Abstract
The cultivation of arid and semi-arid lands is an important global practice, vital to the food security of millions of people, many in developing countries. To successfully convert drylands into arable fields, irrigation projects and soil-enriching, agricultural amendments are often essential. In this thesis, we considered the impacts that these agricultural management practices have on the function and sustainability of cultivated drylands. Using the Magic Valley of southern Idaho – a region once characterized by arid soils and sparsely vegetated landscape – we specifically focused on the mechanisms as they relate to soil ecology and the ecosystem services soil microbial communities provide. We first assessed the impacts of irrigation and antibiotics from manure fertilizer amendments on soils. We used a microcosm approach and incubated soils along a moisture gradient with an antibiotic dose and control for 90 days, tracking microbial respiration responses throughout. At the conclusion of the incubation, we conducted amplicon sequencing on the microbial community DNA. Our findings revealed the existence of alternative stable states and historical legacies associated with the conversion of dryland soils into moist, cultivated fields. Microbial communities are distinct in their structure and abundance once these lands are converted with irrigation. Efforts to restore these lands may encounter serious lags in returning microbial function and soil labile carbon (C) to their original state. Furthermore, we found that antibiotics from manure fertilizer amendments influenced microbial diversity, but not microbial community structure or C-cycling.
To assess the viability of a relatively new approach of ameliorating agricultural soils with biochar – or carbonized organic matter – we used a second microcosm incubation. We were specifically interested in assessing the C-cycling feedback from microbial communities after the amendment of softwood-derived biochar in irrigated dryland soils. Soils were incubated with two rates of biochar amendments (2% and 10%, by mass), and a control. Soils were then dosed with 13C labeled glucose, and we traced the fate of the labeled substrate into the microbial biomass, respiration, and soil organic carbon pools. From this data we were able to assess microbial carbon use efficiency under varying rates of biochar amendment and assess whether this addition impacted the C-cycling function of soil microbial communities. We found that at the high (10%) amendment rate, microbially accessible C was initially withheld from microbial mineralization and released into the soil environment slowly over time. Yet, carbon use efficiency was not impacted at either amendment rate. We concluded that biochar additions may play a role in the temporal dynamics of carbon availability, but that biochar can be an effective C-sequestration approach in soils because microbial C-cycling is not hindered.