Carbon isotope ratios in belowground carbon cycle processes

Journal ArticleAnalyses of carbon isotope ratios (δ13C) in soil organic matter (SOM) and soil respired CO2 provide insights into dynamics of the carbon cycle. δ13C analyses do not provide direct measures of soil CO2 efflux rates but are useful as a constraint in carbon cycle models. In many cases, δ13C analyses allow the identification of components of soil CO2 efflux as well as the relative contribution of soil to overall ecosystem CO2 fluxes

Environmental variables controlling soil respiration on diurnal, seasonal and annual time-scales in a mixed mountain forest in Switzerland

Studies on soil respiration in mountain forests are rather scarce compared to their broad distribution. Therefore, we investigated daily, seasonal and annual soil respiration rates in a mixed forest (Lägeren), located at about 700m in the Swiss Jura mountains, during 2years (2006 and 2007). Soil respiration (SR) was measured continuously with high temporal resolution (half-hourly) at one single point (SRautomated) and periodically with high spatial resolution (SRmanual) at 16 plots within the study site. Both, SRautomated and SRmanual showed a similar seasonal cycle. SR strongly depended on soil temperature in 2007 (R 2=0.82-0.92), but less so in 2006 (R 2=0.56-0.76) when SR was water limited during a summer drought. Including soil moisture improved the fit of the 2006 model significantly (R 2=0.78-0.97). Total annual SR for the study site was estimated as 869g C m−2year−1 for 2006 and as 907g C m−2year−1 for 2007 (uncertainty <10% at the 95% confidence interval, determined by bootstrapping). Selected environmental conditions were assessed in more detail: (1) Rapid, but contrasting changes of SR were found after summer rainfall. Depending on soil moisture at pre-rain conditions, summer rain could either cause a pulse of CO2 from the soil or an abrupt decrease of SRautomated due to water logging of soil pores. (2) Two contrasting winter seasons resulted in SR being about 60-70% (31.2-44.6g C m−2) higher during a mild winter (2007) compared to a harsh winter (2006). (3) Analysing SR for selected periods on a diurnal scale revealed a counter-clockwise hysteresis with soil surface temperatures. This indication of a time-lagged response of SR to temperature was further supported by a very strong relationship (R 2=0.86-0.90) of SR to soil temperature with a time-lag of 2-4

A study of soil methane sink regulation in two grasslands exposed to drought and N fertilization

Oxidation by soil bacteria is the only biological sink for atmospheric methane (CH4). There are substantial uncertainties regarding the global size of this sink, in part because the ecological controls of the involved processes are not well understood to date. We have investigated effects of severe summer drought and of nitrogen inputs (ammonium nitrate or cattle urine) on soil CH4 fluxes in a field experiment. Soil moisture was the most important factor regulating the temporal dynamics of CH4 fluxes. Simulated drought episodes altered the soil's water balance throughout the year, increasing CH4 oxidation by 50% on an annual basis. N fertilizers exerted only small and transient effects at the ecosystem level. Laboratory incubations suggested that effects differed between soil layers, with larger effects of drought and N application in the top soil than in deeper layers. With soil moisture being the primary controlling factor of methanotrophy, a detailed understanding of the ecosystem's water balance is required to predict CH4 budgets under future climatic condition

Effects of Ontogeny on delta C-13 of Plant- and Soil-Respired CO2 and on Respiratory Carbon Fractionation in C-3 Herbaceous Species

Knowledge gaps regarding potential ontogeny and plant species identity effects on carbon isotope fractionation might lead to misinterpretations of carbon isotope composition (delta C-13) of respired CO2, a widely-used integrator of environmental conditions. In monospecific mesocosms grown under controlled conditions, the delta C-13 of C pools and fluxes and leaf ecophysiological parameters of seven herbaceous species belonging to three functional groups (crops, forage grasses and legumes) were investigated at three ontogenetic stages of their vegetative cycle (young foliage, maximum growth rate, early senescence). Ontogeny-related changes in delta C-13 of leaf-and soil-respired CO2 and C-13/C-12 fractionation in respiration (Delta(R)) were species-dependent and up to 7 parts per thousand, a magnitude similar to that commonly measured in response to environmental factors. At plant and soil levels, changes in delta C-13 of respired CO2 and Delta(R) with ontogeny were related to changes in plant physiological status, likely through ontogeny-driven changes in the C sink to source strength ratio in the above-ground plant compartment. Our data further showed that lower Delta(R) values (i.e. respired CO2 relatively less depleted in C-13) were observed with decreasing net assimilation. Our findings highlight the importance of accounting for ontogenetic stage and plant community composition in ecological studies using stable carbon isotopes.Peer reviewe

Afforestation of Tropical Pasture Only Marginally Affects Ecosystem-Scale Evapotranspiration

Evapotranspiration (ET) from tropical ecosystems is a major constituent of the global land-atmosphere water flux and strongly influences the global hydrological cycle. Most previous studies of ecosystem ET have been conducted predominantly in tropical forests, and only few observations cover other tropical land-use types such as pastures, croplands, savannas or plantations. The objectives of our study were: (1) to estimate daily, monthly, and annual ET budgets in a tropical pasture and an adjacent afforestation site, (2) to assess diurnal and seasonal patterns of ET, (3) to investigate environmental controls of ET, and (4) to evaluate the soil infiltration potential. We performed eddy covariance measurements of ecosystem ET in Sardinilla (Panama) from 2007 to 2009. Daily ET (2.6±1.0mmday−1) was significantly lower in the pasture compared to the afforestation site (3.0±0.9mmday−1). The highest ET was observed during the wet-dry transition period in both ecosystems. However, differences in daily ET between sites were relatively small, particularly during the wet season. Radiation was the main environmental control of ET at both sites, however, we observed considerable seasonal variation in the strength of this control, which was stronger during the wet compared to the dry season. In 2008, total annual ET was only slightly higher for the afforestation (1114mmy−1) than the pasture site (1034mmy−1). Our results suggest that afforestation of pasture only marginally increases ecosystem-scale ET 6-8years after establishment. Differences in soil infiltration potentials between our sites seem to explain this patter

Methodological tests of the use of trace elements as tracers to assess root activity

peer-reviewedN.J.H. was funded by the Irish Research Council, co-funded by Marie Curie Actions under FP7. The field experiments A, B and G were supported by the European Community's Seventh Framework Programme (FP7/2007-2013) under the grant agreements FP7-266018 (AnimalChange) and FP7- 244983 (MultiSward). Experiment F was supported by the German Science Foundation (FOR 456).Background and aims There is increasing interest in how resource utilisation in grassland ecosystems is affected by changes in plant diversity and abiotic conditions. Research to date has mainly focussed on aboveground responses and there is limited insight into belowground processes. The aim of this study was to test a number of assumptions for the valid use of the trace elements caesium, lithium, rubidium and strontium as tracers to assess the root activity of several grassland species. Methods We carried out a series of experiments addressing the reliability of soil labelling, injection density, incubation time, application rate and the comparability of different tracers in a multiple tracer method. Results The results indicate that it is possible to achieve a reliable labelling of soil depths. Tracer injection density affected the variability but not the mean level of plant tracer concentrations. Tracer application rates should be based on pilot studies, because of site- and species-specific responses. The trace elements did not meet prerequisites to be used in a multiple tracer method. Conclusions The use of trace elements as tracers is potentially a very useful tool to give insight into plant root activity at different soil depths. This work highlights some of the main benefits and pitfalls of the method and provides specific recommendations to assist the design of tracer experiments and interpretation of the results.N.J.H. was funded by the Irish Research Council, co-funded by Marie Curie Actions under FP7. The field experiments A, B and G were supported by the European Community's Seventh Framework Programme (FP7/2007-2013) under the grant agreements FP7-266018 (AnimalChange) and FP7- 244983 (MultiSward). Experiment F was supported by the German Science Foundation (FOR 456).European Unio

Representative estimates of soil and ecosystem respiration in an old beech forest

Respiration has been proposed to be the main determinant of the carbon balance in European forests and is thus essential for our understanding of the carbon cycle. However, the choice of experimental design strongly affects estimates of annual respiration and of the contribution of soil respiration to total ecosystem respiration. In a detailed study of ecosystem and soil respiration fluxes in an old unmanaged deciduous forest in Central Germany over 3years (2000-2002), we combined soil chamber and eddy covariance measurements to obtain a comprehensive picture of respiration in this forest. The closed portable chambers offered to investigate spatial variability of soil respiration and its controls while the eddy covariance system offered continuous measurements of ecosystem respiration. Over the year, both fluxes were mainly correlated with temperature. However, when soil moisture sank below 23vol.% in the upper 6cm, water limitations also became apparent. The temporal resolution of the eddy covariance system revealed that relatively high respiration rates occurred during budbreak due to increased metabolic activity and after leaf fall because of increased decomposition. Spatial variability in soil respiration rates was large and correlated with fine root biomass (r 2 = 0.56) resulting in estimates of annual efflux varying across plots from 730 to 1,258 (mean 898) g C m−2 year−1. Power function calculations showed that achieving a precision in the soil respiration estimate of 20% of the full population mean at a confidence level of 95%, requires about eight sampling locations. Our results can be used as guidelines to improve the representativeness of soil respiration measurements by nested sampling designs, being applied in long-term and large-scale carbon sequestration projects such as FLUXNET and CarboEurop

Flow of Deposited Inorganic N in Two Gleysol-dominated Mountain Catchments Traced with 15NO3− and 15NH4+

Abstract.: In two mountain ecosystems at the Alptal research site in central Switzerland, pulses of 15NO3 and 15NH4 were separately applied to trace deposited inorganic N. One forested and one litter meadow catchment, each approximately 1600m2, were delimited by trenches in the Gleysols. K15NO3 was applied weekly or fortnightly over one year with a backpack sprayer, thus labelling the atmospheric nitrate deposition. After the sampling and a one-year break, 15NH4Cl was applied as a second one-year pulse, followed by a second sampling campaign. Trees (needles, branches and bole wood), ground vegetation, litter layer and soil (LF, A and B horizon) were sampled at the end of each labelling period. Extractable inorganic N, microbial N, and immobilised soil N were analysed in the LF and A horizons. During the whole labelling period, the runoff water was sampled as well. Most of the added tracer remained in both ecosystems. More NO3− than NH4+ tracer was retained, especially in the forest. The highest recovery was in the soil, mainly in the organic horizon, and in the ground vegetation, especially in the mosses. Event-based runoff analyses showed an immediate response of 15NO3− in runoff, with sharp 15N peaks corresponding to discharge peaks. NO3− leaching showed a clear seasonal pattern, being highest in spring during snowmelt. The high capacity of N retention in these ecosystems leads to the assumption that deposited N accumulates in the soil organic matter, causing a progressive decline of its C:N rati

Temperate tree species show identical response in tree water deficit but different sensitivities in sap flow to summer soil drying

Temperate forests are expected to be particularly vulnerable to drought and soil drying because they are not adapted to such conditions and perform best in mesic environments. Here we ask (i) how sensitively four common temperate tree species (Fagus sylvatica, Picea abies, Acer pseudoplatanus and Fraxinus excelsior) respond in their water relations to summer soil drying and seek to determine (ii) if species-specific responses to summer soil drying are related to the onset of declining water status across the four species. Throughout 2012 and 2013 we determined tree water deficit (TWD) as a proxy for tree water status from recorded stem radius changes and monitored sap flow rates with sensors on 16 mature trees studied in the field at Lägeren, Switzerland. All tree species responded equally in their relative maximum TWD to the onset of declining soil moisture. This implies that the water supply of all tree species was affected by declining soil moisture and that none of the four species was able to fully maintain its water status, e.g., by access to alternative water sources in the soil. In contrast we found strong and highly species-specific responses of sap flow to declining soil moisture with the strongest decline in P. abies (92%), followed by F. sylvatica (53%) and A. pseudoplatanus (48%). F. excelsior did not significantly reduce sap flow. We hypothesize the species-specific responses in sap flow to declining soil moisture that occur despite a simultaneous increase in relative TWD in all species reflect how fast these species approach critical levels of their water status, which is most likely influenced by species-specific traits determining the hydraulic properties of the species tree

Drought alters timing, quantity, and quality of wood formation in Scots pine

Drought has been frequently discussed as a trigger for forest decline. Today, large-scale Scots pine decline is observed in many dry inner-Alpine valleys, with drought discussed as the main causative factor. This study aimed to analyse the impact of drought on wood formation and wood structure. To study tree growth under contrasting water supply, an irrigation experiment was installed in a mature Scots pine (Pinus sylvestris L.) forest at a xeric site in a dry inner-Alpine valley. Inter- and intra-annual radial increments as well as intra-annual variations in wood structure of pine trees were studied. It was found that non-irrigated trees had a noticeably shorter period of wood formation and showed a significantly lower increment. The water conduction cells were significantly enlarged and had significantly thinner cell walls compared with irrigated trees. It is concluded that pine trees under drought stress build a more effective water-conducting system (larger tracheids) at the cost of a probably higher vulnerability to cavitation (larger tracheids with thinner cell walls) but without losing their capability to recover. The significant shortening of the growth period in control trees indicated that the period where wood formation actually takes place can be much shorter under drought than the ‘potential' period, meaning the phenological growth perio