title: Tracing soil carbon and nitrogen cycling in arbuscular and ectomycorrhizal systems
Creators: name: Klink, Saskia
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Arbuscular mycorrhiza
subject: ectomycorrhiza
subject: stable isotopes
subject: soil organic matter
subject: 500
subject: 550
subject: 570

description: Summary The plant-soil-microbe system trades carbon (C) and nutrients and drives the storage or release of these compounds. The intimacy of mycorrhizal fungi with plants supports plant nutrient supply, contributes to soil organic matter (SOM) decomposition and formation. Dominant fungal guilds in temperate systems are arbuscular mycorrhizal (AM), ectomycorrhizal (ECM), and saprotrophic (SAP) fungi. While their disparate nutrient acquisition strategies affect nutrient cycling, SOM formation, and C and nutrient storage of the system, impacts of different fungal guilds on these processes remain obscure. Climate change will greatly affect plant-soil-microbe interactions. Increasing atmospheric carbon dioxide (CO2) enhances photosynthesis, simultaneously aggravating nutrient limitation and competition. Subsequently, changed environmental conditions favor invasive species spread, with unknown consequences for nutrient cycling dynamics. This thesis investigates (i) the role of different fungal guilds on C and nutrient cycling and SOM formation, (ii) the potential of different fungi for nutrient acquisition, and (iii) how impacts of climate change like elevated CO2 levels or plant invasion affect the plant-microbe-soil systems’ C and nutrient cycling dynamics. Stable isotopes possess source and process information, allowing to trace the origin and the fate of compounds in a system. This information was unknown for AM hyphae, thus in manuscript 1 a method to determine AM hyphal isotopic signatures was developed. AM hyphal 13C and 15N separated them from plant tissues and soil. A mixed C source of plant carbohydrates and lipids was deciphered, contrasting with current knowledge on the C-source of ECM fungi. Manuscript 2 used the application of manuscript 1 to analyze the contribution of fungal inputs to specific SOM fractions. Forest plots with dominant tree species associated with ECM or AM fungi were chosen. Main contributions of ECM fungi to stable SOM were predicted in ECM systems, and SAP fungi as major contributors in AM systems, the latter due to activation via AM exudates. The combination of a Bayesian Inference Stable Isotope Mixing model with amino sugar analyzes showed that particulate organic matter fractions were dominated by plant and SAP inputs for both tree species. More stable mineral-associated organic matter was dominated by SAP followed by ECM inputs irrespective of tree species. These findings highlight fungal inputs as dominant source of stable organic matter and a need to consider fungal communities for efforts to increase long-term soil C storage. Manuscript 3 investigated the degradative and nutrient trading potential of ECM fungi and sought to determine nitrogen (N) fractionations in the mycorrhizal nutrient trading. In microcosms, the trading of soil N to Pinus sylvestris was investigated for Paxillus involutus and Pisolithus arhizus with greater enzymatic repertoire relative to Laccaria laccata and Laccaria bicolor with lower degradative capability. Plant biomass was increased due to colonization by P. involutus, P. arhizus, and L. laccata but not by L. bicolor. Under N-limited conditions, P. involutus and P. arhizus tended to trade 15N-enriched N. Laccaria species differed in degradative capabilities and plant benefit. The results highlight the impact of disparate fungal species on SOM decomposition and a need to consider fungal community composition to predict nutrient cycling dynamics. Thus, manuscript 3 supports different ECM fungi varying in their efficiency in accessing nutrients via SOM decomposition and contributing to stable SOM via fungal inputs. Climate change impacts may affect the dynamics of plant-soil-microbe systems’ C and nutrient trading and storage. Invasive plant species benefit from conditions allowing spread and reproduction, dominating over native species. Despite threats by increased litter input rates and a preference to invade AM-associated systems, the magnitude and direction of invasive plant effects on AM- or ECM-systems are obscure. Manuscript 4 investigated the effect of invasive C4 grass Microstegium vimineum on SOM turnover in tree stands with AM- or ECM-association. Invasion by M. vimineum increased decomposition of SOM under ECM-associated trees but decreased decomposition under AM-associated trees. These results highlight the potential of invasive plant species to modify SOM decomposition and that despite more SOC in ECM topsoils, this C is more easily lost under changing environmental conditions. Impacts of nutrient limitation and plant-microbe competition under elevated CO2 were analyzed in manuscript 5. Elevated CO2 often results in growth benefits of plants and microbes but simultaneously aggravates nutrient limitation. Plants with varying microbial interactions to acquire nutrients were grown at ambient vs. elevated CO2 to evaluate trading of N from 15N labeled litter. Despite different microbial interactions to acquire nutrients, plants equally benefitted more from soil nutrients than soil microbes under elevated CO2. These results highlight the need to consider ecosystem constituents as interwoven when discussing measures to decelerate the impacts of climate change. Overall, this thesis highlights the need to consider mycorrhizal fungi for predicting ecosystems’ C and N cycling and storage patterns, and for developing plans to adapt environments for changing conditions.