UC Davis

This dissertation investigates how management with different combinations of practices, namely mineral N fertilizer, cover cropping, compost amendment, furrow irrigation and subsurface drip irrigation have impacted soil carbon, nutrients, and microbial communities in a 25-year long term experiment. First, I investigated how subsoil carbon stocks have increased under a combination of compost and cover crops after 25 years. I found that a potential synergistic effect between soluble C, soluble N and soil pores created by cover crop roots that allowed for greater C and N transport to subsoils. This transport of soluble C and N was associated with reduced microbial stress levels, and potentially longer C residence times. Secondly, I looked at how soil microbial community structure and microbial functional potential changed after 20 years of management practices intended to improve soil health. I found significant changes in functional potential for C breakdown and N transformation, but no significant changes in microbial community structure. These changes in functional potential were associated with changes in soil health measurements and may indicate the potential impact of practices intended to improve soil health. Finally, I examined how soils under a combination of compost and cover crops changed carbon and nutrient distribution under 5 years of subsurface drip irrigation (SDI) and furrow irrigation. I found that soils under SDI and furrow irrigation only differed in carbon and nutrient distribution in the top 30 cm, and that furrow irrigation was associated with a loss of surface organic matter after 5 years while SDI was not. The differences in spatial distribution and amount of water under SDI vs furrow irrigation may not significantly reduce the amount of carbon and nutrients stored in subsoils but may have an impact on surface organic matter content. In conclusion, the impacts of management practices on soils with similar climate and mineralogy may be difficult to measure over the course of a single season, but these impacts may accumulate over multiple years to result in significant shifts in soil edaphic and microbial properties.