The role of forest trees and their mycorrhizal fungi in carbonate weathering and phosphorus biogeochemical cycling

Over millions of years, atmospheric CO2 concentrations, and Earth’s climate, are regulated by continental silicate weathering and associated marine carbonate deposition. On this geological timescale, carbonate weathering has no net effect on CO2 drawdown. However, over the coming decades-to-centuries, accelerated weathering of carbonate rocks may provide a sink for anthropogenic CO2 emissions and increase alkalinity flux to the oceans to counteract ocean acidification. Recent experimental evidence strongly supports trees and their associated mycorrhizal fungi as key drivers of silicate mineral weathering; however, their role in the context of carbonate weathering is largely unknown. Carbonate lithology is abundant globally and underlies many boreal and temperate forest ecosystems in the northern hemisphere. If biological enhancement of carbonate weathering by forests occurs, this might presents a new opportunity for CO2 sequestration. This thesis presents results from a 14-month field experiment at the UK's national pinetum investigating carbonate rock weathering under a common climate. Overall, I find original evidence for biotic enhancement of calcite- and dolomite weathering by an evolutionary diverse range of trees that host either arbuscular (AM) or ectomycorrhizal (EM) root-associating fungal symbionts. Recent soil analyses are integrated with a re-interpretation of historic data to provide an 85-year record of in-situ soil development under different forestry species. This study challenges the classic dogma that divergence of properties is driven by the major tree functional groups, angiosperms and gymnosperms. Instead, we find that over decades, mycorrhizal functional type plays a dominant role in determining soil physico- chemical characteristics, and conditions generated by EM fungi are likely to enhance mineral weathering. Field trials next investigated the impact of tree-mycorrhizal functional group on weathering of the four main carbonate rock types (chalk, limestone, marble and dolomite) and a quartz silicate. Under EM trees carbonate rock grain dissolution was 12 times faster that silicate weathering. In the initial 3 months, calcite weathering intensity increased from gymnosperm to angiosperm species and from AM to later, but independently-evolved EM fungal partnerships. More extensive weathering after 6 months, especially within EM forest soils, confirms the importance of these fungi for carbonate mineral dissolution and nutrient mobilisation. This effect is linked to rhizosphere acidification by EM fungi and is confirmed by a parallel study of tree species’ influence on soil chemistry. Both AM and EM fungi facilitate the mobilisation of nutrient elements, which are provided to their host plants in exchange for carbon from photosynthesis. I applied a suite of nanoscale surface analysis techniques (VSI, SEM) to quantify mineral alteration and provide direct evidence for mycorrhizal involvement in carbonate weathering in the field. Fungal hyphae preferentially colonised chalk and quartz silicate grains, which contained the highest concentrations of phosphorus (P), a growth limiting elemental nutrient. P was selectively depleted from silicate grains, especially in EM forest soils, but accumulated on carbonates. Although the origin of this accumulated P remains uncertain, extensive analyses of different potential P-pools indicated it is likely to be inorganic, but accumulated via active microbial import. These findings lead to new insights linking carbonate weathering with phosphorus biogeochemical cycling in soils. Results show that P from the surrounding environment is concentrated on carbonate grains and this potentially provides a renewable P resource accessible to host trees. Overall, this thesis builds new support for the role of mycorrhizal partnerships in shaping soil properties important for accelerating carbonate rock weathering (Chapter 2); presenting the first field-based evidence for the enhancement of carbonate dissolution by tree roots and their associated mycorrhizal partners (Chapters 3-5) and generating new insights into biogeochemical P cycling in soils (Chapter 4). More broadly, these findings suggest targeted reforestation/afforestation with EM-tree taxa on carbonate-rich terrain as a possible regional-scale land management strategy for promoting short-term anthropogenic CO2 sequestration and perhaps helping ameliorate ocean acidification.