Available at: https://digitalcommons.calpoly.edu/theses/3364
Date of Award
6-2026
Degree Name
MS in Agriculture - Crop Science
Department/Program
Horticulture and Crop Science
College
College of Agriculture, Food, and Environmental Sciences
Advisor
Charlotte Decock
Advisor Department
Natural Resources Management
Advisor College
College of Agriculture, Food, and Environmental Sciences
Abstract
Nitrogen (N) leaching is the primary driver of groundwater contamination in agricultural regions. On California’s Central Coast, Agricultural Order 4.0 regulates this risk by constraining field-level N balances (N applied minus N removed). However, the assumption that unrecovered N is lost via leaching may not hold in organic and regenerative systems that promote soil organic matter accumulation. This study evaluated whether measured N leaching aligns with estimates derived from partial N budgets in organic systems, and whether management practices (intercrops, cover crops, and reduced tillage) can mitigate leaching risk. Four field trials were conducted to investigate N dynamics in organic systems, tracking N inputs, crop uptake, and soil mineral N, as well as measured N leaching in trials 2 and 3. Experimental design differed for each trial which allowed us to address our overarching research questions from different perspectives. Overwintering canola cover crops reduced N leaching by 72.17% compared to the bare ground control, though effects of intercrops on N dynamics in organic systems remain less clear. Across treatments, residue-derived leaching averaged 20.2% of total residue N, indicating high soil retention and/or possible denitrification. Finally, measured N leaching was lower, both during the broccoli crop’s season (10.19 ± 2.32 lbs N acre⁻¹) and in the five months that followed residue incorporation under bare ground management (10.71 ± 4.50 lbs N acre⁻¹), than prediction by partial N budgets representative of current regulations (48.7 lbs N acre⁻¹). Our results suggest that N retention in organic annual systems may, under certain conditions, exceed assumptions embedded in Agricultural Order 4.0, raising the possibility that current mass balance frameworks could overestimate N leaching, particularly in systems using pelletized fertilizers and practices that build soil organic matter. Overwintering cover crops further reduced leaching risk. These findings support adapting regulatory approaches to better reflect N cycling in organic and regenerative systems.