Synergistic Nitrogen Fixation in Indigenous Hawaiian Agriculture: Sugarcane (Saccharum officinarum) Inoculation as a Catalyst for Nitrogen Sovereignty

Abstract

Hawaiʻi imports 85–90% of its food and applies the majority of its nitrogen as synthetic fertilizer, despite a pre-contact agricultural record in which a population of nearly 800,000 was sustained on roughly a quarter of the archipelago's land area without external inputs. Indigenous Hawaiian colluvial systems integrated sugarcane, ʻulu, and kukui residues across stratified polycultures, suggesting that combinations of these species supplied biologically fixed nitrogen at a scale sufficient to support continuous cultivation. This thesis tests whether asymbiotic nitrogen fixation associated with sugarcane (Saccharum officinarum) decomposition is enhanced, suppressed, or unaffected by co-mulching with ʻulu (Artocarpus altilis) and kukui (Aleurites moluccanus), and identifies the mechanisms governing any non-additive response. Mulch mixtures spanning pure monocultures (CCCC, UUUU, KKKK) and four-species blends (CCUU, CCKK, CCKU, KUUU) were incubated in field litterbags for 275 days, with periodic acetylene reduction assays, cumulative nitrogen accounting via trapezoidal integration, and stoichiometric tracking of C:N decay. A two-way ANOVA revealed a significant Treatment × Day interaction (F = 19.56, p < 2 × 10⁻¹⁶), and a linear model identified the proportion of sugarcane in the mixture as the strongest predictor of cumulative nitrogen yield (β = 2.03, R² = 0.62, p = 0.0016), with non-additive residual analysis isolating CCUU as uniquely synergistic (+0.338 gN/kg above expectation) and KUUU and CCKU as antagonistic. Mechanistically, sugarcane functions as an electron-donor substrate for nitrogenase, while ʻulu enables the microbial colonization that retains fixed nitrogen against leaching; the spatial separation of kukui- and ʻulu-dominated agroforests in traditional colluvial systems mirrors the antagonism observed experimentally.