Faith Simitz and Benjamin D. Duval CHANGES IN SOIL ORGANIC MATTER CYCLING IN RESPONSE TO NATIVE OPUNTIA EXTRACT ADDITIONS Faith Simitz and Benjamin D. Duval Department of Biology, New Mexico Institute of Mining and Technology, Socorro, New Mexico SRS 2017-034 ABSTRACT Soils are the largest terrestrial carbon sink, therefore increasing soil carbon stability has implications for atmospheric CO2 and climate change. Adding C to soils from plant residue could increase soil C, however increasing soil C pools requires understanding the response of soil microbes that have a major role in C transformations. In an attempt to understand the C pool stability in arid environments, we added extracts of native prickly pear (Opuntia spp.) as a carbon amendment to soils from two sites and determined CO2 flux and soil aggregate size as evidence of microbial impact on soil C. Extracts were added to both desert and agricultural soils to test for differences in C cycling between important Chihuahuan Desert soil types. Microbial activities were found to increase macroaggregates (>250 µm) from sieved microaggregate (<250 µm) soils indicating the use of added C as a binding agent by-product of microbial processes. To analyze microbial activity, CO2 flux was measured weekly from incubating soils. After two weeks, soils with the cactus amendment showed 57% and 144% greater CO2 flux than controls from agricultural and desert soils, respectively. The increase in desert flux suggests microbial processes are more C limited in these soils. Total CO2 flux was higher from agricultural soils, possibly due to relaxed water and nutrient limitations on microbes. These results suggest limited capacity for C storage in arid land soil in short-term experiments, but highlight important differences between soil types (C limitation differences) that could be important for long-term soil C storage. POSITIVE FEEDBACK NEGATIVE FEEDBACK CO2 Organic C Organic C Figure 3 Soils were sieved to less than 250 µm before being incubated. After incubation, the aggregate size-class for samples was measured. Microaggregates (< 250 µm) decreased in mass proportion while macroaggregates (> 250 µm) increased in mass fraction. Desert macroaggregates increased by 72% while Agricultural macroaggregates increased by 99%. Addition Released to Atmosphere Stored in Macroaggregates Agricultural or Desert Soil Soil Organic C Increased microbial metabolism Expressed by increased CO2 flux Soils are potential C source Unlikely terrestrial C storage Microbial waste (polysaccharides?) act to bind microaggregates Increase in macroaggregates Increase terrestrial C pool FURTHER RESEARCH We aim to predict organic C flux in arid soils in response to climate change. Understanding will require new lines of research, including: new field experiments focused on long-term, controlled manipulations of entire soil profiles identification of the long-term drivers of microbial-cell and microbial-product decomposition METHODS AND MATERIALS Site Description: 10 soil cores per site were collected in late January Desert soil: Box Canyon; 8 mi from NMIMT soil was relatively undisturbed desert soils Agricultural soil: Private agricultural land; 2 mi from NMIMT with land use history of fertilization (urea) and flood-irrigation Pretreatment: Both soil types were wet-sieved using a 2000 µm, 250 µm, and 53 µm sieve then oven-dried at 60oC for 24 hrs Aggregate size-classes were weighed and recorded when dried Incubation: 40 g of soil, ground to <250 µm, was combined with either 6.5 ml of water (control) or 5 ml of cactus extract (C addition) and 1.5 ml of water in specimen cups Specimen cups were then placed in jars with 40 ml of water and sealed tightly Soil Respiration: For one month, gas emissions were measured weekly and recorded using a Gasmet FTIR Gas Analyzer Macroaggregate Formation: Following incubation, samples were wet-sieved as before and aggregate size-classes were determined and weighed Figure 1 CO2 flux plotted against 29 day incubation period with addition of organic carbon (Opuntia spp.). Flux decreased over time, most likely due to decreased labile organic carbon in the system. With standard deviation error bars. Figure 2 Resampled effect size of C additions on Rh. Initial measurements had a 400% increase from control to addition for agricultural soil and a 175% increase for desert soils. Error bars = resampled 95% CI. Acknowledgments Thank you to Dr. Thomas and Sandy Kieft for providing access to their agricultural land for soil core collection. Huge thanks to Dr. Duval for his continual support and expertise on soil science while conducting this project. I would also like to thank Natalie Herrera for her assistance sieving.