The Role of Plant Litter in Driving Plant-Soil Feedbacks

Most studies focusing on plant-soil feedbacks (PSFs) have considered direct interactions between plants, abiotic conditions (e. g., soil nutrients) and rhizosphere communities (e.g., pathogens, mutualists). However, few studies have addressed the role of indirect interactions mediated by plant litter inputs. This is problematic because it has left a major gap in our understanding of PSFs in natural ecosystems, where plant litter is a key component of feedback effects. Here, we propose a new conceptual framework that integrates rhizosphere- and litter-mediated PSF effects. Our framework provides insights into the relative contribution of direct effects mediated by interactions between plants and soil rhizosphere organisms, and indirect effects between plants and decomposer organisms mediated by plant root and shoot litter. We distinguish between three pathways through which senesced root and shoot litter may influence PSFs. Specifically, we examine: (1) physical effects of litter (layer) traits on seed germination, soil structure, and plant growth; (2) chemical effects of litter on concentrations of soil nutrients and secondary metabolites (e.g., allelopathic chemicals); and (3) biotic effects of saprotrophic soil communities that can perform different functional roles in the soil food web, or that may have specialized interactions with litter types, thereby altering soil nutrient cycling. We assess the role of litter in PSF effects via physical, chemical and biotic pathways to address how litter-mediated feedbacks may play out relative to, and in interaction with, feedbacks mediated through the plant rhizosphere. We also present one of the first experimental studies to show the occurrence and species-specificity of litter-mediated feedbacks and we identify critical research gaps. By formally incorporating the plant-litter feedback pathway into PSF experiments, we will further our understanding of PSFs under natural conditions

Essays in Applied Economics

Essay 1 studies physician agency problems, which arise whenever physicians fail to maximize their patients\u27 preferences, given available information. These agency problems are well documented, but the magnitude of their welfare consequences for patients---the losses from suboptimal treatment choice induced by agency---are unclear. I infer patient drug preference from their compliance decisions. I begin by showing that initial prescriptions respond to physician financial incentives to control costs and to pharmaceutical detailing, but compliance does not, pointing to agency problems. I then develop and estimate a model of physician-patient interactions where physician write initial prescriptions, but patients choose whether to comply. Fully eliminating agency problems increases compliance by 6.5 percentage points, and raises patient welfare by 22\% of drug spending. Contracts that better align doctor and patient preferences can improve patient welfare, but attain only half the gains from eliminating agency completely. Although physician agency problems reduce patient welfare, eliminating them is thus likely difficult. Essay 2, co-authored with Alexander M. Gelber and Damon Jones, studies frictions in adjusting earnings to changes in the Social Security Annual Earnings Test (AET) using a panel of Social Security Administration microdata on one percent of the U.S. population from 1961 to 2006. Individuals continue to bunch at the convex kink the AET creates even when they are no longer subject to the AET, consistent with the existence of earnings adjustment frictions in the U.S. We develop a novel estimation framework and estimate in a baseline case that the earnings elasticity with respect to the implicit net-of-tax share is 0.23, and the fixed cost of adjustment is \$152.08. Essay 3 studies the impact of health expenditure risk on annuitization. Theoretical research suggests that such risk can have an ambiguous influence on the annuitization decisions of the elderly. I provide empirical evidence on this linkage, by estimating the impact of supplemental Medicare insurance (Medigap) coverage on the annuity demand of older Americans. Medigap coverage has a strong impact on annuitization: the extensive margin elasticity is 0.39, the overall elasticity of private annuity income with respect to Medigap coverage is 0.56. These results are robust to controls for health, wealth, and preferences, as well as other robustness tests. They suggest that medical expenditure risk has a large impact on underannuitization

Steering the soil microbiome by repeated litter addition

1. Microbial communities drive plant litter breakdown. Litters originating from different plant species are often associated with specialized microbiomes that accelerate the breakdown of that litter, known as home-field advantage. Yet, how and how fast microbial communities specialize towards litter inputs is not known. 2. Here we study effects of repeated litter additions on soil microbial community structure and functioning. We set up a nine-month, full-factorial, reciprocal litter transplant experiment with soils and litters from six plant species (three grasses, three trees). We measured fungal and bacterial community composition, litter mass loss and home-field effects. 3. We found that repeated litter additions resulted in convergence in fungal community composition driven by litter functional group (trees versus grasses). Grasses enriched Sordariomycetes, while Tremellomycetes, Eurotiomycetes, and Leotiomycetes were favored by tree litter. Bacterial community composition, litter mass loss and home-field effects were not affected by litter incubation, but there was a relationship between fungal community composition and mass loss. 4. We conclude that repeated litter incubations can result in directional shifts in fungal community composition, while nine months of litter addition did not change bacterial community composition and the functioning and specialization of microbial communities. 5. Testing further how repeated litter inputs affect microbial functioning is essential for steering decomposer communities for optimal soil carbon and nutrient cycling.For this publication we have uploaded: 1. a file containing the data 2. a READ ME file supporting the data file Funding provided by: Dutch Research Council *Crossref Funder Registry ID: Award Number: 863.14.013Funding provided by: Dutch Research CouncilCrossref Funder Registry ID: Award Number: 863.14.013Field sites To test our hypotheses, we set up two controlled experiments. Soil and litter samples used in all experiments were collected from a long-term field site on the Veluwe, the Netherlands (Hannula et al., 2017; Kardol, Bezemer, & van der Putten, 2006; Veen, Keiser, van der Putten, & Wardle, 2018) situated between Ede (52°04′20″N, 5°44′12″) and Wolfheze (52°00′77″ N, 5°48′58″). We sampled soils from six independent locations within the Veluwe field site. All locations were situated on sandy soils. Mean annual temperature was around 10.7°C and mean annual precipitation approximately 840 mm (Veen et al., 2018) (Royal Netherlands Meteorological Institute (KNMI)). Each location consisted of a semi-natural grassland and a surrounding broad-leaved forest (Veen et al., 2018). Experimental design In the fall of 2016, we collected soils (8th of December) and litter (3-27 October depending on timing of litter fall) from three grass species Agrotis capillaris, Festuca rubra, and Holcus lanatus and three tree species Betula pendula, Fagus sylvatica, and Quercus robur at each site. For the grass species we sampled soils within monoculture patches, for the tree species we sampled soils immediately underneath adult trees. For each plant species at each location we collected ~4 kg of soil from the top 10 cm by pooling ~6-10 individual soil cores. Soils were sieved over a 4-mm sieve. Soils were kept at 4°C until set up of the experiment. Litter, i.e., recently senesced biomass, was collected as a bulk sample from locations where the plant species were highly abundant. Litter was cut into 1-cm fragments and sterilized by gamma-irradiation (25 KGray). Using the soil and litter samples, we set up (i) a reciprocal transplant experiment where soils were incubated with all litter types during three subsequent periods of three month and (ii) we used the incubated soils in a subsequent litter decomposition experiment where soils were confronted with original litter (i.e., litter type as in the field) or with the new litter (i.e., litter type used during the incubation in experiment (i)). Reciprocal litter incubation experiment On 20 December 2016, we set up a full-factorial reciprocal litter transplant experiment with soils from the six replicated field locations. We filled microcosms with 240 g equivalent of dry weight soil. For each plant species at each location we set up seven microcosms which were incubated with 2 g of air-dried-sterilized plant litter from each plant species included in the experiment, according to a full-factorial design; and one mesocosm did not receive any litter (no-litter control). This resulted in a total of 6 replicates × 6 plant species × 7 litter treatments (i.e., 6 litter types, one no-litter control) = 252 microcosms (Fig. 1). Litter addition to the same pots was repeated after three months and after six months (Fig. 1). At each litter addition, litter and soil were gently homogenised; the amount of litter added was similar to average rates of litter fall in temperate ecosystems (Penuelas et al., 2007). At each 3-month litter incubation period we also added 1 g of litter in a nylon mesh bag (mesh size 0.9 × 1.0 mm), which was inserted in the soil, in order to calculate litter mass loss. Microcosms were incubated in the dark at 60% water holding capacity (WHC), 20°C and 80% air humidity. Microcosms were organized to a randomized block design, with each replicated site considered as a block. After each three-month litter incubation period, litter bags were harvested, cleaned and dried at 60°C to measure litter mass loss. Microcosms were weighed and watered to maintain WHC every two weeks. In addition, a soil subsample was collected at the start of the experiment and after each three-month incubation period to measure soil abiotic and biotic conditions (details under "Soil and litter measurements"). At the start of the experiment subsamples of the litter were oven-dried, to be able to correct mass loss calculations for the amount of moisture still present in air-dried litter. Litter decomposition feedback experiment At the end of the reciprocal litter transplant experiment, i.e., after nine months of incubation, we collected a soil subsample from each microcosm from experiment (i) to set up a litter decomposition experiment. Each soil sample was split into two subsamples, used to set up two new microcosms using 50 ml falcon tubes: one microcosm was incubated with the litter type from the plant species where the soil originated from in the field, the other microcosm was incubated with the litter type that the soil had been incubated with during the reciprocal transplant experiment (Fig. 1). Each microcosm received the equivalent of 0.50 g of dry soil and 0.50 g of dry litter (Keiser et al., 2011). This resulted in 252 soil subsamples × 2 litter types (i.e., the historical field litter type and the current incubation litter type; for the no-litter control samples we only incubated with the historical field litter) = 468 microcosms. We used small amounts of soil in this experiment in order to inoculate the soil microbiome, while minimizing effects of soil physical and chemical conditions on litter breakdown (Keiser et al., 2011). Microcosms were incubated in the dark at 20°C, 60% water holding capacity and 80% air humidity for three months and then freeze-dried to determine litter mass loss. Soil and litter measurements At the start of the experiment we measured initial soil and litter chemical properties from all soil and litter types. A soil subsample was dried at 105°C for 24 hours to determine soil moisture content. Soil organic matter content was determined by loss-on-ignition in a muffle furnace (550°C, 4 hr). We determined pH in fresh soil samples with a Mettler Toledo pH meter after shaking the equivalent of 10 g dry weight soil in 25 ml of demi-water for 2 hr at 250 RPM. Inorganic nitrogen content (N-NOx and N-NH4+) were determined with an autoanalyzer (Quaatro, Seal Analytical, Norderstedt, Germany) after shaking the dry weight equivalent of 10 g soil in 50 ml 1M KCl (2 hr, 250 RPM). Soil inorganic nitrogen content was determined again after 9 months of litter incubation, i.e., at the end of experiment (i). A soil subsample was dried at 40°C and ground and used to determine total soil C and N content with an element analyser (Flash 2000, Thermo Fisher Scientific, Bremen, Germany). Soil P availability was measured as P-Olsen and measured with an Auto­Analyzer (Quaatro, Seal Analytical, Norderstedt, Germany) (Olsen, 1954). Litter C and N content was determined with an element analyser (Flash 2000, Thermo Fisher Scientific, Bremen, Germany). Litter P content was determined by digestion with a 2.5% potassium persulfate solution. The obtained extract was measured colorimetrically with an Auto­Analyzer (Quaatro, Seal Analytical, Norderstedt, Germany) (Murphy & Riley, 1962). We determined lignin content using methanol–chloroform extractions and hydrolysis (Rowland & Roberts, 1994). Data analysis Before the analysis of the reciprocal litter incubation experiment (i) we standardized litter mass loss values to 90-day periods, in order to correct for differences in litter incubation length (range between 90-94 days). We then used a general linear mixed model to determine how soil source, litter type and experimental period affected litter mass loss. Site (1|site), experimental period and mesocosm (period|mesocosm) were used a random factors to control for the experimental set up (sites as replicated blocks in field and greenhouse) and for repeated measures, respectively. We tested the effect of soil source, litter type and experimental period on home-field advantage effects (expressed as the percentage of additional decomposition at home; ADH) using a general linear mixed model with site and experimental period as random factors (period|site). For the first experimental period, we tested how home-field effects differed between plant functional groups using a general linear model with transplant type (i.e., transplants between two grass species, two tree species or a grass and a tree species) as a fixed factor and site (1|site) as a random factor. Data from the litter feedback experiment (ii) were analysed from two different perspectives. First, we tested how litter incubation history (from experiment i) affected the mass loss and HFA of the original litter type, allowing us to analyse whether microbial lost affinity and thus HFA for the original litter type. We used general linear mixed models with mass loss and HFA as respective response variables, litter incubation history and litter type as fixed factors and site (1|site) as a random factor. Second, we tested how field history affected the mass loss of the incubation litter type, allowing us to analyse whether microbial communities developed affinity and thus HFA for the incubation litter type. We used general linear mixed models with mass loss and HFA as respective response variables, field history and litter type as fixed factors and site (1|site) as a random factor. We were not able to perform one full model for this experiment, because as a result of logistic constraints we did not include all full-factorial reciprocal transplants in this experiment (see Fig. 1 for set up). For all analysis we used post hoc Tukey HSD tests to test which treatments differed from each other. We explored for a normal distribution of residuals using QQ-plots and a Shapiro-Wilk test and homogeneity of variances using a Levene's test. All data were analysed in R version 3.6 (Team, 2013) using the lme4 (Bates & Maechler, 2009) and lmerTest package (Kuznetsova et al., 2013) package

Contrasting effects of soil microbial interactions on growth–defence relationships between early- and mid-successional plant communities

Plants allocate resources to processes related to growth and enemy defence. Simultaneously, they interact with complex soil microbiomes that also affect plant performance. While the influence of individual microbial groups on single plants is increasingly studied, effects of microbial interactions on growth, defence and growth-defence relationships remain unknown, especially at the plant community level. We investigated how three microbial groups (bacteria, fungi, protists), alone and in full-factorial combinations, affect plant performance and potential growth-defence relationships by measuring phenolics composition in early- and mid-successional grass and forb communities in a glasshouse experiment. Microbial groups did not affect plant growth and only fungi increased defence compounds in early- and mid-successional forbs, while grasses were not affected. Shoot biomass-defence relationships were negatively correlated in most microbial treatments in early-successional forbs, but positively in several microbial treatments in mid-successional forbs. The growth-defence relationship was generally negative in early-successional but not in mid-successional grasses. The presence of different microbiomes commonly removed the observed growth-defence relationships. We conclude that soil microorganisms and their interactions can shift growth-defence relationships differentially for plant functional groups and the relationships vary between successional stages. Microbial interaction-induced growth-defence shifts might therefore underlie distinct plant strategies and fitness

Fungal root-endophytes influence plants in a species-specific manner that depends on plant's growth stage

The mycobiome (fungal microbiome) influences plants— from seed germination to full maturation. While many studies on fungal-plant interaction studies have focused on known mutualistic and pathogenic fungi, the functional role of ubiquitous endophytic fungi remains little explored. We examined how root-inhabiting fungi (endophytes) influence range expanding plant species. We isolated endophytes from three European intra-continental range-expanders and three congenerics that are native both in the range-expander’s original (southern Europe) and new (northern Europe) range. To standardize our collection, endophytes were obtained from all six plant species growing under controlled conditions in northern (new range of the range-expander) and southern (native range of the range-expander) soils. We cultivated, molecularly identified and tested the effects of all isolates on seed germination, and growth of seedlings and older plants. Most of the 34 isolates could not be functionally characterized based on their taxonomic identity and literature information on functions. Endophytes affected plant growth in a plant species-endophyte specific manner, but overall differed between range-expanders and natives. While endophytes reduced germination and growth of range-expanders compared to natives, they reduced seedling growth of natives more than of range-expanders. Synthesis: We conclude that endophytic fungi have a direct effect on plant growth in a plant growth stage-dependent manner. While these effects differed between range expanders and natives, the effect strength and significance varied among the plant genera included in the present study. Nevertheless, endophytes likely influence establishment of newly arriving plants and influence vegetation dynamics,Weighing, counting, microscopy - for details see the main paper.,All information is present.

Fungal root‐endophytes influence plants in a species‐specific manner that depends on plant`s growth stage

The mycobiome (fungal microbiome) influences plants—from seed germination to full maturation. While many studies on fungal‐plant interaction studies have focused on known mutualistic and pathogenic fungi, the functional role of ubiquitous endophytic fungi remains little explored.We examined how root‐inhabiting fungi (endophytes) influence range‐expanding plant species. We isolated endophytes from three European intra‐continental range‐expanders and three congenerics that are native both in the range expander's original (southern Europe) and new (northern Europe) range. To standardize our collection, endophytes were obtained from all six plant species growing under controlled conditions in northern (new range of the range expander) and southern (native range of the range expander) soils. We cultivated, molecularly identified and tested the effects of all isolates on seed germination, and growth of seedlings and older plants.Most of the 34 isolates could not be functionally characterized based on their taxonomic identity and literature information on functions. Endophytes affected plant growth in a plant species–endophyte‐specific manner, but overall differed between range‐expanders and natives. While endophytes reduced germination and growth of range‐expanders compared to natives, they reduced seedling growth of natives more than of range‐expanders.Synthesis. We conclude that endophytic fungi have a direct effect on plant growth in a plant growth stage‐dependent manner. While these effects differed between range expanders and natives, the effect strength and significance varied among the plant genera included in the present study. Nevertheless, endophytes likely influence the establishment of newly arriving plants and influence vegetation dynamics

Temporal dynamics of range-expander and congeneric native plant responses during and after extreme drought events

Climate change is causing range shifts of many species to higher latitudes and altitudes and increasing their exposure to extreme weather events. It has been shown that range-shifting plant species may perform differently in new soil than related natives; however, little is known about how extreme weather events affect range-expanding plants compared to related natives. In this study we used outdoor mesocosms to study how range-expanding plant species responded to extreme drought in live soil from a habitat in a new range with and without live soil from a habitat in the original range (Hungary). During summer drought, the shoot biomass of the range-expanding plant community declined. In spite of this, in the mixed community, range expanders produced more shoot biomass than congeneric natives. In mesocosms with a history of range expanders in the previous year, native plants produced less biomass. Plant legacy or soil origin effects did not change the response of natives or range expanders to summer drought. During rewetting, range expanders had less biomass than congeneric natives but higher drought resilience (survival) in soils from the new range where in the previous year native plant species had grown. The biomass patterns of the mixed plant communities were dominated by Centaurea spp.; however, not all plant species within the groups of natives and of range expanders showed the general pattern. Drought reduced the litter decomposition, microbial biomass, and abundances of bacterivorous, fungivorous, and carnivorous nematodes. Their abundances recovered during rewetting. There was less microbial and fungal biomass, and there were fewer fungivorous nematodes in soils from the original range where range expanders had grown in the previous year. We concluded that in mixed plant communities of range expanders and congeneric natives, range expanders performed better, under both ambient and drought conditions, than congeneric natives. However, when considering the responses of individual species, we observed variations among pairs of congenerics, so that under the present mixed-community conditions there was no uniformity in responses to drought of range expanders versus congeneric natives. Range-expanding plant species reduced soil fungal biomass and the numbers of soil fungivorous nematodes, suggesting that the effects of range-expanding plant species can trickle up in the soil food web

Competition increases sensitivity of wheat (Triticum aestivum) to biotic plant-soil feedback.

Plant-soil feedback (PSF) and plant competition play an important role in structuring vegetation composition, but their interaction remains unclear. Recent studies suggest that competing plants could dilute pathogenic effects, whereas the standing view is that competition may increase the sensitivity of the focal plant to PSF. In agro-ecosystems each of these two options would yield contrasting outcomes: reduced versus enhanced effects of weeds on crop biomass production. To test the effect of competition on sensitivity to PSF, we grew Triticum aestivum (Common wheat) with and without competition from a weed community composed of Vicia villosa, Chenopodium album and Myosotis arvensis. Plants were grown in sterilized soil, with or without living field inoculum from 4 farms in the UK. In the conditioning phase, field inocula had both positive and negative effects on T. aestivum shoot biomass, depending on farm. In the feedback phase the differences between shoot biomass in T. aestivum monoculture on non-inoculated and inoculated soils had mostly disappeared. However, T. aestivum plants growing in mixtures in the feedback phase were larger on non-inoculated soil than on inoculated soil. Hence, T. aestivum was more sensitive to competition when the field soil biota was present. This was supported by the statistically significant negative correlation between shoot biomass of weeds and T. aestivum, which was absent on sterilized soil. In conclusion, competition in cereal crop-weed systems appears to increase cereal crop sensitivity to soil biota

Contrasting effects of soil microbial interactions on growth–defence relationships between early- and mid-successional plant communities

Plants allocate resources to processes related to growth and enemy defence. Simultaneously, they interact with complex soil microbiomes that also affect plant performance. While the influence of individual microbial groups on single plants is increasingly studied, effects of microbial interactions on growth, defence and growth–defence relationships remain unknown, especially at the plant community level. We investigated how three microbial groups (bacteria, fungi, protists), alone and in full-factorial combinations, affect plant performance and potential growth–defence relationships by measuring phenolics composition in early- and mid-successional grass and forb communities in a glasshouse experiment. Microbial groups did not affect plant growth and only fungi increased defence compounds in early- and mid-successional forbs, while grasses were not affected. Shoot biomass–defence relationships were negatively correlated in most microbial treatments in early-successional forbs, but positively in several microbial treatments in mid-successional forbs. The growth–defence relationship was generally negative in early-successional but not in mid-successional grasses. The presence of different microbiomes commonly removed the observed growth–defence relationships. We conclude that soil microorganisms and their interactions can shift growth–defence relationships differentially for plant functional groups and the relationships vary between successional stages. Microbial interaction-induced growth–defence shifts might therefore underlie distinct plant strategies and fitness

Transient negative biochar effects on plant growth are strongest after microbial species loss

Biochar has been explored as an organic amendment to improve soil quality and benefit plant growth. The overall positive effects of biochar on crop yields are generally attributed to abiotic changes, while the alternative causal pathway via changes in soil biota is unexplored. We compared plant growth effects of legumes in sterile soil inoculated with dilutions of soil and soil microbial suspensions to determine the direct effects of biochar-induced changes in soil biota on plant growth. Suspensions and soil were from soil amended with biochar and soil without biochar. By comparing consecutive plant growth phases on the same inoculated soils, we also determined the temporal effects of soil biota from biochar-amended and control soils. Biota from biochar-amended soil was less beneficial for Medicago sativa growth, especially with small amounts of inocula. Flowering was delayed in the presence of biota from biochar plots. Inoculum with either soil or soil suspension gave similar results for plant biomass, indicating that microorganisms play a major role. Vicia villosa growth did not respond to the various inocula, even though the inoculum quantity strongly affected nematode community composition and protozoan abundance. In a later growing phase the negative effect of biochar-associated biota on Medicago growth mostly disappeared, which leads to the conclusion that the benefits of biochar application via abiotic changes may outweigh the negative effects of biochar on soil biota.</p