THE EFFECT OF FREEZE-THAW CYCLES ON MICROBIAL RESILIENCE ALONG A CROP BIODIVERSITY GRADIENT

Freeze-thaw cycles (FTCs) are cyclical periods of soil disturbance that are increasing in number and intensity due to climate change effects on winter precipitation and temperature patterns and are not well characterized within an agroecosystem environment. First, I review the literature and discuss the effects of FTCs on soil properties, explore the nuances of characterizing FTCs in experiments, and assess the knowledge gaps of FTC studies in agroecosystems. I conducted a laboratory experiment using soils from a crop rotational diversity gradient and froze them at three distinct FTC frequencies. My results indicate that increased crop rotational diversity did not moderate FTC disturbance effects at any frequency level. Increased FTC frequencies generally increased soil organic C losses as CO2, decreased ammonium (NH4+), increased nitrate (NO3-) pools, and increased extracellular enzyme activities (EEA). The respiratory burst after each freezing period was the predominant contributor to differences by FTC in cumulative CO2 respiration by the end of the incubation. Interestingly, the medium FTC frequencies facilitated the highest EEA for select enzymes with minimal reductions in microbial biomass. This suggests that microbes and their EEA are impacted too severely with high frequency FTCs to maintain function. My study revealed that the novel microbial communities and soil processes found along a crop rotational diversity gradient are not resilient against climate change effects of FTCs in soils. Accelerated soil organic C loss and nutrient turnover are expected to occur throughout agroecosystems that experience increased FTCs.