Relating soil properties and fallowing length to nitrate reduction in loʻi kalo (flooded taro agroecosystems) for regenerative nitrogen management.

Commodities

• Vegetables: taro

Practices

• Crop Production: fallow, nutrient cycling, nutrient management
• Education and Training: extension, farmer to farmer, focus group, on-farm/ranch research, workshop
• Natural Resources/Environment: wetlands
• Production Systems: agroecosystems, holistic management
• Soil Management: green manures, nutrient mineralization, organic matter, soil analysis, soil chemistry, soil microbiology, soil quality/health
• Sustainable Communities: food sovereignty

Kalo (taro, Colocasia esculenta) is an ancestor to Kānaka ʻŌiwi (Indigenous Hawaiians) and a keystone component of Hawaiian social-ecological systems. Loʻi kalo (wetland taro agroecosystems) require deliberate nitrogen management for sustaining crop productivity amid numerous loss pathways. Dissimilatory nitrate reduction to ammonium (DNRA) and denitrification (DNF) are microbially-mediated pathways that compete over nitrate under flooded soil conditions. Nitrate is lost as nitrogen gas through DNF, and conserved as ammonium through DNRA. DNRA-DNF competition is driven by the ratio of soil-organic-carbon-to-nitrate (SOC:NO₃^-). SOC and NO₃^- are mediated by soil management practices. Fallowing is a regenerative practice that purportedly benefits soil and crop outcomes. However, the direct effects of fallowing on soil and crop outcomes are understudied, and the relationships between fallow length, SOC:NO₃^-, and rates of DNRA and DNF have not been assessed in loʻi kalo soils. Guided by grower priorities and ʻŌiwi agronomic epistemology, the objective of this project is to evaluate the relationships between soil properties, fallow length, and rates of nitrate reduction in loʻi kalo by (1) quantifying rates of DNRA and DNF at different levels of SOC:NO₃^–, and (2) comparing rates of DNRA and DNF across increasing fallow lengths. Insights from this project will help growers understand the biogeochemical effects of fallowing, enhance ecosystem health by preventing nitrogen from becoming harmful airborne and waterborne pollution, reduce costs by minimizing nitrogen fertilizer waste, and increase productivity by optimizing SOC:NO₃^- levels and fallow lengths to maximize nutrient conservation or simply keep loʻi in production.

Research Objectives:

• Quantify rates of DNRA and DNF in loʻi kalo soils and model their relationship with SOC:NO₃^–; and 
• Compare rates of DNRA and DNF across increasing fallow lengths.

Educational Objectives:

• Facilitate the exchange of knowledge about contemporary management practices in loʻi kalo by facilitating Talk Story sessions with kalo growers;
• Empower kalo growers to test their soils, engage with soil reports, and inform their management decisions based on soil reports by providing Soil Report trainings and publishing an accompanying extension publication; and
• Create a practical, relevant, and accessible tool for kalo growers to track management practices over the entire crop cycle.