Faghihinia, Maede
(2020)
Effects of long-term grazing intensity on arbuscular mycorrhizal fungi and soil carbon fluxes in a steppe ecosystem.
PhD thesis, University of Liverpool.
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Abstract
Arbuscular mycorrhizal fungi (AMF) are keystone symbiotic root-associated soil biota that play a major role in maintaining plant biodiversity, productivity and enhancing soil carbon storage in grasslands. Grasslands are currently experiencing increasing grazing pressures, but it is not clear yet whether and how grazing intensity by large herbivores drives AMF function and communities. Moreover, little is also known about the interactive effects of grazing intensity and environmental drivers on AMF communities, and on how these communities transfer carbon through the plant-soil system. The overall aims of this study were to assess the impacts of grazing intensity on AMF communities and their possible outcome on soil carbon storage. This thesis focuses on a typical steppe ecosystem, which is part of the temperate grassland zone of Inner Mongolia, China. These grasslands have been degraded to a large extent mainly because of increasing livestock grazing. This study was carried out within a long-term, farm-scale field experiment with seven levels of field-manipulated grazing intensities which had been maintained over 13 years within two topographies, flat and slope. In Chapter 2, I reviewed the current state of knowledge of livestock grazing impacts on AMF and argued that the present knowledge of the impacts of livestock grazing on mycorrhizal fungal function appeared inconsistent and inadequate mainly due to the context-dependent nature of the plant-AMF symbiosis. The context-dependent nature of the mycorrhizal symbiosis suggests that indirect effects of grazing on AMF occur through changes to the soil and root environment in which the AMF is found. A conceptual framework was, therefore, produced that highlighted potential drivers and mechanistic pathways through which long-term grazing impacts AMF. In addition, it was suggested that grazing effects on AMF communities could be moderated by grazing intensity, seasonality and topography. It was further suggested that measuring a single variable was not sufficient to explain the response of AMF to grazing and that to advance this research field, both internal and external mycorrhizal variables should be included as standard. In Chapter 3, the response of AMF internal and external abundance were investigated with respect to different level of grazing intensity in two topographies, with AMF internal abundance represented by mycorrhizal root colonization (mycorrhizal root frequency, intensity and arbuscule intensity) and external abundance represented by soil hyphal length density. Measurements were made on those drivers of AMF abundance and diversity that were expected to be influenced by grazing including nutrient availability (soil available nitrogen and phosphorus) and edaphic properties (soil organic carbon, soil pH, soil bulk density and soil moisture). The finding showed that external AMF structures were clearly impacted by grazing intensity but that this was not the case for internal mycorrhizal structures indicating that mycorrhizal functioning itself was impacted by grazing intensity. The data also showed AMF root frequency, intensity and arbuscule intensity were lower in the more nutrient-limited slope area compared with the flat landscape. Mycorrhizal root frequency was correlated negatively with topographic-induced changes in soil nitrogen and phosphorus, while arbuscule intensity was marginally-negatively correlated with soil available phosphorus. This study suggested that the impacts of grazing intensity on the plant-AMF association could lead to further knock-on effects on the plant-soil system via the feedbacks that exist between plant and AMF communities. In Chapter 4, the ecology of AMF in response to grazing intensity was compared in different seasons. The AMF internal and external abundance was measured in spring, summer and autumn along the grazing gradient in the two topographic landscapes. Soil nutrients and carbon concentrations were also measured to assess whether seasonal variation in AMF abundance related to seasonal shifts in soil resources. Soil hyphal length density was significantly negatively related to grazing intensity over the course of growing season from spring to autumn. However, AMF root intensity and arbuscule intensity were related negatively to grazing intensity only in spring. The results suggested that external AMF structures in soil were more responsive to seasonal variation and grazing than internal structures in roots. The findings also suggested that early grazing may be detrimental to AMF root colonization of newly-emerging plants. vi In Chapter 5, we focused on the impact of grazing intensity on below-ground carbon flow and particularly the association between recently-fixed photosynthate carbon and soil AMF abundance. AMF are known to act as a pathway for carbon movement through the plant and soil. We applied an in situ 13C stable isotope pulse-chase labeling approach in plots under seven levels of grazing intensity. We quantified allocation of carbon to plant, soil, and soil-respired CO2 along with measurements of mycorrhizal hyphal density in the soil. As grazing intensity increased, carbon assimilation increased per unit shoot biomass, whereas carbon allocation to roots decreased. Soil-respired CO2 appeared unrelated to grazing intensity. Mycorrhizal hyphal density decreased with increasing grazing intensity and was correlated significantly with new carbon inputs in roots two days after labeling and marginally-correlated with that in soil one day after the 13C-CO2 pulse. This indicated that grazing intensity moderated carbon allocation to different carbon pools within the plant-soil system and pointed to the key role of mycorrhizas as a rapid route for carbon translocation from plant to soil. In chapter 6, the aim was to investigate how grazing intensity and host plant grazing preference by large herbivores interactively drive the soil and root-associated AMF communities, by testing whether AMF diversity and community composition can be explained by AMF life-history traits and strategies. DNA was extracted from root and rhizosphere samples of two dominant grasses, Leymus chinensis (Trin.) Tzvel. and Stipa grandis P. Smirn, exhibiting contrasting grazing preferences by sheep. AMF small-subunit (SSU) rRNA gene fragments were then subjected to PCR, cloning, sequencing, and phylogenetic analyses. No variation was detected in AMF α-diversity at OTU level in response to grazing intensity, However, AMF families differed in their response to grazing pressure. The relative abundance of Glomeraceae and Ambisporaceae increased significantly with increased grazing intensity, whereas Paraglomeraceae decreased significantly. Sheep grazing preference for host plants did not explain variation in AMF diversity and community composition. However, root and soil medium had different community composition. AMF communities in roots had a lower α-diversity and higher β-diversity compared to those in soils. Overall, our findings suggested that long-term grazing intensity changed the abundance of functionally-diverse AMF families favoring the ones with disturbance-tolerant traits. Chapter 7 synthesized this research elucidating how grazing intensity impacts AMF structure and function in grassland. This highlighted the importance of landscape topography, seasonality and grazing-induced changes to the plant-soil environment influencing AMF abundance. The association of AMF and below-ground carbon flow was also observed which is critical for soil carbon cycle and storage.
| Item Type: | Thesis (PhD) |
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| Divisions: | Faculty of Science and Engineering > School of Environmental Sciences |
| Depositing User: | Symplectic Admin |
| Date Deposited: | 14 Jan 2021 13:52 |
| Last Modified: | 18 Jan 2023 23:36 |
| DOI: | 10.17638/03098696 |
| Supervisors: |
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| URI: | https://livrepository.liverpool.ac.uk/id/eprint/3098696 |
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