Measuring and modeling how plant-microbe interactions control soil carbon and nitrogen cycling in managed and unmanaged ecosystems

Our understanding of how plant-microbe interactions regulate carbon (C) and nitrogen (N) retention in soil organic matter (SOM) remains uncertain. Conflicting evidence suggests that microbial decomposition can lead to the loss of unprotected particulate SOM, but also that microbial decomposition produces simpler compounds that preferentially form more persistent aggregate- or mineral-protected SOM pools. Further, uncertainty remains in how plant-microbe interactions alter SOM retention in these different pools through litter chemistry controls on microbial decomposition traits and rhizosphere priming. As a result, this uncertainty limits our ability to sustainably manage ecosystems and understand future feedbacks between terrestrial ecosystems and the global climate. In this dissertation, I address these uncertainties to answer the following research questions: 1) How do plant-microbe interactions between plant litter and microbial decomposition traits influence the formation of new soil C for different bioenergy crop litters in the lab?; 2) How do rhizosphere plant-microbe interactions influence soil organic matter stabilization and destabilization depending on nutrient levels in-situ?; and 3) Can empirical measurements help constrain, parameterize, and validate modeled plant-microbe interactions to improve representations of forest ecosystem responses to global change? For question 1, I examined differences between two bioenergy feedstocks, corn and miscanthus, in the ability of their litter to form new unprotected SOM vs. mineral-protected SOM. I traced the fate of isotopically enriched litter C into microbial respiration and SOM pools in the lab and found that chemically simple corn litter promoted higher microbial uptake and carbon use efficiency, forming less unprotected SOM and more mineral-protected SOM than more complex miscanthus litters. I also demonstrated the potential for our measurements to parameterize a microbial SOM model and improve predictions of soil C formation. This link between litter quality, microbial efficiency, and SOM formation bridges empirical uncertainty in how bioenergy crops build soil C. For question 2, I investigated whether living roots and their associated fungi increase or decrease new SOM formation from litter. I traced isotopically enriched litter C and N into SOM pools in root ingrowth cores incubated in a miscanthus field. I found that roots stimulated litter decomposition but balanced this loss by transferring carbon into aggregate-protected SOM. Further, roots selectively mobilized N from litter without additional C release, suggesting that roots efficiently mine N and build persistent soil C. This work expands our mechanistic understanding of how living roots shape agricultural ecosystem processes. For question 3, I investigated if modelling plant-microbe interactions and microbially-explicit N cycling could improve representations of forest soil C and N retention under changes in anthropogenic N deposition. I leveraged decades of C and N cycling data from a whole-watershed N fertilization experiment to run a microbially-explicit plant-microbe interactions model. The model accurately represented key ecosystem C responses to enhanced N availability, including a decline in plant C cost for N acquisition and an increase in soil C. By incorporating new, microbially-explicit N cycling, the model could also capture how enhanced N availability altered N cycling and streamwater N losses. When we ran the model forward under declining N deposition, the model predicted that N losses recovered faster than soil C pools. However, the C sequestered due to N deposition may be vulnerable to future loss, particularly in a warming climate. Collectively, my research shows that vital ecosystem services like soil C and N retention depend on microbially-mediated processes that are regulated by plant-microbe interactions

Ectomycorrhizal Plant-Fungal Co-invasions as Natural Experiments for Connecting Plant and Fungal Traits to Their Ecosystem Consequences

Introductions and invasions by fungi, especially pathogens and mycorrhizal fungi, are widespread and potentially highly consequential for native ecosystems, but may also offer opportunities for linking microbial traits to their ecosystem functions. In particular, treating ectomycorrhizal (EM) invasions, i.e., co-invasions by EM fungi and their EM host plants, as natural experiments may offer a powerful approach for testing how microbial traits influence ecosystem functions. Forests dominated by EM symbiosis have unique biogeochemistry whereby the secretions of EM plants and fungi affect carbon (C) and nutrient cycling; moreover, particular lineages of EM fungi have unique functional traits. EM invasions may therefore alter the biogeochemistry of the native ecosystems they invade, especially nitrogen (N) and C cycling. By identifying “response traits” that favor the success of fungi in introductions and invasions (e.g., spore dispersal and germination) and their correlations with “effect traits” (e.g., nutrient-cycling enzymes) that can alter N and C cycling (and affect other coupled elemental cycles), one may be able to predict the functional consequences for ecosystems of fungal invasions using biogeochemistry models that incorporate fungal traits. Here, we review what is already known about how EM fungal community composition, traits, and ecosystem functions differ between native and exotic populations, focusing on the example of EM fungi associated with species of Pinus introduced from the Northern into the Southern Hemisphere. We develop hypotheses on how effects of introduced and invasive EM fungi may depend on interactions between soil N availability in the exotic range and EM fungal traits. We discuss how such hypotheses could be tested by utilizing Pinus introductions and invasions as a model system, especially when combined with controlled laboratory experiments. Finally, we illustrate how ecosystem modeling can be used to link fungal traits to their consequences for ecosystem N and C cycling in the context of biological invasions, and we highlight exciting avenues for future directions in understanding EM invasion.Fil: Hoeksema, Jason D.. University of Mississippi; Estados UnidosFil: Averill, Colin. No especifíca;Fil: Bhatnagar, Jennifer M.. Boston University; Estados UnidosFil: Brzostek, Edward. West Virginia University; Estados UnidosFil: Buscardo, Erika. Universidade do Brasília; BrasilFil: Chen, Ko Hsuan. University of Florida; Estados UnidosFil: Liao, Hui Ling. University of Florida; Estados UnidosFil: Nagy, Laszlo. Universidade Estadual de Campinas; BrasilFil: Policelli, Nahuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; ArgentinaFil: Ridgeway, Joanna. West Virginia University; Estados UnidosFil: Rojas, J. Alejandro. University of Arkansas for Medical Sciences; Estados UnidosFil: Vilgalys, Rytas. University of Duke; Estados Unido

Lipid‐enhanced Oilcane does not impact soil carbon dynamics compared with wild‐type Sugarcane

Abstract The carbon neutral potential of bioenergy relies in part on the ability of feedstocks to sequester carbon (C) in the soil. Sugarcane is one of the most widely used bioenergy crops, yet there remain unknowns about how it impacts soil C dynamics. In addition, Oilcane, a genetically modified version of Sugarcane has been produced to accumulate more energy‐dense oils and less soluble lignin, which enhances conversion efficiency but may also impact soil C cycling. Thus, our objectives were to examine the impact of Sugarcane litter decomposition on soil C formation and losses and determine if the genetic modifications to produce Oilcane alter these dynamics. To do this, we incubated bagasse (processed stem litter) and leaf litter from Sugarcane and Oilcane in microcosms with forest soil for 11 weeks. We used differences in natural abundance δ13C between C3 forest soil and C4 litter to trace the fate of the litter into respiratory losses as well as stable and unstable soil C pools. Our results show that genetic modifications to Oilcane did not substantially alter soil C dynamics. Sugarcane and Oilcane litter both led to net soil C gains dominated by an accumulation of the added litter as unstable, particulate organic C (POC). Oilcane litter led to small but significantly greater net soil C gains than Sugarcane litter due to greater POC formation, but the formation of stable, mineral associated organic matter (MAOC) did not differ between crop types. Sugarcane and Oilcane had opposing effects on tissue type where Sugarcane bagasse formed more MAOC, while Oilcane leaves preferentially remained as POC which may have important management implications. These results suggest that genetic modifications to Sugarcane will not significantly impact soil C dynamics; however, this may not be universal to other crops particularly if modifications lead to greater differences in litter chemistry

Ectomycorrhizal Plant-Fungal Co-invasions as Natural Experiments for Connecting Plant and Fungal Traits to Their Ecosystem Consequences

Introductions and invasions by fungi, especially pathogens and mycorrhizal fungi, are widespread and potentially highly consequential for native ecosystems, but may also offer opportunities for linking microbial traits to their ecosystem functions. In particular, treating ectomycorrhizal (EM) invasions, i.e., co-invasions by EM fungi and their EM host plants, as natural experiments may offer a powerful approach for testing how microbial traits influence ecosystem functions. Forests dominated by EM symbiosis have unique biogeochemistry whereby the secretions of EM plants and fungi affect carbon (C) and nutrient cycling; moreover, particular lineages of EM fungi have unique functional traits. EM invasions may therefore alter the biogeochemistry of the native ecosystems they invade, especially nitrogen (N) and C cycling. By identifying “response traits” that favor the success of fungi in introductions and invasions (e.g., spore dispersal and germination) and their correlations with “effect traits” (e.g., nutrient-cycling enzymes) that can alter N and C cycling (and affect other coupled elemental cycles), one may be able to predict the functional consequences for ecosystems of fungal invasions using biogeochemistry models that incorporate fungal traits. Here, we review what is already known about how EM fungal community composition, traits, and ecosystem functions differ between native and exotic populations, focusing on the example of EM fungi associated with species of Pinus introduced from the Northern into the Southern Hemisphere. We develop hypotheses on how effects of introduced and invasive EM fungi may depend on interactions between soil N availability in the exotic range and EM fungal traits. We discuss how such hypotheses could be tested by utilizing Pinus introductions and invasions as a model system, especially when combined with controlled laboratory experiments. Finally, we illustrate how ecosystem modeling can be used to link fungal traits to their consequences for ecosystem N and C cycling in the context of biological invasions, and we highlight exciting avenues for future directions in understanding EM invasion