Past, present, and future geo-biosphere interactions on the Tibetan Plateau and implications for permafrost

This manuscript resulted from a Workshop in 2019 at the Senckenberg Research Institute and Natural History Museum Frankfurt, Germany, supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA20100300). J. Liu also thanks the support of the Henan Provincial Key Laboratory of Hydrosphere and Watershed Water Security. T. Ehlers thanks the California Institute of Technology Moore Distinguished Scholar Program for support in completing this manuscript during a sabbatical. J. Liu and T. Bolch thank the support from the Strategic Priority Research Program of the Chinese Academy of Sciences (grants no. XDA20060402, XDA20100300). We thank the German Science Foundation (DFG) for support of the TiP (Tibetan Plateau: Formation-Climate-Ecoystems) priority research program (SPP-1372) for initiating the collaborations that led to this manuscript.Interactions between the atmosphere, biosphere, cryosphere, hydrosphere, and geosphere are most active in the critical zone, a region extending from the tops of trees to the top of unweathered bedrock. Changes in one or more of these spheres can result in a cascade of changes throughout the system in ways that are often poorly understood. Here we investigate how past and present climate change have impacted permafrost, hydrology, and ecosystems on the Tibetan Plateau. We do this by compiling existing climate, hydrologic, cryosphere, biosphere, and geologic studies documenting change over decadal to glacial-interglacial timescales and longer. Our emphasis is on showing present-day trends in environmental change and how plateau ecosystems have largely flourished under warmer and wetter periods in the geologic past. We identify two future pathways that could lead to either a favorable greening or unfavorable degradation and desiccation of plateau ecosystems. Both paths are plausible given the available evidence. We contend that the key to which pathway future generations experience lies in what, if any, human intervention measures are implemented. We conclude with suggested management strategies that can be implemented to facilitate a future greening of the Tibetan Plateau.Publisher PDFPeer reviewe

Positive intercropping effects on biomass production are species-specific and involve rhizosphere enzyme activities: Evidence from a field study

Less attention has been given to soil enzymes that contribute to beneficial rhizosphere interactions in intercropping systems. Therefore, we performed a field experiment by growing faba bean, lupine, and maize in mono and mixed cultures in a moderately fertile soil. We measured shoot biomass and the kinetic parameters (maximal velocity (Vmax) and Michaelis-constant (Km)) of three key enzymes in the rhizosphere: Leucine-aminopeptidase (LAP), β-1,4-N-acetylglucosaminidase (NAG), and phosphomonoesterase (PHO). Faba bean benefitted in mixed cultures by greater shoot biomass production with both maize and lupine compared to its expected biomass in monoculture. Next, LAP and NAG kinetic parameters were less responsive to mono and mixed cultures across the crop species. In contrast, both the Vmax and Km values of PHO increased in the faba bean rhizosphere when grown in mixed cultures with maize and lupine. A positive relative interaction index for shoot P and N uptake for faba bean showed its net facilitative interactions in the mixed cultures. Overall, these results suggest that over-productivity in intercropping is crop-specific and the positive intercropping effects could be modulated by P availability. We argue that the enzyme activities involved in nutrient cycling should be incorporated in further research. [Figure not available: see fulltext.] © 2021, The Author(s)

Oxygen matters: Short- and medium-term effects of aeration on hydrolytic enzymes in a paddy soil

Rapid exposure of anoxic microbial communities to oxygen (O2) can have unpredictable effects, including strong suppression of their enzymatic activity. Nonetheless, most medium- and long-term incubation studies on soil organic matter transformations fail to consider aeration effects during sample post-processing and/or assays. Moreover, it remains unclear whether anoxic enzymatic systems are adapted to quick switch to oxic conditions. We evaluated the effects of short-term (2-h oxic (+O2) vs. anoxic (–O2) assays) and medium-term aeration (after 10-day oxic vs. anoxic pre-incubation) on the kinetic parameters (Vmax, Km) of phosphomonoesterase, β-glucosidase, and leucine aminopeptidase in top bulk, rooted, and bottom bulk paddy soil of flooded rice mesocosms. We hypothesized contrasting short- and medium-term responses of hydrolytic enzyme activities to aeration (i) a negative short-term effect caused by reactive O2 species toxicity and/or other mechanisms, and (ii) adaptation of anoxic microbial communities to medium-term aeration reducing the impact of ongoing O2 exposure. Overall, 2-h aeration suppressed Vmax values by 7–43% and catalytic efficiency Ka (Vmax/Km) by 3–22%, and extended the substrate turnover time Tt (7–33%) of three tested enzymes in all soil compartments pre-incubated without O2. In contrast, no short-term suppressive effect of O2 was observed on three tested enzymes after oxic pre-incubation. Medium-term aeration increased Vmax (by 12–253%) and Ka (by 3–78%) of the enzymes and shortened Tt (4–42%) as compared to the anoxic counterpart. These findings support our hypothesis about anoxic microbial community adaptation over the medium-term aeration. Accordingly, the sensitivity of anoxic hydrolytic enzymes to a short-term O2 exposure and the O2 adaptation mechanisms require strong consideration (i) for enzyme assays of anoxic soils and (ii) for understanding the soil organic matter dynamics in environments with O2 fluctuations. © 2021 Elsevier B.V

Root-o-Mat: A novel tool for 2D image processing of root-soil interactions and its application in soil zymography

We developed a software tool enabling user-friendly and standardized pre- and post-processing of images of rooted soil by combining image processing techniques such as image registration, calibration, and segmentation in a graphical user interface. The added benefits of this image processing approach include an improved workflow in soil zymography. For evaluation, we conducted a rhizobox experiment with maize and determined the activity of leucine-aminopeptidase before and after glucose addition based on soil zymography. The temporal and spatial distribution of enzyme activity at the root-soil interface can be visualized by Root-o-Mat which offers 1) standardized image pre-processing, 2) calibration, 3) identification of hotspots of various intensity thresholds, 4) spatial analysis for selected roots, 5) inter-active illustration of enzyme activity profile lines, 6) image viewer, and 7) detection of temporal changes of enzyme activity. Registering images of the same rhizobox taken in successive periods allows further temporal and spatial analysis. We conclude that Root-o-Mat simplifies and firmly anchors image processing and image analyses in soil zymography. The new software can be downloaded for free (www.root-o-mat.de)

Soil microorganisms exhibit enzymatic and priming response to root mucilage under drought

Although root mucilage plays a prominent role in soil-plant-water relations, especially under drought, its persistence in soil and its microbial decomposition remain unknown. The aim of this study was to investigate: 1) the effect of soil moisture on mucilage decomposition, 2) the effect of mucilage on enzyme activities, and 3) the effect of mucilage on soil organic matter (SOM) decomposition. We hypothesized that mucilage benefits soil microorganisms by compensating for the detrimental effects of drought. Consequently, low water content was expected to reduce SOM mineralization and enzyme activities only in soil without mucilage. High moisture was predicted to support high microbial activities and therefore rapid decomposition of the mucilage. Two doses of maize root mucilage (40 and 200 μg C g−1 soil; C4 plant derived) were added to a C3 soil at optimum moisture (80% WHC) and under drought (30% WHC) to test these hypotheses. Under optimum moisture conditions, CO2 efflux from soil increased in proportion to mucilage addition. In contrast, there was no effect of mucilage on CO2 efflux under drought. At 80% WHC, mucilage was nearly completely decomposed (98% and 88% for low and high dose, respectively) after 15 days. Drought significantly suppressed mucilage mineralization. Microbial uptake of mucilage C was independent of soil moisture, suggesting that its bioavailability is regulated not by the water content of the whole soil, but by the water within the swollen mucilage. The high mucilage dose increased microbial biomass at both moisture levels compared to the soil without mucilage. Positive priming of soil organic matter decomposition was induced by mucilage at 80% WHC, whereas at 30% WHC, mucilage caused slightly negative priming. Mucilage addition counteracted the decrease of enzyme activities at 30% WHC, and so, stabilized the catalytic activity irrespective of soil moisture content. We conclude that mucilage provides biofilm-like properties that maintain microbial and exoenzymatic activities, even under drought. The slow decomposition of mucilage in drying soils suggests that mucilage maintains moist conditions around the roots for a long period, supporting beneficial root-microbial interactions at low water availability. This would result in a positive ecological feedback for microbial life in the rhizosphere and enhance nutrient release for roots under water scarcity. © 201

Organic Nutrients Induced Coupled C- and P-Cycling Enzyme Activities During Microbial Growth in Forest Soils

Besides environmental and soil physical drivers, the functional properties of microbial populations, i. e., growth rate, enzyme production, and maintenance requirements are dependent on the microbes' environment. The soil nutrition status and the quantity and quality of the substrate input, both infer different growth strategies of microorganisms. It is uncertain, how enzyme systems respond during the different phases of microbial growth and retardation in soil. The objective of this study was to uncover the changes of microbial functioning and their related enzyme systems in nutrient-poor and nutrient-rich beech forest soil during the phases of microbial growth. We determined microbial growth via kinetic approach by substrate-induced respiratory response of microorganisms, enabling the estimation of total, and growing biomass of the microbial community. To induce microbial growth we used glucose, while yeast extract simulated additional input of nutrients and factors indicating microbial residues (i.e., necromass compounds). Microbial growth on glucose showed a 12-18 h delay in associated enzyme activity increase or the absence of distinct activity responses (V-max). beta-glucosidase and chitinase (NAG) demonstrated clear differences of V(max)in time and between P-rich and P-poor soils. However, during microbial growth on glucose + yeast extract, the exponential increase in enzymatic activity was clearly stimulated accompanied by a delay of 8-12 h, smoothing the differences in nutrient-acquisition dynamics between the two soils. Furthermore, cross-correlation of beta-glucosidase and acid phosphatase between the two sites demonstrated harmonized time constraints, which reflected the establishment of comparable and balanced enzymatic systems within the decomposition network. The network accelerated nutrient acquisition to maintain microbial growth, irrespective of the contrasting soil properties and initial nutrient stocks, indicating similar tradeoffs of C- and P- cycling enzymes in both soils. This reflects comparable temporal dynamics of activities in nutrient-poor and nutrient-rich soil when the glucose + yeast extract was added. During lag phase and phase of exponential microbial growth, substrate turnover time of all enzymes was shortened in nutrient-poor forest soil exclusively, reflecting that the quality of the added substrate strongly matters during all stages of microbial growth in soil

The tree species matters: Belowground carbon input and utilization in the myco-rhizosphere

Rhizodeposits act as major carbon (C) source for microbial communities and rhizosphere-driven effects on forest C cycling receive increasing attention for maintaining soil biodiversity and ecosystem functions. By in situ 13CO2 pulse labeling we investigated C input and microbial utilization of rhizodeposits by analyzing 13C incorporation into phospholipid fatty acids (PLFA) of beech- (Fagus sylvatica) and ash-associated (Fraxinus excelsior) rhizomicrobial communities. Plant compartments and soil samples were analyzed to quantify the allocation of assimilates. For 1 m high trees, ash assimilated more of the applied 13CO2 (31%) than beech (21%), and ash allocated twice as much 13C belowground until day 20. Approximately 0.01% of the applied 13C was incorporated into total PLFAs, but incorporation varied significantly between microbial groups. Saprotrophic and ectomycorrhizal fungi under beech and ash, but also arbuscular mycorrhizal fungi and Gram negative bacteria under ash, incorporated most 13C. PLFA allowed differentiation of C fluxes from tree roots into mycorrhiza: twice as much 13C was incorporated into the fungal biomarker 18:2ω6,9 under beech than under ash. Within 5 days, 30% of the fungal PLFA-C was replaced by rhizodeposit-derived 13C under beech but only 10% under ash. None of the other microbial groups reached such high C replacement, suggesting direct C allocation via ectomycorrhizal symbioses dominates the C flux under beech. Based on 13CO2 labeling and 13C tracing in PLFA we conclude that ash allocated more C belowground and has faster microbial biomass turnover in the rhizosphere compared to beech. © 2017 Elsevier Masson SA

Root-o-Mat: A novel tool for 2D image processing of root-soil interactions and its application in soil zymography

We developed a software tool enabling user-friendly and standardized pre- and post-processing of images of rooted soil by combining image processing techniques such as image registration, calibration, and segmentation in a graphical user interface. The added benefits of this image processing approach include an improved workflow in soil zymography. For evaluation, we conducted a rhizobox experiment with maize and determined the activity of leucine-aminopeptidase before and after glucose addition based on soil zymography. The temporal and spatial distribution of enzyme activity at the root-soil interface can be visualized by Root-o-Mat which offers 1) standardized image pre-processing, 2) calibration, 3) identification of hotspots of various intensity thresholds, 4) spatial analysis for selected roots, 5) inter-active illustration of enzyme activity profile lines, 6) image viewer, and 7) detection of temporal changes of enzyme activity. Registering images of the same rhizobox taken in successive periods allows further temporal and spatial analysis. We conclude that Root-o-Mat simplifies and firmly anchors image processing and image analyses in soil zymography. The new software can be downloaded for free (www.root-o-mat.de). © 2021 Elsevier Lt

Contribution of the Fenton reaction and ligninolytic enzymes to soil organic matter mineralisation under anoxic conditions

Mechanisms of carbon dioxide (CO2) release from soil in the absence of oxygen were studied considering the Fenton process, which encompasses the reaction of H2O2 with Fe(II) yielding a hydroxyl radical ([rad]OH), in combination with manganese peroxidase (MnP) and lignin peroxidase (LiP). This study aimed to explain the high rate of soil organic matter (SOM) mineralisation and CO2 release from humid temperate rainforest soils under oxygen-limited conditions. The investigated mechanisms challenge the traditional view that SOM mineralisation in rainforest is slow due to anaerobic (micro)environments under high precipitation and explain intensive CO2 release even under oxygen limitation. We hypothesised that the Fenton reaction (FR) greatly contributes to the CO2 released from SOM mineralised under anaerobic conditions especially in the presence of ligninolytic enzymes. We used a novel technique that combines labelled H218O2 and Fe(II) to induce the FR and measured CO18O, Fe(II) solubilisation, and peroxide consumption in a closed gas circulation system for 6 h. Maximal CO2 amount was released when the FR was induced in combination with LiP addition. The CO2 efflux with LiP was 10-fold that of abiotic FR reactions without enzymes, or in soils amended with MnP. This was consistent with i) the contribution of 18O from peroxide to CO2 release, ii) peroxide consumption, and iii) Fe(II) solubilisation by FR. The amount of consumed peroxide was closely correlated with the CO18O derived from soil without enzyme addition or with LiP addition. Concluding, abiotic Fenton Reaction coupled with oxidative enzymes, such as LiP, are crucial for SOM oxidation under anaerobic conditions, e.g. in temperate rainforest soils