Document Type

Thesis

Date of Degree Completion

Summer 2019

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Dr. Carey Gazis

Second Committee Member

Dr. Susan Kaspari

Third Committee Member

Dr. Karl Lillquist

Abstract

Changing agricultural land-use practices to increase soil carbon sequestration contributes to climate change mitigation and improved food security by moving CO2 from the atmosphere into soil as soil organic carbon (SOC). In 2016, a farm in Thorp, Washington, Spoon Full Farm, began converting land historically farmed using conventional methods of tillage and synthetic fertilizers to conservation farming methods with direct seeding and organic soil amendments with a goal of sequestering carbon in the soil. This project evaluates relationships of soil CO2 respiration and net ecological exchange (NEE) with land-use types, seasonal environmental factors (air temperature, relative humidity, soil temperature and soil moisture) and soil carbon and nitrogen properties (SOC, SON, δ13C, and δ15N) on that farm in order to inform land management decisions affecting soil carbon sequestration. Three farm land-use areas studied were: 1) no-till vegetable garden with regular organic matter amendments; 2) no-till hay fields; and 3) historically unfarmed areas. Soil CO2 fluxes were measured on these three land-use areas in spring after snowmelt; summer, when garden and hay fields are irrigated and unfarmed areas are dry; and fall when soil and air temperatures are lower and moisture has returned to soils. Continuous soil CO2 flux measurements of garden soils indicate primary environmental factors influencing soil CO2 flux during summer are air and soil temperature, and during fall are soil temperature and moisture. Garden beds have positive NEE during summer and spring days indicating net CO2 losses from soil. Garden bed respiration is likely dominated by microbial decomposition of compost. Summer period soil CO2 flux correlates with SOC for all land-use types individually, while vegetable garden SOC and SON correlate with CO2 flux annually. This suggests SOC influences summer soil CO2 flux regardless of land-use type, while annual CO2 flux from composted garden soil depends on overall organic content from compost inputs. Hay field CO2 flux during summer shows strong correlation with elevated surface SOC within the crop root zone.

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