Measuring Evapotranspiration of five Alley Cropping systems in Germany using the Eddy-Covariance- and Bowen-Ratio Energy-Balance methods

Measuring evapotranspiration (ET) of heterogeneous ecosystems using methods like the eddy-covariance- (ECEB) and Bowen-Ratio energy-balance (BREB) methods is challenging due to their restrictions to horizontally homogeneous terrain and steady state conditions. The unfavorable ambient conditions lead to a site-specific non-closure of the energy-balance, which is a balance between the incoming net radiation and the ground-, latent- and sensible heat fluxes. Thus, an underestimation of measured heat fluxes leads to an overestimation of the latent heat fluxes inferred from the ECEB method. The aim of our study is to 1) quantify the site-specific non-closure of the energy-balance and 2) characterize the performance of both methods, compared to direct eddy-covariance measurements using a high-frequency infra-red gas analyzer (LI-7200, Licor Inc.). To assess continuous ET rates on a 30-minute time scale we installed a combined ECEB and BREB system at five alley cropping and five agricultural reference sites across Germany. For time periods of four weeks, we performed direct eddy covariance flux measurements for H2O and CO2 over one crop- and one grassland alley cropping- and their respective reference systems during the growing season of 2016. We found a non-closure between 21 and 26 % for all sites, with the residual energy being highest during the morning and lowest in the afternoon. Related to that the energy-balance ratio (EBR), i.e. the ratio between the turbulent heat fluxes and available energy, was below one in the morning hours and increased slightly during the day up to 1.8, until the EBR decreased sharply after sunset. The EBR correlated to the daily cycle of solar radiation, the main driver of turbulent fluxes. Corresponding, we found an increasing EBR with increasing friction velocity, indicating, that the energy-balance closure improves under turbulent condition. Further our analysis reveal that turbulent fluxes estimated by the BREB method compared well with direct eddy-covariance measurements. An accuracy improvement was found with increasing sensor distance. We conclude, when calculating ET rates on a 30-minute time scale using the ECEB method the site-specific non-closure should be assessed beforehand by eddy-covariance. In the current study, ignoring the non-closure would have lead to an overestimation of the ET rates of about 25 % for the ECEB method

Response of CO₂ and H₂O fluxes in a mountainous tropical rainforest in equatorial Indonesia to El Niño events

The possible impact of El Niño–Southern Oscillation (ENSO) events on the main components of CO₂ and H₂O fluxes between the tropical rainforest and the atmosphere is investigated. The fluxes were continuously measured in an old-growth mountainous tropical rainforest in Central Sulawesi in Indonesia using the eddy covariance method for the period from January 2004 to June 2008. During this period, two episodes of El Niño and one episode of La Niña were observed. All these ENSO episodes had moderate intensity and were of the central Pacific type. The temporal variability analysis of the main meteorological parameters and components of CO₂ and H₂O exchange showed a high sensitivity of evapotranspiration (ET) and gross primary production (GPP) of the tropical rainforest to meteorological variations caused by both El Niño and La Niña episodes. Incoming solar radiation is the main governing factor that is responsible for ET and GPP variability. Ecosystem respiration (RE) dynamics depend mainly on the air temperature changes and are almost insensitive to ENSO. Changes in precipitation due to moderate ENSO events did not have any notable effect on ET and GPP, mainly because of sufficient soil moisture conditions even in periods of an anomalous reduction in precipitation in the region

Carbon allocation and carbon isotope fluxes in the plant-soil-atmosphere continuum: a review

The terrestrial carbon (C) cycle has received increasing interest over the past few decades, however, there is still a lack of understanding of the fate of newly assimilated C allocated within plants and to the soil, stored within ecosystems and lost to the atmosphere. Stable carbon isotope studies can give novel insights into these issues. In this review we provide an overview of an emerging picture of plant-soil-atmosphere C fluxes, as based on C isotope studies, and identify processes determining related C isotope signatures. The first part of the review focuses on isotopic fractionation processes within plants during and after photosynthesis. The second major part elaborates on plant-internal and plant-rhizosphere C allocation patterns at different time scales (diel, seasonal, interannual), including the speed of C transfer and time lags in the coupling of assimilation and respiration, as well as the magnitude and controls of plant-soil C allocation and respiratory fluxes. Plant responses to changing environmental conditions, the functional relationship between the physiological and phenological status of plants and C transfer, and interactions between C, water and nutrient dynamics are discussed. The role of the C counterflow from the rhizosphere to the aboveground parts of the plants, e.g. via CO<sub>2</sub> dissolved in the xylem water or as xylem-transported sugars, is highlighted. The third part is centered around belowground C turnover, focusing especially on above- and belowground litter inputs, soil organic matter formation and turnover, production and loss of dissolved organic C, soil respiration and CO<sub>2</sub> fixation by soil microbes. Furthermore, plant controls on microbial communities and activity via exudates and litter production as well as microbial community effects on C mineralization are reviewed. A further part of the paper is dedicated to physical interactions between soil CO<sub>2</sub> and the soil matrix, such as CO<sub>2</sub> diffusion and dissolution processes within the soil profile. Finally, we highlight state-of-the-art stable isotope methodologies and their latest developments. From the presented evidence we conclude that there exists a tight coupling of physical, chemical and biological processes involved in C cycling and C isotope fluxes in the plant-soil-atmosphere system. Generally, research using information from C isotopes allows an integrated view of the different processes involved. However, complex interactions among the range of processes complicate or currently impede the interpretation of isotopic signals in CO<sub>2</sub> or organic compounds at the plant and ecosystem level. This review tries to identify present knowledge gaps in correctly interpreting carbon stable isotope signals in the plant-soil-atmosphere system and how future research approaches could contribute to closing these gaps

Contrasting drought legacy effects on gross primary productivity in a mixed versus pure beech forest

Droughts affect terrestrial ecosystems directly and concurrently and can additionally induce lagged effects in subsequent seasons and years. Such legacy effects of drought on vegetation growth and state have been widely studied in tree ring records and satellite-based vegetation greenness, while legacies on ecosystem carbon fluxes are still poorly quantified and understood. Here, we focus on two ecosystem monitoring sites in central Germany with a similar climate but characterised by different species and age structures. Using eddy covariance measurements, we detect legacies on gross primary productivity (GPP) by calculating the difference between random forest model estimates of potential GPP and observed GPP. Our results showed that, at both sites, droughts caused significant legacy effects on GPP at seasonal and annual timescales, which were partly explained by reduced leaf development. The GPP reduction due to drought legacy effects is of comparable magnitude to the concurrent drought effects but differed between two neighbouring forests with divergent species and age structures. The methodology proposed here allows the quantification of the temporal dynamics of legacy effects at the sub-seasonal scale and the separation of legacy effects from model uncertainties. The application of the methodology at a larger range of sites will help us to quantify whether the identified lag effects are general and on which factors they may depend.ISSN:1726-4170ISSN:1726-417

The impact of extreme summer drought on the short-term carbon coupling of photosynthesis to soil CO2 efflux in a temperate grassland

Abstract. Along with predicted climate change, increased risks for summer drought are projected for Central Europe. However, large knowledge gaps exist in terms of how drought events influence the short-term ecosystem carbon cycle. Here, we present results from 13 CO 2 pulse labeling experiments at an intensively managed lowland grassland in Switzerland. We investigated the effect of extreme summer drought on the short-term coupling of freshly assimilated photosynthates in shoots to roots as well as to soil CO 2 efflux. Summer drought was simulated using rainout shelters during two field seasons (2010 and 2011). Soil CO 2 efflux and its isotopic composition were measured with custombuilt chambers coupled to a quantum cascade laser spectrometer (QCLAS-ISO, Aerodyne Research Inc., MA, USA). During the 90 min pulse labeling experiments, we added 99.9 atom % 13 CO 2 to the grass sward. In addition to the isotopic analysis of soil CO 2 efflux, this label was traced over 31 days into bulk shoots, roots and soil. Drought reduced the incorporation of recently fixed carbon into the shoots, but increased the relative allocation of fresh assimilates below ground compared to the control grasslands. Contrary to our hypothesis, we did not find a change of allocation speed in response to drought. Although drought clearly reduced soil CO 2 efflux rates, about 75 % of total tracer uptake in control plots was lost via soil CO 2 efflux during 19 days after pulse labeling, compared to only about 60 % under drought conditions. Thus, the short-term coupling of above-and below-ground processes was reduced in response to summer drought. The occurrence of a natural spring drought in 2011 lead to comparable albeit weaker drought responses increasing the confidence in the generalizability of our findings

Carbon allocation and carbon isotope fluxes in the plant-soil-atmosphere continuum: A review

The terrestrial carbon (C) cycle has received increasing interest over the past few decades, however, there is still a lack of understanding of the fate of newly assimilated C allocated within plants and to the soil, stored within ecosystems and lost to the atmosphere. Stable carbon isotope studies can give novel insights into these issues. In this review we provide an overview of an emerging picture of plant-soil-atmosphere C fluxes, as based on C isotope studies, and identify processes determining related C isotope signatures. The first part of the review focuses on isotopic fractionation processes within plants during and after photosynthesis. The second major part elaborates on plant-internal and plant-rhizosphere C allocation patterns at different time scales (diel, seasonal, interannual), including the speed of C transfer and time lags in the coupling of assimilation and respiration, as well as the magnitude and controls of plant-soil C allocation and respiratory fluxes. Plant responses to changing environmental conditions, the functional relationship between the physiological and phenological status of plants and C transfer, and interactions between C, water and nutrient dynamics are discussed. The role of the C counterflow from the rhizosphere to the aboveground parts of the plants, e.g. via CO2 dissolved in the xylem water or as xylem-transported sugars, is highlighted. The third part is centered around belowground C turnover, focusing especially on above- and belowground litter inputs, soil organic matter formation and turnover, production and loss of dissolved organic C, soil respiration and CO2 fixation by soil microbes. Furthermore, plant controls on microbial communities and activity via exudates and litter production as well as microbial community effects on C mineralization are reviewed. A further part of the paper is dedicated to physical interactions between soil CO2 and the soil matrix, such as CO2 diffusion and dissolution processes within the soil profile. Finally, we highlight state-of-the-art stable isotope methodologies and their latest developments. From the presented evidence we conclude that there exists a tight coupling of physical, chemical and biological processes involved in C cycling and C isotope fluxes in the plant-soil-atmosphere system. Generally, research using information from C isotopes allows an integrated view of the different processes involved. However, complex interactions among the range of processes complicate or currently impede the interpretation of isotopic signals in CO2 or organic compounds at the plant and ecosystem level. This review tries to identify present knowledge gaps in correctly interpreting carbon stable isotope signals in the plant-soil-atmosphere system and how future research approaches could contribute to closing these gaps

Winter respiratory C losses provide explanatory power for net ecosystem productivity

Accurate predictions of net ecosystem productivity (NEPc) of forest ecosystems are essential for climate change decisions and requirements in the context of national forest growth and greenhouse gas inventories. However, drivers and underlying mechanisms determining NEPc (e.g., climate and nutrients) are not entirely understood yet, particularly when considering the influence of past periods. Here we explored the explanatory power of the compensation day (cDOY)defined as the day of year when winter net carbon losses are compensated by spring assimilationfor NEPc in 26 forests in Europe, North America, and Australia, using different NEPc integration methods. We found cDOY to be a particularly powerful predictor for NEPc of temperate evergreen needleleaf forests (R-2=0.58) and deciduous broadleaf forests (R-2=0.68). In general, the latest cDOY correlated with the lowest NEPc. The explanatory power of cDOY depended on the integration method for NEPc, forest type, and whether the site had a distinct winter net respiratory carbon loss or not. The integration methods starting in autumn led to better predictions of NEPc from cDOY then the classical calendar method starting 1 January. Limited explanatory power of cDOY for NEPc was found for warmer sites with no distinct winter respiratory loss period. Our findings highlight the importance of the influence of winter processes and the delayed responses of previous seasons' climatic conditions on current year's NEPc. Such carry-over effects may contain information from climatic conditions, carbon storage levels, and hydraulic traits of several years back in time.Peer reviewe

Implementing a new rubber plant functional type in the Community Land Model (CLM5) improves accuracy of carbon and water flux estimation

Rubber plantations are an economically viable land-use type that occupies large swathes of land in Southeast Asia that have undergone conversion from native forest to intensive plantation forestry. Such land-use change has a strong impact on carbon, energy, and water fluxes in ecosystems, and uncertainties exist in the modeling of future land-use change impacts on these fluxes due to the scarcity of measured data and poor representation of key biogeochemical processes. In this current modeling effort, we utilized the Community Land Model Version 5 (CLM5) to simulate a rubber plant functional type (PFT) by comparing the baseline parameter values of tropical evergreen PFT and tropical deciduous PFT with a newly developed rubber PFT (focused on the parameterization and modification of phenology and allocation processes) based on site-level observations of a rubber clone in Indonesia. We found that the baseline tropical evergreen and baseline tropical deciduous functions and parameterizations in CLM5 poorly simulate the leaf area index, carbon dynamics, and water fluxes of rubber plantations. The newly developed rubber PFT and parametrizations (CLM-rubber) showed that daylength could be used as a universal trigger for defoliation and refoliation of rubber plantations. CLM-rubber was able to predict seasonal patterns of latex yield reasonably well, despite highly variable tapping periods across Southeast Asia. Further, model comparisons indicated that CLM-rubber can simulate carbon and energy fluxes similar to the existing rubber model simulations available in the literature. Our modeling results indicate that CLM-rubber can be applied in Southeast Asia to examine variations in carbon and water fluxes for rubber plantations and assess how rubber-related land-use changes in the tropics feedback to climate through carbon and water cycling

Validation of 3D-CMCC Forest Ecosystem Model (v.5.1) against eddy covariance data for 10 European forest sites

This study evaluates the performances of the new version (v.5.1) of 3D-CMCC Forest Ecosystem Model (FEM) in simulating gross primary productivity (GPP), against eddy covariance GPP data for 10 FLUXNET forest sites across Europe. A new carbon allocation module, coupled with new both phenological and autotrophic respiration schemes, was implemented in this new daily version. Model ability in reproducing timing and magnitude of daily and monthly GPP fluctuations is validated at intra-annual and inter-annual scale, including extreme anomalous seasons. With the purpose to test the 3D-CMCC FEM applicability over Europe without a site-related calibration, the model has been deliberately parametrized with a single set of species-specific parametrizations for each forest ecosystem. The model consistently reproduces both in timing and in magnitude daily and monthly GPP variability across all sites, with the exception of the two Mediterranean sites. We find that 3D-CMCC FEM tends to better simulate the timing of inter-annual anomalies than their magnitude within measurements' uncertainty. In six of eight sites where data are available, the model well reproduces the 2003 summer drought event. Finally, for three sites we evaluate whether a more accurate representation of forest structural characteristics (i.e. cohorts, forest layers) and species composition can improve model results. In two of the three sites results reveal that model slightly increases its performances although, statistically speaking, not in a relevant way.Peer reviewe

Atmospheric deposition, CO2, and change in the land carbon sink

Concentrations of atmospheric carbon dioxide (CO2) have continued to increase whereas atmospheric deposition of sulphur and nitrogen has declined in Europe and the USA during recent decades. Using time series of flux observations from 23 forests distributed throughout Europe and the USA, and generalised mixed models, we found that forest-level net ecosystem production and gross primary production have increased by 1% annually from 1995 to 2011. Statistical models indicated that increasing atmospheric CO2 was the most important factor driving the increasing strength of carbon sinks in these forests. We also found that the reduction of sulphur deposition in Europe and the USA lead to higher recovery in ecosystem respiration than in gross primary production, thus limiting the increase of carbon sequestration. By contrast, trends in climate and nitrogen deposition did not significantly contribute to changing carbon fluxes during the studied period. Our findings support the hypothesis of a general CO2-fertilization effect on vegetation growth and suggest that, so far unknown, sulphur deposition plays a significant role in the carbon balance of forests in industrialized regions. Our results show the need to include the effects of changing atmospheric composition, beyond CO2, to assess future dynamics of carbon-climate feedbacks not currently considered in earth system/climate modelling.Peer reviewe