enhanced carbon mineralization by 2,800% and enzymatic activity by 250-350% in the previously dry soil compartment within 168 h. Our results demonstrate that hydraulic redistribution can partly compensate water deficiency if water is available in other zones of the mycel...

Journal of Plant Nutrition and Soil Science, 2014

ABSTRACT In temperate forest soils, N net mineralization has been extensively investigated during... more ABSTRACT In temperate forest soils, N net mineralization has been extensively investigated during the growing season, whereas N cycling during winter was barely addressed. Here, we quantified net ammonification and nitrification during the dormant season by in situ and laboratory incubations in soils of a temperate European beech and a Norway spruce forest. Further, we compared temperature dependency of N net mineralization in in situ field incubations with those from laboratory incubations at controlled temperatures. From November to April, in situ N net mineralization of the organic and upper mineral horizons amounted to 10.9 kg N (ha · 6 months)–1 in the spruce soil and to 44.3 kg N (ha · 6 months)–1 in the beech soil, representing 65% (beech) and 26% (spruce) of the annual above ground litterfall. N net mineralization was largest in the Oi/Oe horizon and lowest in the A and EA horizons. Net nitrification in the beech soil [1.5 kg N (ha · 6 months)–1] was less than in the spruce soil [5.9 kg N (ha · 6 months)–1]. In the range of soil temperatures observed in the field (0–8°C), the temperature dependency of N net mineralization was generally high for both soils and more pronounced in the laboratory incubations than in the in situ incubations. We suggest that homogenization of laboratory samples increased substrate availability and, thus, enhanced the temperature response of N net mineralization. In temperate forest soils, N net mineralization during the dormant season contributes substantially to the annual N cycling, especially in deciduous sites with large amounts of litterfall immediately before the dormant season. High Q10 values of N net mineralization at low temperatures suggest a huge effect of future increasing winter temperature on the N cycle in temperate forests.

Biogeochemistry, 2015

Temperate forests provide favorable conditions for carbonate bedrock weathering as the soil CO 2 ... more Temperate forests provide favorable conditions for carbonate bedrock weathering as the soil CO 2 partial pressure is high and soil water is regularly available. As a result of weathering, abiotic CO 2 can be released and contribute to the soil CO 2 efflux. We used the distinct isotopic signature of the abiotic CO 2 to estimate its contribution to the total soil CO 2 efflux. Soil cores were sampled from forests on dolomite and limestone and were incubated under the exclusion of atmospheric CO 2 . Efflux and isotopic signatures of CO 2 were repeatedly measured of cores containing the whole mineral soil and bedrock material (heterotrophic respiration ? CO 2 from weathering) and of cores containing only the mineral top-soil layer (A-horizon; heterotrophic respiration). An aliquot of the cores were let dry out during incubation to assess effects of soil moisture. Although the d 13 C values of the CO 2 efflux from the dolomite soil cores were within a narrow range (A-horizon -26.2 ± 0.1 %; whole soil profile wet -25.8 ± 0.1 %; whole soil profile dry -25.5 ± 0.1 %) the CO 2 efflux from the separated A-horizons was significantly depleted in 13 C when compared to the whole soil profiles (p = 0.015). The abiotic contribution to the total CO 2 efflux from the dolomite soil cores was 2.0 ± 0.5 % under wet and 3.4 ± 0.5 % under dry conditions. No abiotic CO 2 efflux was traceable from the limestone soil cores. An overall low contribution of CO 2 from weathering was affirmed by the amount and 13 C signature of the leached dissolved inorganic carbon (DIC) and the radiocarbon signature of the soil CO 2 efflux in the field. Together, our data point towards no more than 1-2 % contribution of abiotic CO 2 to the growing season soil CO 2 efflux in the field.

Ecological Studies, 2009

ABSTRACT Net uptake of atmospheric methane by methanotrophic bacteria in forest soils is generall... more ABSTRACT Net uptake of atmospheric methane by methanotrophic bacteria in forest soils is generally less than 20 kg CH4 ha−1 per year and therefore has almost no impact on the carbon budget of forest ecosystems. However, terrestrial soils are the most important biological sink for atmospheric methane, consuming between 20 and 45 Tg CH4 per year (Smith et al. 2000; Dutaur and Verchot 2007). The methane concentration in the atmosphere increased from about 700 to 1,774 ppb (by volume) during the last 150 years, though the concentration has possibly stabilised during the past decade (IPCC 2007). Methane currently contributes approximately 18% to the anthropogenic greenhouse effect and has a global warming potential 25 times higher than that of CO2 based on a time horizon of 100 years (IPCC 2007). Human activities have reduced the soil sink for atmospheric methane by converting natural forests and grasslands to coniferous plantations, agricultural and urban land (Ojima et al. 1993; Dobbie et al. 1996; Smith et al. 2000; Borken et al. 2003; Borken and Beese 2006) and by causing acid deposition in temperate forests (Brumme and Borken 1999). Undisturbed forest soils generally consume much higher rates of atmospheric methane than agricultural and urban soils, although the methane uptake rates may vary considerably within and among forests over all climatic regions. Comparisons of adjacent forest and agricultural soils showed that converting forest land to agricultural use reduced methane uptake rates by two-thirds in both temperate and tropical regions (Smith et al. 2000). The mechanisms for long-term reduction in methane uptake are not completely understood. Disturbances of the soil structure as well as changes in vegetation, micro-climate, and nutrient status may have diminished the population of methane-utilising bacteria in several ways after land conversion to agricultural use. It is believed that these still unidentified bacteria have an extremely slow growth rate due to the small atmospheric methane concentration. Methanotrophs have a high affinity for methane as indicated by low K m values and low threshold concentrations (Bender and Conrad 1993). Smith et al. (2000) pointed out that it probably takes more than 200 years for methane utilising bacteria to recover after reversion of agricultural land to forest or grassland.

Journal of Geophysical Research, 2009

1] This experiment investigated the effects of prolonged summer drought on soil respiration (SR) ... more 1] This experiment investigated the effects of prolonged summer drought on soil respiration (SR) in a mountainous Norway spruce forest in south Germany. On three manipulation plots we excluded summer throughfall in the years of 2006/2007 and measured SR fluxes in comparison to three control plots. Using radiocarbon measurements we quantified the contribution of rhizosphere (RR) and heterotrophic respiration (HR) to total SR. In both manipulation years, mean CO 2 emissions (±SE) from the throughfall exclusion (TE) plots were smaller than from the control plots with 5.7 t C ha À1 (±0.3) compared to 6.7 t C ha À1 (±0.2) in 2006 and 5.9 t C ha À1 (±0.3) compared to 7.0 t C ha À1 (±0.4) in 2007. Under control conditions, CO 2 originated mainly from HR (60-95% of SR). Prolonged drought reduced HR, whereas RR was not affected or even increased slightly. Reduction of CO 2 emissions on the TE plots was found up to 6 weeks after differences in matric potential conditions disappeared, possibly either because water repellency inhibited homogeneous rewetting of the organic horizons or because of severe damage to the microbial population. No evidence was found for the release of new, formerly protected substrates by preceding drought. Continuous measurements in 2008 (no manipulation) did not reveal increased CO 2 emissions on the TE plots that could compensate for the reduction during the years 2006/2007. Based on our results, we postulate a negative feedback between increased frequency and magnitude of summer droughts and SR in Norway spruce stands.

Global change biology, Jan 26, 2014

Nitrogen (N) nutrition in pristine peatlands relies on the natural input of inorganic N through a... more Nitrogen (N) nutrition in pristine peatlands relies on the natural input of inorganic N through atmospheric deposition or biological dinitrogen (N2 ) fixation. However, N2 fixation and its significance for N cycling, plant productivity, and peat buildup are mostly associated with the presence of Sphagnum mosses. Here, we report high nonsymbiotic N2 -fixation rates in two pristine Patagonian bogs with diversified vegetation and natural N deposition. Nonsymbiotic N2 fixation was measured in samples from 0 to 10, 10 to 20, and 40 to 50 cm depth using the (15) N2 assay as well as the acetylene reduction assay (ARA). The ARA considerably underestimated N2 fixation and can thus not be recommended for peatland studies. Based on the (15) N2 assay, high nonsymbiotic N2 -fixation rates of 0.3-1.4 μmol N2 g(-1) day(-1) were found down to 50 cm under micro-oxic conditions (2 vol.%) in samples from plots covered by Sphagnum magellanicum or by vascular cushion plants, latter characterized by de...