We examined the impact of arbuscular mycorrhizal fungi and rhizobia on the living microbial community and microbial necromass under different long-term fertilization treatments at the long-term ...Static Fertilization Experiment Bad Lauchstädt (Germany). Phospholipid fatty acids (PLFA) and amino sugars plus muramic acid, were used as biomarkers for soil microbial bio- and necromass, respectively, and analyzed from six treatments imposed on two crop rotations, varying only in the inclusion/non-inclusion of a legume. Treatments included: two levels of only farmyard manure (FYM), only mineral fertilizer (NPK), the combined application of both fertilizer types and a non-fertilized control. PLFA profiles differed clearly between the investigated crop rotations and were significantly related to labile C, mineral N, and soil pH. This emphasizes the role of carbon, and of mycorrhizal and rhizobial symbioses, as driver for changes in the microbial community composition due to effects on the living conditions in soil. We found some evidence that legume associated symbiosis with arbuscular mycorrhizal fungi and rhizobia act as a buffer, reducing the impact of varying inputs of mineral nutrients on the decomposer community. While our results support former findings that living microbial populations vary within short-term periods and are reflective of a given crop grown in a given year, soil necromass composition indicates longer term changes across the two crop rotation types, mainly shaped by fertilizer related effects on the community composition and C turnover. However, there was some evidence that specifically the presence of a legume, affects the soil necromass composition not only over the whole crop rotation but even in the short-term.
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•We extracted PLFA and amino sugars from six fertilization treatments under two crops.•Both, microbial bio- and necromass compositions were shaped by manure addition.•Short-term effects on microbial biomass result from mineral fertilizers.•Strength of mineral fertilizer effects depends on crop type and plant-symbionts.•Turnover of microbial necromass may depend on N fertility.
Dissolved organic carbon (DOC) and nitrogen (DON) represent an important part of the C and N cycles in forest ecosystems. Little is known about the controls on fluxes and concentrations of these ...compounds in soils under field conditions. Here we compiled published data on concentrations and fluxes of DOC and DON from 42 case studies in forest ecosystems of the temperate zone in order to evaluate controls on a larger temporal and spatial scale. The focus was on annual fluxes and concentrations in throughfall, forest floor leachates and soil solutions. In all compartments considered, concentrations and fluxes differed widely between the sites. Highest concentrations of DOC and DON were generally observed in forest floor leachates and in A horizons. Highest fluxes occurred in forest floor leachates. The fluxes of DOC and DON in forest floor leachates increased with increasing annual precipitation and were also positively related to DOC and DON fluxes with throughfall. Variation in throughfall fluxes could explain 46% and 65% of the variation in DOC and DON fluxes from the forest floor, respectively. No general difference in DOC and DON concentrations and fluxes in forest floor leachates was found when comparing coniferous and hardwood sites. Concentrations of DOC in forest floor leachates were positively correlated to the pH of the forest floor. Furthermore, there was no relationship between organic C and N stocks, soil C/N, litterfall or mineral N inputs and concentrations and fluxes of DOC and DON in forest floor leachates. Including all compartments, fluxes of DOC and DON were highly correlated. Ratios of DOC to DON calculated from fluxes from the forest floor were independent of the amount of annual precipitation, pointing to a similar response of DOC and DON to precipitation conditions. A decrease in the ratio of DOC to DON with soil depth as observed on a plot-scale, was not confirmed by data analysis on a large scale. The controls observed on annual fluxes and concentrations of DON and DOC at regional scale differed from those reported for smaller time and space scales.
We present the first investigation of the composition of dissolved organic matter (DOM) compared to total organic matter (TOM, consisting of DOM, < 0.45 μm and particulate organic matter 0.45 μm < ...POM < 500 μm) in throughfall, stemflow and forest floor leachate of common beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) H. Karst.) forests using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. We hypothesized that the composition and properties of organic matter (OM) in forest ecosystem water samples differ between DOM and TOM and between the two tree species. The 13C NMR results, derived from 21 samples, point to pronounced differences in the composition of DOM and TOM in throughfall solution at the beech sites, with TOM exhibiting higher relative intensities for the alkyl C region, which represents aliphatic C from less decomposed organic material compared to DOM. Furthermore, TOM shows lower intensities for lignin-derived and aromatic C of the aryl C region resulting in lower aromaticity indices and a diminished degree of humification. Across the ecosystem compartments, differences in the structural composition of DOM and TOM under beech lessened in the following order: throughfall > stemflow ≈ forest floor leachate. In contrast to the broadleaved sites, differences between DOM and TOM in throughfall solution under spruce were less pronounced and spectra were, overall, dominated by the alkyl C region, representing aliphatic C. Explanations of the reported results might be substantiated in differences in tree species-specific structural effects, leaching characteristics or differences in the microbial community of the tree species' phyllosphere and cortisphere. However, the fact that throughfall DOM under beech showed the highest intensities of recalcitrant aromatic and phenolic C among all samples analysed likely points to a high allelopathic potential of beech trees negatively affecting other organisms and hence ecosystem processes and functions.
Summary
Dissolved organic nitrogen and carbon (DOC) are significant in the C and N cycle in terrestrial ecosystems. Little is known about their dynamics in the field and the factors regulating their ...concentrations and fluxes. We followed the fluxes and concentrations of the two in a Norway spruce (Picea abies (L.) Karst.) forest ecosystem in Germany from 1995 to 1997 by sampling at fortnightly intervals. Bulk precipitation, throughfall, forest floor percolates from different horizons and soil solutions from different depths were analysed for major ions, dissolved organic N and DOC. The largest fluxes and concentrations were observed in percolates of the Oi layer, which contain amino N and amino sugar N as the major components. The average ratio of dissolved organic C to N in forest floor percolates corresponded to the C/N ratio of the solid phase. Concentrations and fluxes were highly dynamic with time and decreased with depth. The largest fluxes in forest floor percolates occurred when the snow melted. The concentrations and fluxes of dissolved organic N were significantly correlated with DOC, but the correlation was weak, indicating different mechanisms of release and consumption. The dynamics of dissolved organic N and DOC in forest floor percolates were not explained by pH and ionic strength of the soil solution nor by the water flux, despite large variations in these. Furthermore, the release of these fractions from the forest floor was not related to the quality and amount of throughfall. Concentrations of dissolved organic N in forest floor percolates increased with soil temperature, while temperature effects on DOC were less pronounced, but their fluxes from the forest floor were not correlated with temperature. In the growing season concentrations of both dissolved organic N and C in forest floor percolates decreased with increasing intensity of throughfall. Thus, the average throughfall intensity was more important than the amount of percolate in regulating their concentrations in forest floor percolates. Our data emphasize the role of dissolved organic N and DOC in the N and C cycle of forest ecosystems.
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 <2 mm) and 86% of coarse root biomass (>2 mm) was located in the upper 40 cm 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.
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 DO 14C 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 DO 14C, which are in approximate agreement with observations.
The emission of arsenic (As) with leachate from mechanically biologically pretreated municipal solid waste (MBP-MSW) was quantified over one year using landfill simulation reactors. Arsenic ...mobilization and transformation processes were studied by simulating different environmental conditions (anoxic conditions with underlying soil or oxic/anoxic conditions). Amounts of mono-, di-, and trimethylated As in MBP-MSW prior to simulation were <48 μg As kg−1 and were magnified to 300−390 μg As kg−1 under anoxic conditions, whereas methylated As was undetectable in the oxic setup. The highest leachate concentrations (up to 84 μg L−1) occurred during the first four weeks of manipulation. The annual Astotal release with leachates averaged 19.6, 7.6, and 4.5 μg kg−1 under an anoxic environment with underlying soil, oxic conditions, and anoxic conditions, respectively, with 15−50% occurring as organic As. The annually released As represented 0.2−0.8% of the Astotal pool, suggesting that As mobilization from waste is a slow process. The anoxia diminished As release rates, whereas anoxic conditions with underlying soil material elevated the As mobilization, probably due to reductive dissolution of soil-derived Fe and Mn (hydr)oxides. The mass balance of methylated As in MBP-MSW and leachates before and after the treatments highlights As methylation under anoxic conditions and demethylation under oxic landfill conditions.
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 microorganisms 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.
Dissolved organic matter (DOM) is part of the biogeochemical cycles of carbon and nutrients, carries pollutants and drives soil formation. The DOM concentration and properties along the water flow ...path through forest ecosystems depend on its sampling location and transformation processes. To improve our understanding of the effects of forest management, especially tree species selection and management intensity, on DOM concentrations and properties of samples from different ecosystem fluxes, we studied throughfall, stemflow, litter leachate and mineral soil solution at 26 forest sites in the three regions of the German Biodiversity Exploratories. We covered forest stands with three management categories (coniferous, deciduous age class and unmanaged beech forests). In water samples from these forests, we monitored DOC concentrations over 4 years and characterized the quality of DOM with UV-vis absorption, fluorescence spectroscopy combined with parallel factor analysis (PARAFAC) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Additionally, we performed incubation-based biodegradation assays. Multivariate statistics revealed strong significant effects of ecosystem fluxes and smaller effects of main tree species on DOM quality. Coniferous forests differed from deciduous forests by showing larger DOC concentrations, more lignin- and protein-like molecules, and fewer tannin-like molecules in throughfall, stemflow, and litter leachate. Cluster analysis of FT-ICR-MS data indicated that DOM compositions, which varied in aboveground samples depending on tree species, become aligned in mineral soil. This alignment of DOM composition along the water flow path in mineral soil is likely caused by microbial production and consumption of DOM in combination with its interaction with the solid phase, producing a characteristic pattern of organic compounds in forest mineral soils. We found similarly pronounced effects of ecosystem fluxes on the biodegradability of DOM, but surprisingly no differences between deciduous and coniferous forests. Forest management intensity, mainly determined by biomass extraction, contribution of species, which are not site-adapted, and deadwood mass, did not influence DOC concentrations, DOM composition and properties significantly.
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