Drought resistance and resilience of rhizosphere communities in forest soils from the cellular to ecosystem scale - insights from 13C pulse labeling.

The link between above- and belowground communities is a key uncertainty in drought and rewetting effects on forest carbon (C) cycle. In young beech model ecosystems and mature naturally dry pine forest exposed to 15-yr-long irrigation, we performed 13C pulse labeling experiments, one during drought and one 2 wk after rewetting, tracing tree assimilates into rhizosphere communities. The 13C pulses applied in tree crowns reached soil microbial communities of the young and mature forests one and 4 d later, respectively. Drought decreased the transfer of labeled assimilates relative to the irrigation treatment. The 13C label in phospholipid fatty acids (PLFAs) indicated greater drought reduction of assimilate incorporation by fungi (-85%) than by gram-positive (-43%) and gram-negative bacteria (-58%). 13C label incorporation was more strongly reduced for PLFAs (cell membrane) than for microbial cytoplasm extracted by chloroform. This suggests that fresh rhizodeposits are predominantly used for osmoregulation or storage under drought, at the expense of new cell formation. Two weeks after rewetting, 13C enrichment in PLFAs was greater in previously dry than in continuously moist soils. Drought and rewetting effects were greater in beech systems than in pine forest. Belowground C allocation and rhizosphere communities are highly resilient to drought.

Rhizosphere activity in an old-growth forest reacts rapidly to changes in soil moisture and shapes whole-tree carbon allocation.

Drought alters carbon (C) allocation within trees, thereby impairing tree growth. Recovery of root and leaf functioning and prioritized C supply to sink tissues after drought may compensate for drought-induced reduction of assimilation and growth. It remains unclear if C allocation to sink tissues during and following drought is controlled by altered sink metabolic activities or by the availability of new assimilates. Understanding such mechanisms is required to predict forests' resilience to a changing climate. We investigated the impact of drought and drought release on C allocation in a 100-y-old Scots pine forest. We applied 13CO2 pulse labeling to naturally dry control and long-term irrigated trees and tracked the fate of the label in above- and belowground C pools and fluxes. Allocation of new assimilates belowground was ca. 53% lower under nonirrigated conditions. A short rainfall event, which led to a temporary increase in the soil water content (SWC) in the topsoil, strongly increased the amounts of C transported belowground in the nonirrigated plots to values comparable to those in the irrigated plots. This switch in allocation patterns was congruent with a tipping point at around 15% SWC in the response of the respiratory activity of soil microbes. These results indicate that the metabolic sink activity in the rhizosphere and its modulation by soil moisture can drive C allocation within adult trees and ecosystems. Even a subtle increase in soil moisture can lead to a rapid recovery of belowground functions that in turn affects the direction of C transport in trees.

A simple method for the removal of dissolved organic matter and δ15N analysis of NO3(-) from freshwater.

RATIONALE: Stable isotopes of nitrogen in nitrate (NO(3)(-)) are frequently used to identify nitrate sources and to study nitrogen (N) transformation processes, but the measurement methods available are generally rather labor intensive and/or costly, and dissolved organic matter (DOM) can interfere with the δ(15)N signature of nitrate. We therefore have developed a simple cleanup procedure for freshwater samples with low nitrate and high DOM concentrations. METHODS: Nitrate and DOM are extracted from a freeze-dried water sample by using a concentrated sodium hydroxide solution. By the subsequent addition of acetone, two liquid layers are formed, and nitrate migrates into the acetone while DOM remains in the concentrated NaOH solution, thus separating the nitrate from the DOM. For nitrogen isotope analysis, purified nitrate salts are combusted at 1030 °C to produce N(2) gas in an elemental analyzer (EA) coupled to an isotope ratio mass spectrometer (IRMS). RESULTS: With this novel technique up to 99% of DOM could be removed from river water and soil solutions. The method has been tested for sample amounts as small as 4 µmol NO(3)(-) with a precision of <0.1‰ (1SD). Nitrate standards are reproduced accurately without any blank correction. CONCLUSIONS: The benefits of this method are the lack of interferences derived from DOM on the δ(15)N signature and the ease of sample preparation.

Bacterial, archaeal and fungal succession in the forefield of a receding glacier.

Glacier forefield chronosequences, initially composed of barren substrate after glacier retreat, are ideal locations to study primary microbial colonization and succession in a natural environment. We characterized the structure and composition of bacterial, archaeal and fungal communities in exposed rock substrates along the Damma glacier forefield in central Switzerland. Soil samples were taken along the forefield from sites ranging from fine granite sand devoid of vegetation near the glacier terminus to well-developed soils covered with vegetation. The microbial communities were studied with genetic profiling (T-RFLP) and sequencing of clone libraries. According to the T-RFLP profiles, bacteria showed a high Shannon diversity index (H) (ranging from 2.3 to 3.4) with no trend along the forefield. The major bacterial lineages were Proteobacteria, Actinobacteria, Acidobacteria, Firmicutes and Cyanobacteria. An interesting finding was that Euryarchaeota were predominantly colonizing young soils and Crenarchaeota mainly mature soils. Fungi shifted from an Ascomycota-dominated community in young soils to a more Basidiomycota-dominated community in old soils. Redundancy analysis indicated that base saturation, pH, soil C and N contents and plant coverage, all related to soil age, correlated with the microbial succession along the forefield.

Soil phosphorus status and P nutrition strategies of European beech forests on carbonate compared to silicate parent material.

Sustainable forest management requires understanding of ecosystem phosphorus (P) cycling. Lang et al. (2017) [Biogeochemistry, https://doi.org/10.1007/s10533-017-0375-0] introduced the concept of P-acquiring vs. P-recycling nutrition strategies for European beech (Fagus sylvatica L.) forests on silicate parent material, and demonstrated a change from P-acquiring to P-recycling nutrition from P-rich to P-poor sites. The present study extends this silicate rock-based assessment to forest sites with soils formed from carbonate bedrock. For all sites, it presents a large set of general soil and bedrock chemistry data. It thoroughly describes the soil P status and generates a comprehensive concept on forest ecosystem P nutrition covering the majority of Central European forest soils. For this purpose, an Ecosystem P Nutrition Index (ENI P ) was developed, which enabled the comparison of forest P nutrition strategies at the carbonate sites in our study among each other and also with those of the silicate sites investigated by Lang et al. (2017). The P status of forest soils on carbonate substrates was characterized by low soil P stocks and a large fraction of organic Ca-bound P (probably largely Ca phytate) during early stages of pedogenesis. Soil P stocks, particularly those in the mineral soil and of inorganic P forms, including Al- and Fe-bound P, became more abundant with progressing pedogenesis and accumulation of carbonate rock dissolution residue. Phosphorus-rich impure, silicate-enriched carbonate bedrock promoted the accumulation of dissolution residue and supported larger soil P stocks, mainly bound to Fe and Al minerals. In carbonate-derived soils, only low P amounts were bioavailable during early stages of pedogenesis, and, similar to P-poor silicate sites, P nutrition of beech forests depended on tight (re)cycling of P bound in forest floor soil organic matter (SOM). In contrast to P-poor silicate sites, where the ecosystem P nutrition strategy is direct biotic recycling of SOM-bound organic P, recycling during early stages of pedogenesis on carbonate substrates also involves the dissolution of stable Ca-Porg precipitates formed from phosphate released during SOM decomposition. In contrast to silicate sites, progressing pedogenesis and accumulation of P-enriched carbonate bedrock dissolution residue at the carbonate sites promote again P-acquiring mechanisms for ecosystem P nutrition. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10533-021-00884-7.

A new isolation procedure of nitrate from freshwater for nitrogen and oxygen isotope analysis.

The nitrogen (δ(15)N) and oxygen isotope (δ(18)O) analysis of nitrate (NO(3)(-)) from aqueous samples can be used to determine nitrate sources and to study N transformation processes. For these purposes, several methods have been developed; however, none of them allows an accurate, fast and inexpensive analysis. Here, we present a new simple method for the isolation of nitrate, which is based on the different solubilities of inorganic salts in an acetone/hexane/water mixture. In this solvent, all major nitrate salts are soluble, whereas all other oxygen-bearing compounds such as most inorganic carbonates, sulfates, and phosphates are not. Nitrate is first concentrated by freeze-drying, dissolved in the ternary solvent and separated from insoluble compounds by centrifugation. Anhydrous barium nitrate is then precipitated in the supernatant solution by adding barium iodide. For δ(18)O analysis, dried Ba(NO(3))(2) samples are directly reduced in a high-temperature conversion system to CO and measured on-line using isotope ratio mass spectrometry (IRMS). For δ(15)N analysis, samples are combusted in an elemental analyzer (EA) coupled to an IRMS system. The method has been tested down to 20 µmol NO(3)(-) with a reproducibility (1SD) of 0.1‰ for nitrogen and 0.2-0.4‰ for oxygen isotopes. For nitrogen we observed a small consistent (15) N enrichment of +0.2‰, probably due to an incomplete precipitation process and, for oxygen, a correction for the incorporation of water in the precipitated Ba(NO(3))(2) has to be applied. Apart from being robust, this method is highly efficient and low in cost.

Effects of drought on nitrogen uptake and carbon dynamics in trees.

Research on drought impact on tree functioning is focussed primarily on water and carbon (C) dynamics. Changes in nutrient uptake might also affect tree performance under drought and there is a need to explore underlying mechanisms. We investigated effects of drought on (a) in situ nitrogen (N) uptake, accounting for both, N availability to fine roots in soil and actual N uptake, (b) physiological N uptake capacity of roots and (c) the availability of new assimilates to fine roots influencing the N uptake capacity using 15N and 13C labelling. We assessed saplings of six different tree species (Acer pseudoplatanus L., Fagus sylvatica L., Quercus petraea (Mattuschka) Liebl., Abies alba Mill., Picea abies (L.) H.Karst. and Pinus sylvestris L.). Drought resulted in significant reduction of in situ soil N uptake in deciduous trees accompanied by reduced C allocation to roots and by a reduction in root biomass available for N uptake. Although physiological root N uptake capacity was not affected by drought in deciduous saplings, reduced maximum ammonium but not nitrate uptake was observed for A. alba and P. abies. Our results indicate that drought has species-specific effects on N uptake. Even water limitations of only 5 weeks as assessed here can decrease whole-plant inorganic N uptake, independent of whether the physiological N uptake capacity is affected or not.

Phosphorus Allocation to Leaves of Beech Saplings Reacts to Soil Phosphorus Availability.

Decreasing phosphorus (P) concentrations in leaves of beech (Fagus sylvatica L.) across Europe raise the question about the implications for forest health. Considering the distribution of beech forests on soils encompassing a broad range of nutrient availability, we hypothesized that this tree species exhibits high phenotypic plasticity allowing it to alter mass, and nutrient allocation in response to local nutrient availability. To test this, we grew two groups of 12-15 year old beech saplings originating from sites with high and low soil P availability for 2 years in mineral soil from their own site and in soil from the other site. After two growing seasons, P concentrations in leaves and stem, as well as mass allocation to leaves and fine roots were affected by both soil and plant origin. By contrast, relative P allocation to leaves and fine roots, as well as P concentrations in fine roots, were determined almost entirely by the experimental soil. Independent of the P nutritional status defined as average concentration of P in the whole plant, which still clearly reflected the soil conditions at the site of plant origin, relative P allocation to leaves was a particularly good indicator of P availability in the experimental soil. Furthermore, a high plasticity of this plant trait was indicated by a large difference between plants growing in the two experimental soils. This suggests a strong ability of beech to alter resource allocation in response to specific soil conditions.

Heavy metal accumulation and phytostabilisation potential of tree fine roots in a contaminated soil.

Root systems of Norway spruce (Picea abies) and poplar (Populus tremula) were long-term exposed to metal-contaminated soils in open-top chambers to investigate the accumulation of the heavy metals in the fine roots and to assess the plants suitability for phytostabilisation. The heavy metals from the contaminated soil accumulated in the fine roots about 10-20 times more than in the controls. The capacity to bind heavy metals already reached its maximum after the first vegetation period. Fine roots of spruce tend to accumulate more heavy metals than poplar. Copper and Zinc were mainly detected in the cell walls with larger values in the epidermis than in the cortex. The heavy metals accumulated in the fine roots made up 0.03-0.2% of the total amount in the soils. We conclude that tree fine roots adapt well to conditions with heavy metal contamination, but their phytostabilisation capabilities seem to be very low.

The C:N:P:S stoichiometry of soil organic matter.

The formation and turnover of soil organic matter (SOM) includes the biogeochemical processing of the macronutrient elements nitrogen (N), phosphorus (P) and sulphur (S), which alters their stoichiometric relationships to carbon (C) and to each other. We sought patterns among soil organic C, N, P and S in data for c. 2000 globally distributed soil samples, covering all soil horizons. For non-peat soils, strong negative correlations (p < 0.001) were found between N:C, P:C and S:C ratios and % organic carbon (OC), showing that SOM of soils with low OC concentrations (high in mineral matter) is rich in N, P and S. The results can be described approximately with a simple mixing model in which nutrient-poor SOM (NPSOM) has N:C, P:C and S:C ratios of 0.039, 0.0011 and 0.0054, while nutrient-rich SOM (NRSOM) has corresponding ratios of 0.12, 0.016 and 0.016, so that P is especially enriched in NRSOM compared to NPSOM. The trends hold across a range of ecosystems, for topsoils, including O horizons, and subsoils, and across different soil classes. The major exception is that tropical soils tend to have low P:C ratios especially at low N:C. We suggest that NRSOM comprises compounds selected by their strong adsorption to mineral matter. The stoichiometric patterns established here offer a new quantitative framework for SOM classification and characterisation, and provide important constraints to dynamic soil and ecosystem models of carbon turnover and nutrient dynamics.

Recovery of trees from drought depends on belowground sink control.

Climate projections predict higher precipitation variability with more frequent dry extremes(1). CO2 assimilation of forests decreases during drought, either by stomatal closure(2) or by direct environmental control of sink tissue activities(3). Ultimately, drought effects on forests depend on the ability of forests to recover, but the mechanisms controlling ecosystem resilience are uncertain(4). Here, we have investigated the effects of drought and drought release on the carbon balances in beech trees by combining CO2 flux measurements, metabolomics and (13)CO2 pulse labelling. During drought, net photosynthesis (AN), soil respiration (RS) and the allocation of recent assimilates below ground were reduced. Carbohydrates accumulated in metabolically resting roots but not in leaves, indicating sink control of the tree carbon balance. After drought release, RS recovered faster than AN and CO2 fluxes exceeded those in continuously watered trees for months. This stimulation was related to greater assimilate allocation to and metabolization in the rhizosphere. These findings show that trees prioritize the investment of assimilates below ground, probably to regain root functions after drought. We propose that root restoration plays a key role in ecosystem resilience to drought, in that the increased sink activity controls the recovery of carbon balances.

Metal fractionation in a contaminated soil after reforestation: temporal changes versus spatial variability.

In a lysimeter experiment, topsoils were polluted with filter dust from a non-ferrous metal smelter and then planted with trees. Sequential extractions were used to follow the changes in metal fractionation of Cu, Zn, Cd, and Pb over 42 months. Plant-free and uncontaminated soils served as reference. In the contaminated and planted soils, the largest changes in speciation occurred within the first 6 months. The relative amounts of certain metal fractions were linearly related to each other, indicating systematic redistribution between fractions. The results indicate that under natural conditions with high heterogeneity in total metal contents spatial differences are more important than temporal variations in determining the fractionation and solubility of metals in contaminated soils. In the absence of plants soils exhibited a completely different fractionation 30 months after pollution, with much higher proportions in the more refractory phases. This suggests that plant activity kept the metals in a more soluble form.

Decrease of labile Zn and Cd in the rhizosphere of hyperaccumulating Thlaspi caerulescens with time.

By using a rhizobox micro-suction cup technique we studied in-situ mobilization and complexation of Zn and Cd in the rhizosphere of non-hyperaccumulating Thlaspi perfoliatum and two different Thlaspi caerulescens ecotypes, one of them hyperaccumulating Zn, the other Zn and Cd. The dynamic fraction (free metal ions and small labile complexes) of Zn and Cd decreased with time in the rhizosphere solution of the respective hyperaccumulating T. caerulescens ecotypes, and at the end of the experiment, it was significantly smaller than in the other treatments. Furthermore, the rhizosphere solutions of the T. caerulescens ecotypes exhibited a higher UV absorptivity than the solution of the T. perfoliatum rhizosphere and the plant-free soil. Based on our findings we suggest that mobile and labile metal-dissolved soil organic matter complexes play a key role in the rapid replenishment of available metal pools in the rhizosphere of hyperaccumulating T. caerulescens ecotypes, postulated earlier.

Acidification of soil solution in a chestnut forest stand in southern Switzerland: are there signs of recovery?

In a previous study, a rapid acidification of soil solution was observed between 1987 and 1997 in a cryptopodzolic soil in southern Switzerland despite a reduction in acidic deposition. The molar ratio of base nutrient cations to aluminum (BC/Al) in the soil solution was used to assess acidification. The monitoring of the soil solution chemistry was continued at the same site between 1998 and 2003 to find out how long the delay in reaction to reduced deposition would last and whether the BC/Al ratios would recover. The reevaluation of all data collected during the 16-year observation period showed no clear improvement in the BC/Al ratios, except below the litter layer where the ratios greatly increased after 1998. Initial signs of recovery were also detected in the mineral horizons, the ratios stabilizing in the second part of the observation period. Sulfate concentrations decreased significantly below the litter mat in response to decreased S deposition. BC concentrations markedly declined below the litter layer and in the mineral horizons, which was attributed to the depletion of the BC exchangeable pool as a result of continued acidic deposition.