Defoliation induced modifications of arbuscular mycorrhizal symbiosis and soil carbon dynamics under drought in grassland ecosystems

Abstract

Grasslands are vital ecosystems, providing a wide range of ecosystem services, notably acting as significant carbon (C) sinks that absorb and release carbon dioxide. Among various factors that regulate grassland functioning under climate change, the mutualistic relationship between plants and arbuscular mycorrhizal (AM) fungi is of paramount importance. AM fungi, the predominant mycorrhizal type in grassland ecosystems, play a crucial role in plant nutrient supply, but also, they help mitigate stress in host plants resulting from climate extremes, such as drought, which are expected to become more severe and frequent with climate change. Grazing, as the most extensive land management practice of grassland globally, can significantly influence the C dynamics. However, crucial knowledge gaps remain regarding how grazing affects the plant-fungi response to drought conditions. This thesis combines multi-factorial glasshouse and field-based experiments, and state-of-the-art in situ isotope tracing technology to test how defoliation, a key aspect of grazing in grasslands, influences below-ground AM fungi symbiosis and its response to drought focusing on AM fungi colonization, extraradical hyphal length, C availability in neutral lipid fatty acids (NLFA), and the allocation of recently assimilated C within the system. In the first chapter, I provide a comprehensive review of the current understanding of the individual effects of defoliation and drought on AM fungi, followed by an exploration of their combined impacts. The second chapter presents findings from a glasshouse study simulating defoliation, highlighting that C availability in AM fungi is crucial for determining the effects of defoliation on below-ground AM fungi symbiosis. Here, I observed that defoliation significantly reduces AM fungi root colonization, extraradical hyphal length, and C storage in NLFAs. In the third chapter, the focus shifts to an in situ field study in a typical species-rich meadow, where I discovered that high intensity defoliation significantly reduces AM fungal root colonization and exacerbates the negative impact of drought on extraradical hyphal length. I also found that the increase in AM fungi C storage induced by drought was suppressed by defoliation. Lastly, the fourth chapter reveals that the combination of defoliation and drought significantly suppressed transfer of recently assimilated photosynthetic C below-ground . While drought reduced 13C enrichment in plant shoots, roots, and soil respiration, it had no effect on 13C enrichment in extraradical AM fungal hyphae and stimulated 13C transfer relative to initial plant uptake. The findings from this thesis indicate that under controlled glasshouse conditions, defoliation can negatively impact below-ground AM fungi, yet root colonization and C storage in neutral lipids do not further decline with increased defoliation intensity. In contrast, field conditions reveal that high-intensity defoliation can significantly disrupt positive plant-fungi interactions under drought conditions, potentially leading to long-term effects on soil carbon storage. This research addresses a significant knowledge gap regarding how defoliation alters crucial plant-soil microbiome associations and their impact on grassland community dynamics and functionality. Furthermore, these insights offer valuable references for policymakers, grassland managers, and ecologists focused on preserving healthy plant-soil microbiome relationships in the context of anticipated climate change.

Date of Award	29 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	David Johnson (Supervisor) & Richard Bardgett (Supervisor)