Abstract: Many landscapes experience either natural (e.g., volcanic eruptions) or anthropogenic (e.g., urban development) disturbances that reduce them to states at or near primary succession. These areas often require hundreds of years or more before they once again provide habitat for nearby species or are productive landscapes for human use. The developing field of Landscape Terraformation seeks a holistic understanding of primary succession by integrating geochemistry, hydrology, microbiology, and ecology to understand how landscapes change as they undergo primary succession. The ultimate goals of the field are to both improve our understanding of primary succession and to develop best practices to rapidly restore degraded landscapes.
We established a greenhouse experiment that mimicked an early-successional soil environment to assess how variation in root morphology, soil microbes, and soil moisture interact to affect plant performance, geochemical weathering, and hydrological properties of incipient soils. We selected 17 genotypes of alfalfa (Medicago sativa) for their variation in root morphology (taproot depth & fine root density) and grew them for three months in a crushed basalt substrate that had either been inoculated with soil microbes or sterilized (live vs. sterile). Plants were then divided into three soil moisture conditions: low (30-50% of soil water holding capacity; WHC), medium (55-75% WHC), and high (80-100% WHC). Prior to harvest, we conducted a discharge experiment to understand the effects of each treatment on hydrology, then harvested the plants to quantify plant performance, and collected soil samples for geochemical and microbial community analysis.
We expected that live soils would enhance plant performance, that plant genotypes with a high density of fine roots would outperform other genotypes, and that these patterns would intensify with increasing soil moisture.
Alfalfa were smaller and 36% less likely to survive when grown in live soil compared to sterile soil. They had no root nodules to indicate presence of their nitrogen-fixing mutualist rhizobia, which suggests that the microbial community consisted of competitors rather than mutualists. We found genotype-specific effects: two genotypes had higher survival rates and three genotypes varied in their biomass allocation, but these patterns were not related to root morphology. Finally, increased soil moisture improved alfalfa performance, but did not interact with other experimental factors. Overall, the early-successional basaltic substrate presented a challenging environment for plant growth and suggests that the intentional addition of mutualists may be critical to the success of terraformed landscapes.
