Beate Michalzik

Controls on the dynamics of dissolved organic matter in soils: a review.

Dissolved organic matter (DOM) in soils plays an important role in the biogeochemistry of carbon, nitrogen, and phosphorus, in pedogenesis, and in the transport of pollutants in soils. The aim of this review is to summarize the recent literature about controls on DOM concentrations and fluxes in soi

Fluxes and concentrations of dissolved organic carbon and nitrogen - a synthesis for temperate forests

Dissolved organic carbon (DOC) and nitrogen (DON)represent an important part of the C and N cycles inforest ecosystems. Little is known about the controlson fluxes and concentrations of these compounds insoils under field conditions. Here we compiledpublished data on concentrations and fluxes of DOC andDON from 42 case studies in forest ecosystems of thetemperate zone in order to evaluate controls on alarger temporal and spatial scale. The focus was onannual fluxes and concentrations in throughfall,forest floor leachates and soil solutions. In allcompartments considered, concentrations and fluxesdiffered widely between the sites. Highestconcentrations of DOC and DON were generally observedin forest floor leachates and in A horizons. Highestfluxes occurred in forest floor leachates. The fluxesof DOC and DON in forest floor leachates increasedwith increasing annual precipitation and were alsopositively related to DOC and DON fluxes withthroughfall. Variation in throughfall fluxes couldexplain 46% and 65% of the variation in DOC and DONfluxes from the forest floor, respectively. No generaldifference in DOC and DON concentrations and fluxes inforest floor leachates was found when comparingconiferous and hardwood sites. Concentrations of DOCin forest floor leachates were positively correlatedto the pH of the forest floor. Furthermore, there wasno relationship between organic C and N stocks, soilC/N, litterfall or mineral N inputs and concentrationsand fluxes of DOC and DON in forest floor leachates.Including all compartments, fluxes of DOC and DON werehighly correlated. Ratios of DOC to DON calculatedfrom fluxes from the forest floor were independent ofthe amount of annual precipitation, pointing to asimilar response of DOC and DON to precipitationconditions. A decrease in the ratio of DOC to DON withsoil depth as observed on a plot-scale, was notconfirmed by data analysis on a large scale. Thecontrols observed on annual fluxes and concentrationsof DON and DOC at regional scale differed from thosereported for smaller time and space scales.

Modelling the production and transport of dissolved organic carbon in forest soils

DyDOC describes soil carbon dynamics, with a focus on dissolved organic carbon (DOC). The model treats the soil as a three-horizon profile, and simulates metabolic carbon transformations, sorption reactions and water transport. Humic substances are partitioned into three fractions, one of which is immobile, while the other two (hydrophilic and hydrophobic) can pass into solution as DOC. DyDOC requires site-specific soil characteristics, and is driven by inputs of litter and water, and air and soil temperatures. The model operates on hourly and daily time steps, and can simulate carbon cycling over both long (hundreds-to-thousands of years) and short (daily) time scales. An important feature of DyDOC is the tracking of 14C, from its entry in litter to its loss as DO14C in drainage water, enabling information about C dynamics to be obtained from both long-term radioactive decay, and the characteristic 14C pulse caused by thermonuclear weapon testing during the 1960s (bomb carbon). Parameterisation is performed by assuming a current steady state. Values of a range of variables, including C pools, annual DOC fluxes, and 14C signals, are combined into objective functions for least-squares minimisation. DyDOC has been applied successfully to spruce forest sites at Birkenes (Norway) and Waldstein (Germany), and most of the parameters have similar values at the two sites. The results indicate that the supply of DOC from the surface soil horizon to percolating water depends upon the continual metabolic production of easily leached humic material. In contrast, concentrations and fluxes of DOC in the deeper soil horizons are controlled by sorption processes, involving comparatively large pools of leachable organic matter. Times to reach steady state are calculated to be several hundred years in the organic layer, and hundreds-to-thousands of years in the deeper mineral layers. It is estimated that DOC supplies 89% of the mineral soil carbon at Birkenes, and 73% at Waldstein. The model, parameterised with steady state data, simulates short-term variations in DOC concentrations and fluxes, and in DO14C, which are in approximate agreement with observations.

Insect herbivores and the nutrient flow from the canopy to the soil in coniferous and deciduous forests

Phytophagous insects can have severe impacts on forested ecosystems in outbreak situations but their contribution to flows of energy and matter is otherwise not so well known. Identifying the role of phytophagous insects in forested ecosystems is partly hindered by the difficulty of combining results from population and community ecology with those from ecosystem ecology. In our study we compared the effects of aphids and leaf-feeding lepidopterous larvae on the epiphytic micro-organisms in the canopies of spruce, beech and oak, and on the vertical flow of energy and nutrients from the canopies down to the forest floor. We particularly searched for patterns resulting from endemic herbivory rather than outbreak situations. Excreta of lepidopterous larvae and aphids promoted the growth of epiphytic micro-organisms (bacteria, yeasts, filamentous fungi) on needles and leaves, which suggests that micro-organisms were energy limited. Leachates from needles and leaves of infested trees contained higher concentrations of dissolved organic C and lower concentrations of NH4-N and NO3-N, relative to uninfested trees. The seasonal abundance of herbivores and micro-organisms significantly affected the dynamics of throughfall chemistry; for instance, concentrations of inorganic N were lower underneath infested than uninfested trees during June and July. There was little difference between the chemistry of soil solutions collected from the forest floor beneath infested and uninfested trees. Thus, under moderate to low levels of infestation the effects of above-ground herbivory seems to be obscured in the soil through buffering biological processes.

Factors controlling decomposition rates of fine root litter in temperate forests and grasslands

Background and aimsFine root decomposition contributes significantly to element cycling in terrestrial ecosystems. However, studies on root decomposition rates and on the factors that potentially influence them are fewer than those on leaf litter decomposition. To study the effects of region and land use intensity on fine root decomposition, we established a large scale study in three German regions with different climate regimes and soil properties. Methods In 150 forest and 150 grassland sites we deployed litterbags (100xa0μmxa0mesh size) with standardized litter consisting of fine roots from European beech in forests and from a lowland mesophilous hay meadow in grasslands. In the central study region, we compared decomposition rates of this standardized litter with root litter collected on-site to separate the effect of litter quality from environmental factors.ResultsStandardized herbaceous roots in grassland soils decomposed on average significantly faster (24u2009±u20096xa0% mass loss after 12xa0months, mean ± SD) than beech roots in forest soils (12u2009±u20094xa0%; pu2009<u20090.001). Fine root decomposition varied among the three study regions. Land use intensity, in particular N addition, decreased fine root decomposition in grasslands. The initial lignin:N ratio explained 15xa0% of the variance in grasslands and 11xa0% in forests. Soil moisture, soil temperature, and C:N ratios of soils together explained 34xa0% of the variance of the fine root mass loss in grasslands, and 24xa0% in forests.ConclusionsGrasslands, which have higher fine root biomass and root turnover compared to forests, also have higher rates of root decomposition. Our results further show that at the regional scale fine root decomposition is influenced by environmental variables such as soil moisture, soil temperature and soil nutrient content. Additional variation is explained by root litter quality.

Response of cocoa trees (Theobroma cacao) to a 13-month desiccation period in Sulawesi, Indonesia

In South-east Asia, ENSO-related droughts represent irregularly occurring hazards for agroforestry systems containing cocoa which are predicted to increase in severity with expected climate warming. To characterize the drought response of mature cocoa trees, we conducted the Sulawesi Throughfall Displacement Experiment in a shaded (Gliricidia sepium) cocoa agroforestry system in Central Sulawesi, Indonesia. Three large sub-canopy roofs were installed to reduce throughfall by about 80% over a 13-month period to test the hypotheses that (i) cocoa trees are sensitive to drought due to their shallow fine root system, and (ii) bean yield is more sensitive to drought than leaf or stem growth. As 83% of fine root (diameter <2xa0mm) and 86% of coarse root biomass (>2xa0mm) was located in the upper 40xa0cm of the soil, the cocoa trees examined had a very shallow root system. Cocoa and Gliricidia differed in their vertical rooting patterns, thereby reducing competition for water. Despite being exposed for several months to soil water contents close to the conventional wilting point, cocoa trees showed no significant decreases in leaf biomass, stem and branch wood production or fine root biomass. Possible causes are active osmotic adjustment in roots, mitigation of drought stress by shading from Gliricidia or other factors. By contrast, production of cocoa beans was significantly reduced in the roof plots, supporting reports of substantial reductions in bean yields during ENSO-related drought events in the region. We conclude that cocoa possesses traits related to drought tolerance which enable it to maintain biomass production during extended dry periods, whereas bean yield appears to be particularly drought sensitive.

Investigation of gas exchange processes in peat bog ecosystems by means of innovative Raman gas spectroscopy.

Highly sensitive Raman gas spectroscopy is introduced for simultaneous real time analysis of O(2), CO(2), CH(4), and N(2) in order to elucidate the dynamics of greenhouse gases evolving from climate-sensitive ecosystems. The concentrations and fluxes of this suite of biogenic gases were quantified in the head space of a water-saturated, raised peat bog ecotron. The intact peat bog, exhibiting various degradation stages of peat and sphagnum moss, was exposed to various light regimes in order to determine important ecosystem parameters such as the maximum photosynthesis rate of the sphagnum as well as the extent of soil and plant respiration. Miniaturized Raman gas spectroscopy was proven to be an extremely versatile analytical technique that allows for onsite multigas analysis in high temporal resolution. Therefore it is an urgently needed tool for elucidation of complex biochemical processes especially in climate-sensitive ecosystems and consequently for the estimation of climate-relevant gas budgets.

Different effect of drying on the fluxes of dissolved organic carbon and nitrogen from a Norway spruce forest floor

The forest floor represents the major source of dissolved organic carbon (DOC) and nitrogen (DON) in forest soils. The release mechanisms of DOC and DON from forest floors and their environmental controls as well as the dynamics of concentrations and fluxes are still poorly understood. We investigated the effect of drying and rewetting on the release of DOC and DON from a Norway spruce forest floor. Undisturbed soil columns of 17 cm diameter and 15-20 cm height were taken with 7 replicates from the forest floor of a mature Norway spruce (Picea abies [L.] Karst.) site and established at 10 degreesC in the laboratory. Columns were exposed to different periods of drying (3, 5, 10, 20 days). Each drying period was followed by a rewetting for 5 days at an irrigation rate of 10 mm d(-1) with a natural throughfall solution. The percolates from the forest floor were collected daily and analyzed for DOC, total N, NH4, NO3, pH, electrical conductivity and major ions. Drying for 10 and 20 days decreased the water content of the Oi horizon from 280% dry weight to about 30%. The water content of the Oe and the Oa horizon only changed from about 300% to 200%. The fluxes of DOC from the forest floor were moderately effected by drying and rewetting with an increase after 3 and 5 days of drying, but a decrease after 10 and 20 days. On the contrary, the drying for 10 and 20 days resulted in a drastic increase of the DON fluxes and a subsequent decrease of the DOC/DON ratios in the forest floor percolates from about 50 to 3.3. These results suggest that the mechanisms for DOC release in forest floors differ from those for DON and that drying and rewetting cause temporal variations in the DOC/DON ratios in forest floor percolates.

Controls on soil carbon storage and turnover in German landscapes

Soil organic carbon (OC) storage across regions is influenced by climate and parent materials, which determine soil properties like clay content and mineralogy. Within homogeneous soil regions, land use and management practices are further important controls for soil OC contents and turnover. Here, we studied the impact of study region, land use (forest, grassland), forest management (spruce and beech forest under age-class management, unmanaged beech forest), and grassland management (meadow, mown pasture, unmown pasture) on stocks and turnover (based on Δ14C values) of soil OC in density frations of topsoil horizons. Samples were taken from 36 plots in the regions Hainich–Dün (HAI) and the Schwäbische Alb (ALB) in Germany. They were separated into two light fractions (free light fraction (LF1), occluded light fraction (LF2)) and the mineral-associated organic matter (MOM) fraction using sodium polytungstate with a density of 1.6 g cm−3. Overall most soil OC was stored in the MOM fraction (73%). Soil OC concentrations and stocks in the MOM fraction differed between study regions, probably due to larger amounts of pedogenic Al- and Fe-oxides in the ALB than in the HAI region. Within each region, forest soils stored significantly higher proportions of total OC in the two LF (33±1.9 %) than grassland soils (20±2.3 %). Different management of forests and grasslands affected the C:N ratio of density fractions, but not OC storage. While modelled soil OC turnover in the MOM was longest of all fractions, all fractions had average Δ14C values above atmospheric values, suggesting a significant fast-cycling component in all of them. Different from stocks, turnover of OC in the MOM fraction were not affected by study region or contents of pedogenic oxides. Radiocarbon contents in the LF were higher for forest than for grassland sites, indicating faster turnover of OC at grassland sites. However, some of the observed difference could originate from different average lifetimes of roots in forests and grasslands. Applying different lag-times for OC input for forests and grasslands significantly reduced the differences in modelled turnover times. Lower Δ14C values of mown pastures than pasture soils in both regions suggest a management effect on soil C turnover in grasslands.We conclude that OC storage in the MOM of topsoil layers is more affected by regional differences in soil texture and mineralogy than by land use and management, while its turnover could not be explained with the studied soil properties. Soil OC storage and turnover in the two LFs is influenced by land use (forest or grassland) and management, but ecosystem specific lag-times have to be considered for modelling OC turnover in these fractions.

The importance of canopy-derived dissolved and particulate organic matter (DOM and POM) - comparing throughfall solution from broadleaved and coniferous forests

Abstract• Dissolved organic matter (DOM) and its main constituents carbon (DOC) and nitrogen (DON) represent an important part of the C and N cycles in forest ecosystems. Although many investigations have been addressing this issue, the knowledge on particulate organic matter (0.45 μm < POM < 500 μm) dynamics, its origin and involvement in organic matter cycling in forest ecosystems is still imperfect.• In this paper, we report on dissolved and particulate organic carbon and nitrogen fractions in throughfall solutions collected from a broadleaved and coniferous forest stand in Central Germany. Over a period of 2.5 y (2005–2007) we followed the concentrations and fluxes of DOM and POM at a mature beech (Fagus sylvatica L.) and a Norway spruce (Picea abies L.) forest site. Bulk and throughfall precipitation were sampled in weekly (2005) and fortnightly (2006–2007) intervals and analyzed for dissolved (< 0.45 μm, filtered) and total (< 500 μm, unfiltered) amounts of organic carbon (DOC, TOC, POC) and nitrogen (TN, DN, PON, NO3-N) species. Proportions of particulate organic C and N were determined by difference between total and dissolved fractions.• Under spruce, throughfall concentrations of most C and N fractions were twice as high as under beech. At both sites, concentrations and fluxes were significantly higher during the growing than the dormant season. At the broadleaved site, 80% of the annual fluxes of the DOC and TOC and 70% of the DN and TN were released during the growing season, compared to 60% for C and N at the coniferous site. POC under beech contributes with up to 30% to TOC compared to less than 20% at the spruce site.• We suggest that pollen deposition, insect excretions and accumulated organic matter mobilised by dry/wet precipitation patterns play a supreme role for the formation of DOM and POM in forest canopies. The study demonstrates that the canopy is an important source for POM. Dynamics of DOM and POM are mainly driven by tree species effects and seasonality as well as by biotic agents.Résumé• La matière organique dissoute (DOM) et ses principaux constituants carbonés (DOC) et l’azote (DON) représentent une part importante des cycles du carbone et de l’azote dans les écosystèmes forestiers. Bien que de nombreuses enquêtes se soient penché sur ce problème, les connaissances sur les dynamiques des particules de matière organique (0,45 μm < POM < 500 μm), leur origine et leur rôle dans les cycles de la matière organique dans les écosystèmes forestiers sont encore imparfaites.• Dans ce papier, nous présentons un rapport sur les particules de carbone organique et les fractions d’azote dissoutes dans les solutions de pluie arrivant directement au sol sous le couvert, récoltées dans un peuplement forestier feuillu et dans un peuplement de conifères en Allemagne centrale. Sur une période de 2,5 ans (2005–2007) nous avons suivi les concentrations et les flux de DOM et de POM dans une hêtraie arrivée à maturité (Fagus sylvatica L.) et un peuplement d’épicéa commun (Picea abies L.). Le volume des pluies et les précipitations au sol ont été échantillonnés à des intervalles de temps d’une semaine (2005) et d’une quinzaine (2006–2007) et analysés pour les quantités de carbone organique dissoutes (< 0,45 μm, filtrées) et totales (< 500 μm, non filtrées) (DOC, TOC, POC) et d’azote (TN, DN, PON, NO3-N). Les proportions des particules organiques de carbone et d’azote ont été déterminées par différence entre les fractions totales et dissoutes.• Sous la pessière, les concentrations de la plupart des fractions de C et de N, dans la pluie au sol, étaient deux fois plus élevées que sous la hêtraie. Sur les deux sites, les concentrations et les flux ont été significativement plus élevés pendant la période de croissance que durant la période de dormance. Sur le site feuillu, 80 % des flux annuels de DOC et de TOC et 70 % de DN et TN ont été libérés au cours de la saison de croissance, contre 60 % pour C et N pour les conifères. Dans la hêtraie POC contribue à hauteur de 30 % de TOC, comparativement à moins de 20 % dans la pessière.• Nous suggérons que les dépôts de pollen, les déjections d’insectes et la matière organique accumulée, mobilisés par les séquences de périodes sèches et de précipitations jouent un rôle suprême pour la formation des DOM et POM dans les canopées forestières. L’étude démontre que la canopée est une source importante pour POM. Les dynamiques de DOM et POM sont principalement conduites par les effets des espèces d’arbres et par la saisonnalité ainsi que par des agents biotiques.