Effect of Organic Amendment Application Rate on Greenhouse Gas Emissions at an Organic Farm in Santa Barbara County, California

Abstract:

Though greenhouse gas emissions (GHGEs) from mineral fertilizer application in agriculture have been well studied, the effect of organic amendment (OA) application rate on GHGEs is not yet understood. Application of multiple OAs can improve different properties that control soil fertility, including nutrient availability, aggregate stability, and water-holding capacity. We measured nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄) flux at an organic farm in Goleta, CA in order to understand how OA application rate affects GHGEs and crop yield from agricultural soils. Based on management practices in the region, we asked farm managers to establish high compost (HC) and low compost (LC) treatments during the growing season of an annual crop (18.2, 9.13 Mg ha^-1, respectively), and we measured GHGEs in beds and furrows using static chambers. Organic fertilizer (672 kg ha^-1) was applied equally to HC and LC beds six weeks after compost application. Overall, emissions of N₂O and CO₂ were higher in HC than LC, but yield-scaled emissions were higher in LC. Importantly, treatment differences in both N₂O and CO₂ emissions were not apparent until after mid-season fertilizer application. Net CH₄ uptake was higher in HC than LC in the furrows, but there was no difference in the beds. Our data suggest that high compost application rates likely increased SOM mineralization, soil water content, and nitrification and denitrification rates in HC relative to LC, which led to higher N₂O emissions during the growing season. Fertilization primed SOM decomposition and increased soil respiration, which led to increased CO₂ emissions. Our results suggest that improved management of application rate and timing during use of multiple OAs could reduce GHGEs while maintaining high crop yield. Understanding the mechanisms by which OA application rates alter the balance between GHGEs and yield is an important step toward reducing agriculture’s contribution to climate change through policy.