Application of organic amendments (OAs) has the potential to act as a greenhouse gas (GHG) emission mitigation strategy and increase soil carbon (C) storage in row crop agriculture. However, OA application may stimulate an increase in emissions of nitrous oxide (N2O), methane (CH 4), and carbon dioxide (CO2). To determine how compost application rate affects both GHG emissions and crop yield, a field study was conducted at an organic vegetable farm in Santa Barbara County (SBC), California. Additionally, a targeted survey of compost-using farmers (n = 14) in SBC and adjacent counties was conducted to understand OA application practices of compost users in the region. In our field experiment, farm managers were asked to apply their normal, high compost (HC) treatment of 18.2 Mg ha -1 and a low compost (LC) treatment of 9.1 Mg ha-1 immediately prior to seeding of carrots (Daucus carota subsp. sativus).

A commercial organic fertilizer (OF) was applied to beds of both HC and LC treatments 43 days after compost application (DAC) at the rate of 672 kg ha-1. N2O, CO2, and CH 4 emissions from static flux chambers, along with soil nitrate (NO 3-), soil ammonium (NH4 +), gravimetric soil moisture, and soil temperature in beds and furrows of each treatment, were measured approximately once weekly for the entire 95 day carrot growing season. Soil total C, total N, bulk density, organic matter, pH, plant tissue C and N, and crop yield were also measured during the study. Seasonal N2O and CO2 emissions were higher in HC than LC in beds (p < 0.001 N2O, p = 0.035 CO2); seasonal net uptake of CH4 was not different in beds (p = 0.901), but was significantly higher in LC than HC in furrows (p = 0.027), though CH4 fluxes were often highly variable. Significantly higher emissions of N2O and CO2 in HC beds than LC beds were measured only after OF application (except CO2 5 DAC).

Crop yield was higher in HC (2.82+/-0.21 Mg ha-1) than LC (2.17+/-0.06 Mg ha-1). Yield-scaled global warming potential (GWP), an indicator of agronomic efficiency, was not significantly different between treatments. CO2 was not included in GWP due to its biogenic origin and an inability to accurately account for net soil C loss during this short time period. A higher amount of biochemically available organic matter and soil moisture in HC than LC may have led to higher decomposition and denitrification rates following OF application, suggesting that C storage benefits of OA application can be reduced by fertilizer addition. Because GWP per unit crop yield was not different between LC and HC, HC may better concurrently address concerns regarding atmospheric GHG emissions and global food demand. Our survey results indicate that average compost application rate in and around SBC (12.45+/-6.54 Mg ha-1) was lower than HC in our study.

While farmers understood the benefits of compost for long-term soil fertility and C storage, economic limitations to increasing compost application rate exist within this region of major crop production. To determine whether OA application affects decomposition of native soil organic matter for more accurate GWP calculation, future studies would benefit from including a control treatment and identifying origins of CO2-C emissions, potentially through use of isotopic labeling. New insights about the effects of OA application rate on GHG emissions can aid in assessment of OA use in agricultural C sequestration or GHG mitigation policies.