Biology subjects

Kahmen, A.

Publications and source records attributed to Kahmen, A..

10 recordsLinked to original sources

bioRxiv2026

Seasonal dynamics in the trophic ecology and condition of a marine, benthic mesopredator, the southern stingray, Hypanus americanus

Mesopredators contribute to food web stability and as such, understanding their trophic ecology can help to predict potential consequences of ongoing ecosystem modification. Here, multi-tissue carbon and nitrogen stable isotope analysis ({delta}13C and {delta}15N) and biochemical blood parameters ({beta}-hydroxybutyrate, glucose, lactate, and osmolality) were used to assess sex, size, spatial and seasonal differences in trophic ecology and condition of southern stingrays, Hypanus americanus, in Bimini, The Bahamas. Stingrays exhibited a dietary preference for molluscs and annelids, with an ontogenetic shift towards lower {delta}13C with increasing body size indicating a shift towards more mangrove associated prey. Nitrogen isotope values showed minimal seasonal changes, but higher {delta}15N values in males indicated foraging at a higher trophic level than females. Blood {beta}-hydroxybutyrate concentrations and osmolality revealed a similar energetic state and condition between sex, size, location and season. Our results advance our understanding of the seasonal trophic ecology of a benthic, marine mesopredator and identify the southern stingray as an important trophic link in seagrass and mangrove habitats in Bimini.

ecology↗

bioRxiv2025

Decoupling of stomatal conductance from net assimilation at high temperature as a mechanism to increase transpiration

O_LIPhotosynthetic assimilation (Anet) and stomatal conductance (gs) are usually strongly coupled, but this relationship is decreased or even lost at high temperatures (Tair). The contributions of environmental drivers (Tair, vapour pressure deficit (VPD), and soil moisture) in interaction with the physiological mechanisms behind this process are still unclear. C_LIO_LIWe exposed saplings of three temperate and tropical species to rising Tair (20 to 40{degrees}C) at low (1.2 to 1.9 kPa) and increasing VPD (1.1 to 5.6 kPa), and at stable Tair (35{degrees}C) to increasing VPD (1.4 to 4.3 kPa) under well-watered or chronic soil drought conditions ([≤]10 %). Anet, gs, and transpiration (E) in the light and the dark and leaf thermoregulation were tracked throughout the experiment. C_LIO_LIWhen VPD remained low, gs continued to increase while Anet decreased at Tair > 35{degrees}C, leading to stomatal decoupling. In contrast, under rising VPD, trees maintained the coupling between Anet and gs at high Tair. C_LIO_LIWhile a decoupling of Anet and gs only occurred when VPD was low, Anet and E decoupled under both VPD regimes at high Tair. C_LIO_LIOur results indicate that, since gs and VPD collectively drive E, stomatal decoupling is needed to increase E when VPD is not sufficiently high. C_LI

plant biology↗

bioRxiv2025

Batch culture and species effects modulate the decoupling between diatom frustule-bound and biomass nitrogen isotope signatures

The stable nitrogen (N) isotope composition of organic matter encapsulated in diatom silica frustules ({delta}15NDB) from sedimentary records has been used as a proxy for reconstructing N consumption dynamics in the ocean over geologic timescales. This proxy relies on the assumption that {delta}15NDB tracks biomass {delta}15N without being affected by internal N-isotope fractionation. However, recent ground-truthing efforts have shown that {delta}15NDB can diverge from biomass {delta}15N values, though the extent and mechanisms behind this decoupling remain unclear. In this study, we cultured two freshwater and two marine diatom species in batch cultures to test whether {delta}15NDB (1) is subject to species-dependent internal 15N fractionation, and (2) reflects the {delta}15N of source nitrate to the same extent as biomass {delta}15N values, assessing potential asynchronous integration of the N isotope signal. We monitored the N-isotope systematics during diatom growth by measuring {delta}15N values of nitrate, bulk biomass and frustule-bound organic N throughout batch culture progression. We found that {delta}15NDB did not follow typical Rayleigh fractionation dynamics and remained relatively stable, while biomass {delta}15N increased predictably with progressive fractional nitrate consumption. The observed divergence could only be partially explained by asynchronous integration of source-nitrate {delta}15N values into biomass versus frustule-bound organic N (i.e., delayed incorporation into frustule-bound material), as newly formed frustules predominantly recorded the {delta}15N of 15N-labeled nitrate added during growth. This demonstrates that {delta}15NDB values capture the isotopic signature of newly assimilated nitrate rather than N derived from internal, or legacy, pools. We hypothesize that shifts in growth conditions during batch culture progression alter the coupling between carbon and nitrogen metabolism, leading to physiologically driven variation in internal 15N fractionation and corresponding offsets between {delta}15NDB and biomass {delta}15N. Such sensitivity to internal isotope fractionation during biosynthesis implies that the interpretation of sedimentary {delta}15NDB records is more intricate than previously assumed.

plant biology↗

bioRxiv2025

Who is who in necromass formation and stabilization in soil? Unraveling the role of fungi and bacteria as complementary players of biogeochemical functioning

Multiple global change drivers have caused a large carbon (C) debt in our soils. To remedy this debt, understanding the role of microorganisms in soil C cycling is crucial to tackle the C soil loss. Microbial carbon use efficiency (CUE) is a parameter that captures the formation of microbially-derived soil organic matter (SOM). While it is known that biotic and abiotic drivers influence CUE, it remains unclear how distinct microbial communities and abiotic conditions influence the formation of microbially-derived SOC and its persistence in soils. Here, we combined the inoculation of distinct communities (a biotic factor) grown at different moisture levels (an abiotic factor) to manipulate the formation of microbial necromass in model soil. In a follow-up experiment, we then evaluated the persistence of this previously formed microbially-derived C to decomposition. While we show that necromass formation reflects the microbial community composition, the SOC formed within the most complex community of bacteria and fungi seems to be more resistant to decomposition compared to the SOC formed within the simpler communities (bacteria and fungi simple community, bacteria only and fungi only communities). Moreover, fungal necromass proved to be more thermally-stable than bacterial necromass, if this necromass is formed under the presence of bacteria. Our findings reveal that although abiotic factors can influence microbial physiology, the biological origin of microbially-derived C and the co-occurrence of fungal and bacterial growth were the stronger drivers explaining SOM persistence in these soils, suggesting the importance of microbial succession in SOC stabilization.

ecology↗

bioRxiv2024

Aerosol deposition affects water uptake and water loss of beech leaves

The deposition of aerosols on leaves could significantly influence plant-atmosphere-interaction through the formation of very thin aqueous films that allow the transport of liquid water through the stomata. Such films can be formed by deliquescence and dynamic expansion of hygroscopic aerosols ( hydraulic activation of stomata). Two processes that may be associated with stomatal liquid water transport are foliar water uptake (FWU) and the contribution of leaky stomata to minimum epidermal conductance (gmin). We investigated whether ambient aerosols affect FWU and gmin of Fagus sylvatica seedlings. Plants were grown in ventilated greenhouses with ambient air or filtered, almost aerosol-free air. The gmin was determined using leaf drying curves. FWU was investigated gravimetrically and with deuterium- enriched water, starting from different leaf water potentials, by spraying freshly-cut or pre-dried leaves (60 minutes). The presence of aerosols in the environment increased gmin by about 47%, confirming previous measurements in other species. Aerosols also increased FWU measured by deuterium uptake. FWU was higher for freshly-cut leaves than for pre-dried leaves, despite the lower leaf water potential. No gravimetric weight gain could be detected. Both the gmin and FWU results are consistent with bidirectional stomatal transport of liquid water along aerosol-induced pathways. The FWU result could also have been generated by water vapor through reverse transpiration, although the functional contribution of the aerosols would remain unclear. At low leaf water potential, the pathway may dry out and become less functional for FWU, whereas it may still be noticeable as stomatal leakage, given the strong gradient of water potential from the leaf interior to the atmosphere.

plant biology↗

bioRxiv2023

Soil fungi remain active and invest in storage compounds during drought independent of future climate conditions

Microbial growth is central to soil carbon cycling. However, how microbial communities grow under climate change is still largely unexplored. In an experiment simulating future climate conditions (increased atmospheric CO2 and temperature) and drought, we traced 2H or 18O applied via water-vapor exchange into fatty acids or DNA, respectively, allowing to measure community- and group-level adjustments in soil microbial physiology (replication, storage product synthesis, and carbon use efficiency, CUE). We show, that while overall community-level growth decreased by half during drought, fungal growth remained stable demonstrating an astonishing resistance of fungal activity against soil moisture changes. In addition, fungal investment into storage triglycerides increased more than five-fold under drought. CUE (the balance between anabolism and catabolism) was unaffected by drought but decreased in future climate conditions. Our results highlight that accounting for different growth strategies can foster our understanding of soil microbial contribution to C cycling and feedback to climate change.

microbiology↗

bioRxiv2023

Unsaturated intercellular vapor pressure is relevant for leaf water heavy isotope enrichment

Leaf intercellular vapor pressure (ei) can be unsaturated, but its effect on leaf water heavy isotope enrichment (LWE) has not yet been quantified. We evaluated the ecological relevance of unsaturated ei for LWE, i.e., for leaf water oxygen-18 and deuterium enrichment, using data from a boreal forest stand and a large-scale dataset. Unsaturated ei can firstly affect LWE by directly decreasing ei in the Craig Gordon model (Mechanism 1), which leads to an increased influence of atmospheric vapor isotopic enrichment above source water ({Delta}v), and a decreased influence of kinetic fractionation by diffusion through the stomata and boundary layer ({varepsilon}k). Unsaturated ei can secondly affect LWE by changing {varepsilon}k (Mechanism 2). To evaluate the effect of Mechanism 1 to LWE, we employed sensitivity tests on LWE model performance using varying measured intercellular relative humidity (RHcellular), or RHcellular fitted to observed LWE. To explore the effects of Mechanism 2 to LWE, we modified the calculation of {varepsilon}k and observed consequences to LWE predictions. Unsaturated ei is relevant to LWE by Mechanism 1, since a lowered RHcellular noticeably changed LWE predictions. It clearly improved deuterium predictions and conditionally improved oxygen-18 predictions. Isotope fractionation by Mechanism 2 is unlikely relevant to oxygen-18 and deuterium enrichment. Unsaturated ei must now be recognized as a variable that introduces error to heavy isotope enrichment models and reconstructions from organic material, via Mechanism 1. We suggest a correction for unsaturated ei for both oxygen-18 and deuterium enrichment using a variable RHcellular calculated from atmospheric relative humidity.

plant biology↗

bioRxiv2022

Microbial diversity ensures the stability of carbon cycling processes under increasing temperature in model soils

Biodiversity loss and climate change are two of the most alarming threats to ecosystem functioning and stability. These factors are, however, typically investigated separately, overlooking potential interactions between species extinctions and climate change on ecosystems. Here, we evaluated how different temperature regimes impact the relationship between microbial diversity and ecosystem functioning, in terms of temperature sensitivity of carbon (C) cycling functions. We hypothesized that more diverse soil communities promote the stability of C cycling functions under both temperature regimes. We did not observe a ubiquitous response of all C-cycling processes to temperature increases within the distinct community diversities. While growth was stable with increasing temperatures among the diversity levels, respiration rates increased more strongly at low diversity levels than high diversity levels at higher temperatures. Carbon use efficiency, which integrates both growth and respiration, tended to decrease with temperature at lower levels of diversity. Jointly, these results demonstrate the importance of belowground soil community diversity for maintaining C-cycling thermal response under changing climate.

microbiology↗

bioRxiv2020

Influence of spatial arrangement, biofertilizers and bioirrigation on the performance of legume-millet intercropping system in rainfed areas of southern India

Biofertilization via the inoculation with arbuscular mycorrhizal fungi (AMF), combined with rhizobia and plant growth promoting rhizobacteria (PGPR), are beginning to become established as an effective and sustainable measure to improve yields. Biofertilization might have a particular potential to boost the yield of intercropping systems in rainfed areas because AMF can form a common mycorrhizal network (CMN) that can transfer nutrients and water between two plants and balance as such belowground competition. In this study, we tested if biofertilizers can enhance the yield of intercropping systems using a pigeon pea (PP) - finger millet (FM) intercropping system grown for two consecutive growing seasons (2016/17 and 2017/18) at two contrasting sites in Bengaluru and Kolli Hills, India. To validate the process of bioirrigation (transfer of water between rhizosphere of two plants), we tested, for the first time, if the spatial arrangement of intercropped plants using either a row-wise or a mosaic design affected yield and water relations with and without biofertilizers. Our results demonstrate that intercropping can improve the straw and grain yield in PP-FM intercropping compared to the respective monocultures but that intercropping effects vary depending on the site characteristic such as climate and soil type. Spatial arrangement of component plants affected the total, straw and grain biomass in intercropping treatments, but this effect also varied across sites. Most importantly, the results from the 2017-18 growing season clearly demonstrated a positive effect of biofertilizer on biomass yield, and this effect was irrespective of site, spatial arrangement, mixed or monoculture. Despite a yield increase in intercropping, we did not see a positive effect of biofertilization on water relations of FM possibly due to interspecific competition for soil moisture where PP dominated. In summary, our study shows the potential of biofertilizers to increase the yield of intercropping systems in rainfed dryland agriculture.

ecology↗

bioRxiv2020

Soil nutrient availability alters tree carbon allocation dynamics during drought

Drought alters allocation patterns of carbon (C) and nutrients in trees and eventually impairs tree functioning. Elevated soil nutrient availability might alter the response of trees to drought. We hypothesize that increased soil nutrient availability stimulates root metabolism and carbon allocation to belowground tissues under drought stress. To test this hypothesis, we subjected three-year-old Pinus sylvestris saplings in open-top cambers during two subsequent years to drought using three different water treatments (100%, 20% and 0% plant available water in the soil) and two soil nutrient regimes (ambient and nitrogen-phosphorus-potassium (N-P-K) fertilization corresponding to 5 g N/m2/yr) and released drought thereafter. We conducted a 15N and 13C labelling experiment during the peak of the first-year drought by injecting 15N labelled fertilizer in the soil and exposing the tree canopies to 13C labelled CO2. The abundance of the N and C isotopes in the roots, stem and needles was assessed during the following year. C uptake was slightly lower in drought stressed trees, and extreme drought inhibited largely the N uptake and transport. Carbon allocation to belowground tissues was decreased under drought, but not in combination with fertilization. Our results indicate a potential positive feedback loop, where fertilization improved the metabolism and functioning of the roots, stimulating the source activity and hence C allocation to belowground tissues. This way, soil nutrients compensated for drought-induced loss of root functioning, mitigating drought stress of trees.

plant biology↗