Designing a Regenerative Systems Approach for Sustainable Turmeric Production

Detailed results including tables, graphs and figures are presented in the attached bulletin. Results from this study underscore one of the major advantages of employing mixed species cover crops in agroecosystems. As expected, cover crop species that are already known to be well-adapted to Hawaiʻi, specifically sudangrass, sunn hemp, cowpea and radish, outperformed species not widely utilized in Hawaiʻi, but recommended by mainland suppliers, such as millet, chia and sunflower. Planting a mixture of sudangrass, sunn hemp, and radish stood out as a strong candidate for the most successful cover crop mix in terms of generating cover crop biomass with C:N ratios < 20:1, which can support net nitrogen mineralization. However, treatment 1 did not outperform treatments 3, 4, and 5, each of which contained only 2 dominant cover crop species, yet exhibited numerically higher total percent coverage. Notably, treatment 3, consisting of sudangrass, sunn hemp, and chia (a weak broadleaf), exhibited numerically highest % cover crop and lowest % weed among treatments 1-5. Contrary to our initial hypothesis based on the ‘Three Sisters’ concept, this outcome suggests that a mixture’s overall success does not necessarily rely on the presence of a greater number of robust species, and two species may be sufficient. However, our findings do not suggest that a lesser number of species in a mixture guarantees a superior outcome, as exemplified by the suboptimal performance observed in treatment 2, which only included 1 well-adapted species (cowpea). 

Intraspecific and interspecific competition may have influenced the outcomes observed in this study. Treatments with only 2 strong cover crop species were observed to have initial germination and establishment, which might have eased competitive pressures due to more available space. To mitigate competition and encourage maximum provision of ecosystem services, seeding rates can be adjusted based on the success of each species (Bybee-Finley et al., 2022). As seeding rates can be a barrier for producers when it comes to cover crop implementation, determining optimal seeding rate becomes crucial for ensuring ecosystem functionality while minimizing production expenses (Bybee-Finley et al., 2022). Prior research indicates that decreasing seeding rates for highly competitive species, while keeping rates for less competitive species near monoculture rates, could mitigate competition in cover crop mixtures (White et al., 2017). 

Nematodes are recognized as valuable bioindicators of soil health (Ferris et al., 2001). Results presented here indicated better soil food web structure with higher SI, EI, and CI in the bare ground control compared to all the cover crop mix treatments.  

Our results may have been shaped by two key factors. First, cover crop biomass was not left to decompose in cover crop alleys where soil samples were taken. Instead, the cover crop biomass was blown into the turmeric ridges. It is unclear if the cover crop biomass blown into the turmeric rows benefits turmeric growth, since it was not part of the measurements taken in this study. This would have lessened the benefits of cover cropping, let alone cover cropping mixtures, on soil health. Second, the bare ground control alleys, which were covered by black plastic weed mat, were not subjected to tillage between trials, unlike the cover crop treatment alleys where cover crop residues were disk plowed and seeds were drilled into the soil. In addition, the use of wood chips in the field prior to covering the bare ground control alleys with weed mat resulted in high carbon inputs with slow decomposition rate, thus potentially allowing for a more structured soil food web due to a sustained nutrient availability from the wood chips. Consequently, the control treatment may have had more carbon inputs and less disturbance, leading to a better soil ecosystem. 

Bacterivores and fungivores are essential biological elements in the soil nutrient cycle, facilitating the release of soil nutrients (such as nitrogen) from bacteria and fungi, and promoting mineralization (Ingham et al. 1985). Conversely, predators and omnivores indirectly affect nutrient cycling via predation of bacterivores and fungivores, thus enhancing overall soil health and productivity (Yeates & Wardle, 1996). Interestingly, our results indicate that predator abundance was positively associated with % legume cover. This is consistent with prior research conducted by Wang et al. (2011), which documented that predatory nematode abundance was augmented by sunn hemp when grown in greenhouse trials. Fungivore abundance, conversely, was positively correlated to % broadleaf cover in this study. 

Relating nematode indices against cover crop cover percentages suggested that in our tested agroecosystem, the cover crop alleys did not optimize nutrient enrichment (EI) or soil food web structure (SI). This outcome may be a consequence of tilling and the removal of cover crop residues. This result emphasizes the importance of incorporating cover crop biomass directly into the soil prior to subsequent cash crop planting in order to benefit from cover cropping. While our cover crop alleys did allow the root systems of the cover crop alleys to sustain beneficial microorganisms within the root rhizosphere, the incorporation of green manure, or keeping the cover crop biomass in place, is an indispensable aspect of cover cropping.