Root Mortality Research Articles

Increased soil respiration in response to experimentally reduced snow cover and increased soil freezing in a temperate deciduous forest

Winter snowpack in seasonally snow-covered regions plays an important role in moderating ecosystem processes by insulating soil from freezing air temperatures. However, climate models project a decline in snowpack at mid and high latitudes over the next century. We conducted a snow removal experiment in a temperate deciduous forest at Harvard Forest in Massachusetts, USA to quantify the effects of a reduced winter snowpack and increased soil freezing on total soil respiration and its bulk (i.e. heterotrophic) and root-rhizosphere components. Snow removal increased soil freezing severity by more than three-fold, which resulted in a 27.6% increase in annual total soil respiration (p = 0.058). Across our plots and years of this study, we found that the severity, rather than simply the presence of soil freezing, was the primary driver of the soil respiration response to reduced winter snowpack. Bulk soil respiration made the largest contribution to total soil respiration with root-rhizosphere respiration contributing up to 26.1 ± 6.5% of total soil respiration across plot types and years. Snow removal significantly increased fine root mortality (p = 0.03), which was positively correlated with soil frost depth and duration (p = 0.068, $${\text{R}}_{{{\text{LMM}}(m)}}^{ 2}$$ = 0.46), rates of total soil respiration (p = 0.075; $${\text{R}}_{{{\text{LMM}}(m)}}^{ 2}$$ = 0.27) and the contribution of root-rhizosphere respiration to total soil respiration (p = 0.004; $${\text{R}}_{{{\text{LMM}}(m)}}^{ 2}$$ = 0.58). We conclude that increased rates of soil respiration in response to soil freezing are driven by plant-mediated processes, whereby soil frost-induced root mortality stimulates respiration through decomposition of root necromass with additional enhancements possibly related to priming of soil organic matter decomposition and elevated rates of root respiration associated with growth.

Drought-induced changes in root biomass largely result from altered root morphological traits: Evidence from a synthesis of global field trials.

Extreme drought is likely to become more frequent and intense as a result of global climate change, which may significantly impact plant root traits and responses (i.e., morphology, production, turnover, and biomass). However, a comprehensive understanding of how drought affects root traits and responses remains elusive. Here, we synthesized data from 128 published studies under field conditions to examine the responses of 17 variables associated with root traits to drought. Our results showed that drought significantly decreased root length and root length density by 38.29% and 11.12%, respectively, but increased root diameter by 3.49%. However, drought significantly increased root:shoot mass ratio and root cortical aerenchyma by 13.54% and 90.7%, respectively. Our results suggest that drought significantly modified root morphological traits and increased root mortality, and the drought-induced decrease in root biomass was less than shoot biomass, causing higher root:shoot mass ratio. The cascading effects of drought on root traits and responses may need to be incorporated into terrestrial biosphere models to improve prediction of the climate-biosphere feedback.

Evaluating changes of biomass in global vegetation models: the role of turnover fluctuations and ENSO events

This paper evaluates the ability of eight global vegetation models to reproduce recent trends and inter-annual variability of biomass in natural terrestrial ecosystems. For the purpose of this evaluation, the simulated trajectories of biomass are expressed in terms of the relative rate of change in biomass (RRB), defined as the deviation of the actual rate of biomass turnover from its equilibrium counterpart. Cumulative changes in RRB explain long-term changes in biomass pools. RRB simulated by the global vegetation models is compared with its observational equivalent, derived from vegetation optical depth reconstructions of above-ground biomass (AGB) over the period 1993–2010. According to the RRB analysis, the rate of global biomass growth described by the ensemble of simulations substantially exceeds the observation. The observed fluctuations of global RRB are significantly correlated with El Niño Southern Oscillation events (ENSO), but only some of the simulations reproduce this correlation. However, the ENSO sensitivity of RRB in the tropics is not significant in the observation, while it is in some of the simulations. This mismatch points to an important limitation of the observed AGB reconstruction to capture biomass variations in tropical forests. Important discrepancies in RRB were also identified at the regional scale, in the tropical forests of Amazonia and Central Africa, as well as in the boreal forests of north-western America, western and central Siberia. In each of these regions, the RRBs derived from the simulations were analyzed in connection with underlying differences in net primary productivity and biomass turnover rate ̶as a basis for exploring in how far differences in simulated changes in biomass are attributed to the response of the carbon uptake to CO2 increments, as well as to the model representation of factors affecting the rates of mortality and turnover of foliage and roots. Overall, our findings stress the usefulness of using RRB to evaluate complex vegetation models and highlight the importance of conducting further evaluations of both the actual rate of biomass turnover and its equilibrium counterpart, with special focus on their background values and sources of variation. In turn, this task would require the availability of more accurate multi-year observational data of biomass and net primary productivity for natural ecosystems, as well as detailed and updated information on land-cover classification.

A belowground perspective of temperate old-growth forests: Fine root system structure in beech primeval and production forests

Old-growth forests differ from managed forests by a generally higher biodiversity, larger carbon stores, and greater heterogeneity of aboveground structures. It is not known whether the aboveground structural diversity of old-growth forests is mirrored in root system structure, e.g. by greater root biomass, the occurrence of root gaps, and a different fine root morphology. We studied the fine root system of beech (roots <2 mm in diameter) in three development stages (initial, optimum, terminal) of a primeval beech (Fagus sylvatica) forest in the Slovakian Carpathians and compared it to a nearby beech production forest close to harvest. We conducted root coring to 30 cm depth in 16 plots (84 sampling locations) with subsequent detailed analysis of fine root bio- and necromass, and fine root morphology. Despite lower aboveground wood mass, the production forest had on average about 15% larger beech fine root biomass and necromass totals than the primeval forest, though the differences were not significant. Contrary to expectation, the spatial variation of fine root biomass in the 0–30 cm layer in the structurally diverse primeval forest was not higher than in the more homogeneous production forest. Also, fine root morphology did not differ between primeval and production forest.Across the three primeval forest stages, fine root biomass tended to peak in the optimum stage, but the difference was not significant, and no alteration in fine root morphology across the stages was detected. Fine root necromass increased significantly from the initial to the terminal stage, pointing at higher fine root mortality and/or reduced root decomposition in forest patches with many senescent trees and canopy gaps. Yet, soil fertility did not decrease toward the terminal stage of forest development. From the fine root biomass data, no root gaps could be detected in the terminal stage, perhaps due to rapid gap colonization by beech saplings. We conclude that the structural differences between the fine root systems of beech primeval and production forest were relatively small. Canopy heterogeneity seems to be a less important factor determining root distribution in the primeval forest than soil heterogeneity, which can be high in production forests as well.

Frontiers in root ecology: recent advances and future challenges

Background and aims: Plant phenology is a sensitive indicator of plant response to climate change. Observations of phenological events belowground for most ecosystems are difficult to obtain and very little is known about the relationship between tree shoot and root phenology. We examined the influence of environmental factors on fine root production and mortality in relation with shoot phenology in hybrid walnut trees (Juglans sp.) growing in three different climates (oceanic, continental and Mediterranean) along a latitudinal gradient in France. Methods : Eight rhizotrons were installed at each site for 21 months to monitor tree root dynamics. Root elongation rate (RER), root initiation quantity (RIQ) and root mortality quantity (RMQ) were recorded frequently using a scanner and time-lapse camera. Leaf phenology and stem radial growth were also measured. Fine roots were classified by topological order and 0–1 mm, 1–2 mm and 2–5 mm diameter classes and fine root longevity and risk of mortality were calculated during different periods over the year. Results : Root growth was not synchronous with leaf phenology in any climate or either year, but was synchronous with stem growth during the late growing season. A distinct bimodal pattern of root growth was observed during the aerial growing season. Mean RER was driven by soil temperature measured in the month preceding root growth in the oceanic climate site only. However, mean RER was significantly correlated with mean soil water potential measured in the month preceding root growth at both Mediterranean (positive relationship) and oceanic (negative relationship) sites. Mean RIQ was significantly higher at both continental and Mediterranean sites compared to the oceanic site. Soil temperature was a driver of mean RIQ during the late growing season at continental and Mediterranean sites only. Mean RMQ increased significantly with decreasing soil water potential during the late aerial growing season at the continental site only. Mean root longevity at the continental site was significantly greater than for roots at the oceanic and Mediterranean sites. Roots in the 0–1 mm and 1–2 mm diameter classes lived for significantly shorter periods compared to those in the 2–5 mm diameter class. First order roots (i.e. the primary or parents roots) lived longer than lateral branch roots at the Mediterranean site only and first order roots in the 0–1 mm diameter class had 44.5% less risk of mortality than that of lateral roots for the same class of diameter. Conclusions: We conclude that factors driving root RER were not the same between climates. Soil temperature was the best predictor of root initiation at continental and Mediterranean sites only, but drivers of root mortality remained largely undetermined

Effect of a Bacterial Grass Culture on the Plant Growth and Disease Control in Tomato

This study aimed to investigate the plant growth-promoting and biocontrol potential of a grass culture with Paenibacillus ehimensis KWN8 on tomato. For this experiment, treatments of a chemical fertilizer (F), a bacterial grass culture (G), a 1/3 volume of G plus 2/3 F (GF), and F plus a synthetic fungicide (FSf) were applied to tomato leaves and roots. The result showed that the severity of Alternariasolani and Botrytiscinerea symptoms were significantly reduced after the application of the bacterial grass culture (G and GF) and FSf. In addition, root mortality in G and GF was lower compared to F. Tomato plants treated with G or GF had better vegetative growth and yield compared to F. Application of G affected the fungal and bacterial populations in the soil. In conclusion, treatment with a bacterial grass culture decreased disease severity and increased tomato growth parameters. However, there were no statistically significant correlations between disease occurrence and tomato yields. This experiment presents the possibility to manage diseases of tomato in an environmentally friendly manner and to also increase the yield of tomato by using a grass culture broth containing P. ehimensis KWN38.

Effects of Warming and Precipitation Manipulation on Fine Root Dynamics of Pinus densiflora Sieb. et Zucc. Seedlings

Air warming (TC: control; TW: +3 °C) and precipitation manipulation (PC: control; PD: −30%; PI: +30%) were established to examine effects of these treatments on fine root production (FRP), fine root mortality (FRM), and total root (coarse and fine root) biomass in 33- to 59-month-old Pinus densiflora Sieb. et Zucc. seedlings for two years. We hypothesized that warming and altered precipitation would affect the growth, death, and biomass of fine roots by changing soil temperature and soil water availability. Mean annual FRP and total root biomass were significantly altered by only precipitation manipulation: they were 29.3% (during the two-year period) and 69.0% (after the entire two years) higher, respectively, in PD plots than in PC plots, respectively. In contrast, only warming had a significant effect on mean annual FRM, being 13.2% lower in TW plots than TC plots during the two-year period. Meanwhile, fine root biomass was affected negatively and simultaneously by both soil temperature and soil moisture. It seemed that fine root dynamics have changed so that they maintain their systems in response to the altered soil temperature and moisture. The current study adds significant knowledge for understanding the fine root dynamics of P. densiflora seedlings under altered temperature and precipitation regimes.

Linking above- and belowground phenology of hybrid walnut growing along a climatic gradient in temperate agroforestry systems

Plant phenology is a sensitive indicator of plant response to climate change. Observations of phenological events belowground for most ecosystems are difficult to obtain and very little is known about the relationship between tree shoot and root phenology. We examined the influence of environmental factors on fine root production and mortality in relation with shoot phenology in hybrid walnut trees (Juglans sp.) growing in three different climates (oceanic, continental and Mediterranean) along a latitudinal gradient in France. Eight rhizotrons were installed at each site for 21 months to monitor tree root dynamics. Root elongation rate (RER), root initiation quantity (RIQ) and root mortality quantity (RMQ) were recorded frequently using a scanner and time-lapse camera. Leaf phenology and stem radial growth were also measured. Fine roots were classified by topological order and 0–1 mm, 1–2 mm and 2–5 mm diameter classes and fine root longevity and risk of mortality were calculated during different periods over the year. Root growth was not synchronous with leaf phenology in any climate or either year, but was synchronous with stem growth during the late growing season. A distinct bimodal pattern of root growth was observed during the aerial growing season. Mean RER was driven by soil temperature measured in the month preceding root growth in the oceanic climate site only. However, mean RER was significantly correlated with mean soil water potential measured in the month preceding root growth at both Mediterranean (positive relationship) and oceanic (negative relationship) sites. Mean RIQ was significantly higher at both continental and Mediterranean sites compared to the oceanic site. Soil temperature was a driver of mean RIQ during the late growing season at continental and Mediterranean sites only. Mean RMQ increased significantly with decreasing soil water potential during the late aerial growing season at the continental site only. Mean root longevity at the continental site was significantly greater than for roots at the oceanic and Mediterranean sites. Roots in the 0–1 mm and 1–2 mm diameter classes lived for significantly shorter periods compared to those in the 2–5 mm diameter class. First order roots (i.e. the primary or parents roots) lived longer than lateral branch roots at the Mediterranean site only and first order roots in the 0–1 mm diameter class had 44.5% less risk of mortality than that of lateral roots for the same class of diameter. We conclude that factors driving root RER were not the same between climates. Soil temperature was the best predictor of root initiation at continental and Mediterranean sites only, but drivers of root mortality remained largely undetermined.

Integrating belowground carbon dynamics into Yield-SAFE, a parameter sparse agroforestry model

Agroforestry combines perennial woody elements (e.g. trees) with an agricultural understory (e.g. wheat, pasture) which can also potentially be used by a livestock component. In recent decades, modern agroforestry systems have been proposed at European level as land use alternatives for conventional agricultural systems. The potential range of benefits that modern agroforestry systems can provide includes farm product diversification (food and timber), soil and biodiversity conservation and carbon sequestration, both in woody biomass and the soil. Whilst typically these include benefits such as food and timber provision, potentially, there are benefits in the form of carbon sequestration, both in woody biomass and in the soil. Quantifying the effect of agroforestry systems on soil carbon is important because it is one means by which atmospheric carbon can be sequestered in order to reduce global warming. However, experimental systems that can combine the different alternative features of agroforestry systems are difficult to implement and long-term. For this reason, models are needed to explore these alternatives, in order to determine what benefits different combinations of trees and understory might provide in agroforestry systems. This paper describes the integration of the widely used soil carbon model RothC, a model simulating soil organic carbon turnover, into Yield-SAFE, a parameter sparse model to estimate aboveground biomass in agroforestry systems. The improvement of the Yield-SAFE model focused on the estimation of input plant material into soil (i.e. leaf fall and root mortality) while maintaining the original aspiration for a simple conceptualization of agroforestry modeling, but allowing to feed inputs to a soil carbon module based on RothC. Validation simulations show that the combined model gives predictions consistent with observed data for both SOC dynamics and tree leaf fall. Two case study systems are examined: a cork oak system in South Portugal and a poplar system in the UK, in current and future climate.

Changes of Scots Pine Phyllosphere and Soil Fungal Communities during Outbreaks of Defoliating Insects

Outbreaks of forest pests increase with climate change, and thereby may affect microbial communities and ecosystem functioning. We investigated the structure of phyllosphere and soil microbial communities during defoliation by the nun moth (Lymantria monacha L.) (80% defoliation) and the pine tree lappet (Dendrolimus pini L.) (50% defoliation) in Scots pine forests (Pinus sylvestris L.) in Germany. Ribosomal RNA genes of fungi and bacteria were amplified by polymerase chain reaction (PCR), separated by denaturing gradient gel electrophoresis (DGGE), and subsequently sequenced for taxonomic assignments. Defoliation by both pests changed the structure of the dominant fungal (but not bacterial) taxa of the phyllosphere and the soil. The highly abundant ectomycorrhizal fungal taxon (Russula sp.) in soils declined, which may be attributed to insufficient carbohydrate supply by the host trees and increased root mortality. In contrast, potentially pathogenic fungal taxa in the phyllosphere increased during pest outbreaks. Our results suggest that defoliation of pines by insect pest, change the structure of fungal communities, and thereby indirectly may be contributing to aggravation of tree health.

Dual Fumigant and Herbicide Use Optimizes Replanting Preparation in a Virus- and Nematode-Affected Vineyard

Summary Goal: An exploratory study evaluated best-management techniques for replanting a commercial vineyard suffering from both a high incidence of Grapevine leafroll associated virus-3 and high densities of northern root-knot nematode ( Meloidogyne hapla ). The grower had two goals: to reduce the density of plant-parasitic nematodes, which can delay establishment of young vines, using a pre-plant fumigant, and to reduce survival of virus-infected vine tissue by applying an herbicide to the foliage prior to vine removal. Key Findings: Fall-applied metam sodium reduced densities of M. hapla in the following growing season. Its herbicidal properties also resulted in high root mortality. In cases where vines survived, only a few shoots (and suckers) emerged, displaying typical morphology. Fall-applied foliar glyphosate significantly reduced shoot and sucker regrowth, but did not kill root tissue. In the few instances where shoots and suckers grew the following spring, that tissue expressed typical symptoms of glyphosate phytotoxicity. While low rates of abnormal growth would eliminate much of the aerial reservoir of virus, the presence of live roots indicates that rogue vines and living root debris could contribute to infection of new, virus-free planting stock. Combining fumigant and foliar glyphosate was additive: nematode control, root mortality, and a lack of healthy shoot/crown sucker regrowth. Impact and Significance: This preliminary work showed that the combination of fall fumigation and foliar glyphosate application reduced nematode populations and vine survival. In situations where plant-parasitic nematodes are the sole issue facing replanting, fumigation alone suffices. Where virus alone is the impetus for replanting, fall-applied foliar glyphosate may provide sufficient overall vine mortality, with a caveat of low root mortality and its attendant risk of root-feeding vectors spreading the virus to new planting stock.

Soil freeze–thaw with root litter alters N transformations during the dormant season in soils under two temperate forests in northern Japan

Extreme soil freeze–thaw regimes caused by winter climate change can alter nitrogen (N) transformations through plant root mortality. In this study, we conducted a snow removal experiment to simulate extreme soil freeze–thaw to understand the effect of soil freeze–thaw on N transformations in soils under two northern temperate forests (oak and larch) in northern Japan. We also investigated the effect of root input on microbial biomass and transformations of dissolved organic carbon (DOC) and N by experimentally adding root litter to obtain mechanistic insight. Snow removal significantly reduced net nitrification and N mineralization, while it increased net ammonification in both soils. Root litter addition provided contrasting effects of vegetation type on net DOC change; it significantly decreased net DOC production in the oak soil, while it was increased in the larch soil. The contrasting responses of DOC change with vegetation were possibly related to specific microbial composition and physiology of the vegetation. Further, root addition eliminated the differences in net N mineralization and microbial biomass between the two vegetation types in the snow removal plots. Our results indicated that soil freeze–thaw could reduce the differences in vegetation type on soil N dynamics through root litter input.

Fine Root Abundance and Dynamics of Stone Pine (Pinus cembra) at the Alpine Treeline Is Not Impaired by Self-shading.

Low temperatures are crucial for the formation of the alpine treeline worldwide. Since soil temperature in the shade of tree canopies is lower than in open sites, it was assumed that self-shading may impair the trees’ root growth performance. While experiments with tree saplings demonstrate root growth impairment at soil temperatures below 5–7°C, field studies exploring the soil temperature – root growth relationship at the treeline are missing. We recorded soil temperature and fine root abundance and dynamics in shaded and sun-exposed areas under canopies of isolated Pinus cembra trees at the alpine treeline. In contrast to the mentioned assumption, we found more fine root biomass and higher fine root growth in colder than in warmer soil areas. Moreover, colder areas showed higher fine root turnover and thus lower root lifespan than warmer places. We conclude that P. cembra balances enhanced fine root mortality in cold soils with higher fine root activity and by maintaining higher fine root biomass, most likely as a response to shortage in soil resource supply. The results from our study highlight the importance of in situ measurements on mature trees to understand the fine root response and carbon allocation pattern to the thermal growth conditions at the alpine treeline.

The effect of simulated warming on root dynamics and soil microbial community in an alpine meadow of the Qinghai-Tibet Plateau

Previous studies have explored the effects of global change on above-ground vegetation in grassland ecosystems. Few, however, have investigated the responses of root dynamics and soil microbes to simulated warming in alpine meadows of the Qinghai-Tibet Plateau. This research used field open top chamber experiment to examine warming's impacts. Root production, mortality, and turnover were observed in situ using minirhizotron tubes, while soil microbial community composition and diversity were assessed by using phospholipid-derived fatty acids (PLFA) and Biolog-Eco plates. The results showed that 1) warming shifted the root standing crop distribution to deeper soil while increasing the accumulated root mortality and root production at 0–10cm soil depth; 2) warming increased the abundance of fungal PLFAs while decreasing the abundance of other functional groups; 3) warming decreased the soil microbial functional diversity at the different soil layers; 4) bacterial and fungal PLFA contents were positively correlated with root mortality and turnover but were negatively correlated with root standing crop; 5) root dynamics significantly affect carbon utilization of the soil microbial community; and warming induced changes in root dynamics are associated with soil microbial community structure and function. Overall, these findings indicate that warming alters root production, mortality, and turnover and the relative abundance of different microorganisms in different soil layers. As a result, warming tended to shift microbial communities from bacteria toward fungi, leading to changes in the microbial community structure in different soil layers.

Reduced snow cover alters root-microbe interactions and decreases nitrification rates in a northern hardwood forest.

Snow cover is projected to decline during the next century in many ecosystems that currently experience a seasonal snowpack. Because snow insulates soils from frigid winter air temperatures, soils are expected to become colder and experience more winter soil freeze-thaw cycles as snow cover continues to decline. Tree roots are adversely affected by snowpack reduction, but whether loss of snow will affect root-microbe interactions remains largely unknown. The objective of this study was to distinguish and attribute direct (e.g., winter snow- and/or soil frost-mediated) vs. indirect (e.g., root-mediated) effects of winter climate change on microbial biomass, the potential activity of microbial exoenzymes, and net N mineralization and nitrification rates. Soil cores were incubated in situ in nylon mesh that either allowed roots to grow into the soil core (2mm pore size) or excluded root ingrowth (50μm pore size) for up to 29months along a natural winter climate gradient at Hubbard Brook Experimental Forest, NH (USA). Microbial biomass did not differ among ingrowth or exclusion cores. Across sampling dates, the potential activities of cellobiohydrolase, phenol oxidase, and peroxidase, and net N mineralization rates were more strongly related to soil volumetric water content (P<0.05; R2 =0.25-0.46) than to root biomass, snow or soil frost, or winter soil temperature (R2 <0.10). Root ingrowth was positively related to soil frost (P<0.01; R2 =0.28), suggesting that trees compensate for overwinter root mortality caused by soil freezing by re-allocating resources towards root production. At the sites with the deepest snow cover, root ingrowth reduced nitrification rates by 30% (P<0.01), showing that tree roots exert significant influence over nitrification, which declines with reduced snow cover. If soil freezing intensifies over time, then greater compensatory root growth may reduce nitrification rates directly via plant-microbe N competition and indirectly through a negative feedback on soil moisture, resulting in lower N availability to trees in northern hardwood forests.

Injury and Recovery of Maize Roots Affected by Flooding

<abstract> Proper soil environment for adequate root growth is crucial to maintain crop yields. Excessively wet soil conditions cause poor root growth and restrict plant water and nutrient uptake. The purpose of this study was to investigate corn root response to flooding and the root system ability to recover after saturation. Corn plants were grown in 1.3 m tall acrylic cylinders under controlled conditions. Plants were subjected to flooding for 1, 3, or 5 days at growth stages V4, V8, V12, and R1. Data were collected throughout the growing season on: root growth, root depth, and root mass. Root mortality was noted after the flooding treatment. Roots responded quickly to saturation (flooding) and all growth ceased within 24 h. Recovery to normal growth rates occurred within five days unless the saturation caused severe plant damage. One day of flooding did not affect root mass at any stage of growth. Root mass was not affected by three days of flooding during the V4 and V8 stages. A significant reduction of root mass was noted for more than 35 days following 3-day inundation during V12 and R1. Flooding for five days during stage V4, V12, and R1 caused a significant reduction in root mass. Plants flooded during V8 suffered no significant reduction in root mass. Assessment of root response to saturated soil conditions is necessary to improve groundwater table management practices in high water table soils, as well as to enhance the performance of simulation model that predict plant growth and yield on drained lands.

Forest burning affects quality and quantity of soil organic matter

Fire alters ecosystem carbon cycling and generates pyrogenic matter such as charcoal, which can be incorporated into soils. The incorporation and cycling of charcoal in soils is a potential carbon sink, but studies investigating charcoal and carbon dynamics in soils are still lacking. We investigated soil carbon, charcoal and nitrogen dynamics in the top 20cm of a sandy soil within a eucalypt forest in eastern Australia at three sites representing a chronosequence from 3months to 14years post-fire. In the short-term, fire removed litter, but resulted in an increase in both the charcoal and non-charcoal SOC content of the soils, which we attribute to above-ground charcoal generation and its incorporation into the soil profile, as well as below-ground root mortality. On a decadal timeframe, charcoal was preferentially retained in the sandy soil, in which other stabilisation mechanisms are limited, so that the influx of dead root carbon had no remnant effects. The incorporation and retention of charcoal in the soil profile is highly important to carbon cycling in such sandy soils with high fire frequency. It is highly likely that these effects are not limited to the upper 20cm of soil and future studies should investigate deep soil charcoal cycling.

Mechanical Failure of Fine Root Cortical Cells Initiates Plant Hydraulic Decline during Drought.

Root systems perform the crucial task of absorbing water from the soil to meet the demands of a transpiring canopy. Roots are thought to operate like electrical fuses, which break when carrying an excessive load under conditions of drought stress. Yet the exact site and sequence of this dysfunction in roots remain elusive. Using in vivo x-ray computed microtomography, we found that drought-induced mechanical failure (i.e. lacunae formation) in fine root cortical cells is the initial and primary driver of reduced fine root hydraulic conductivity (Lpr) under mild to moderate drought stress. Cortical lacunae started forming under mild drought stress (-0.6 MPa Ψstem), coincided with a dramatic reduction in Lpr, and preceded root shrinkage or significant xylem embolism. Only under increased drought stress was embolism formation observed in the root xylem, and it appeared first in the fine roots (50% loss of hydraulic conductivity [P50] reached at -1.8 MPa) and then in older, coarse roots (P50 = -3.5 MPa). These results suggest that cortical cells in fine roots function like hydraulic fuses that decouple plants from drying soil, thus preserving the hydraulic integrity of the plant's vascular system under early stages of drought stress. Cortical lacunae formation led to permanent structural damage of the root cortex and nonrecoverable Lpr, pointing to a role in fine root mortality and turnover under drought stress.

Acid and calcareous soils affect nitrogen nutrition and organic nitrogen uptake by beech seedlings (Fagus sylvatica L.) under drought, and their ectomycorrhizal community structure

The role of different soil types for beech productivity and drought sensitivity is unknown. The aim of this experimental study was to compare mycorrhizal diversity between acid sandy and calcareous soils and to investigate how this diversity affects tree performance, nitrogen uptake and use efficiency (NUE). Beech trees were germinated and grown in five different soil types (pH 3.8 to 6.7). One-and-a-half-year-old plants were exposed for 6 weeks to sufficient or low soil humidity. Tree biomass, root tip mycorrhizal colonization and community structure, root tip mortality, leaf area, photosynthesis, nitrogen concentrations, NUE and short-term 15N uptake from glutamine were determined. Soil type did not affect photosynthesis or biomass formation, with one exception in calcareous soil, where root mortality was higher than in the other soil types. Beech in acid soils showed lower mycorrhizal colonization, higher nitrogen tissue concentrations, and lower NUE than those in calcareous soils. Drought had no effect on nitrogen concentrations or NUE but caused reductions in mycorrhizal colonization. Mycorrhizal species richness correlated with nitrogen uptake and NUE. Nitrogen uptake was more sensitive to drought in calcareous soils than in acid soils. Beech may be more drought-susceptible on calcareous sites because of stronger decrease of organic nitrogen uptake than on acid soils.

Estimating the production and mortality of fine roots using minirhizotrons in a Pinus densiflora forest in Gwangneung, Korea

The aim of this study was to estimate fine root production (FP) and fine root mortality (FM) at 0–10, 10–20, and 20–30 cm soil depths using minirhizotrons in a 75-year-old Pinus densiflora Sieb. et Zucc. forest located in Gwangneung, Korea. We developed the conversion factors (frame cm−2) of three soil depths (0.158 for 0–10 cm, 0.120 for 10–20 cm, and 0.131 for 20–30 cm) based on soil coring and minirhizotron data. FP and FM were estimated using conversion factors from March 26, 2013 to March 2, 2014. The annual FP and FM values at the 0–30 cm soil depth were 3200.2 and 2271.5 kg ha−1 yr −1, respectively. The FP estimate accounted for approximately 17 % of the total net primary production at the study site. FP was highest in summer (July 31–September 26), and FM was highest in autumn (September 27–November 29). FP was positively correlated with seasonal change in soil temperature, while FM was not related to that change. The seasonality of FP and FM might be linked to above-ground photosynthetic activity. Both FP and FM at the 0–10 cm depth were significantly higher than at 10–20 and 20–30 cm depths, and this resulted from the decrease in nutrient availability with increasing soil depth. The minirhizotron approach and conversion factors developed in this study will enable fast and accurate estimation of the fine root dynamics in P. densiflora forest ecosystems.