Nitrogen Dynamics and Sweet Potato Production under Indigenous Soil Moisture Conservation Practices in the Leeward Kohala Field System, Hawai’i Island

Abstract

Intensive cultivation of ‘uala (sweet potato) in the Leeward Kohala field system on Hawai’i Island supported substantial populations of Native Hawaiians prior to its abandonment in the 19th century. Productivity is influenced by the heterogeneity of the climate and biogeochemical soil characteristics across the substantial ecological gradient. Agricultural infrastructure and associated practices were developed to manage crop production eventuating from the variation in inter- and intra-annual rainfall. Mechanisms of sustaining soil nitrogen (N) are still unclear; however, a pronounced source of N is soil organic matter decomposition. This study investigated in situ the effects of indigenous Hawaiian mulching practices against two control treatments on soil moisture and temperature dynamics to facilitate N mineralization in soil mounds cultivated with ‘uala. Field experiments were set in two agricultural restoration plots with distinct climatic and soil characteristics. Data included soil moisture and temperature, soil and plant N, growth and development of ‘uala, and real-time weather data. Concurrently, N mineralization was also investigated under controlled conditions. All indigenous mulching treatments were found to significantly increase soil moisture, regulate temperature variation, and improve N availability compared to control. Differences in soil properties between treatments translated to significant differences in above-ground biomass. The data suggest that these differences would extend to tuber production, but the use of a long-gestation variety limited tuber production in the study. Increased temperatures in the controlled experiments were observed to increase inorganic N significantly, but less substantially than soil moisture. Indigenous practices in the region could have greatly mitigated plant stress due to moisture, temperature, and N availability, increasing productivity and reducing the variability of the Leeward Kohala Field System.

Microclimatic Effects of Kō (Sugarcane, Saccharum officinarum) Row Planting in the Leeward Kohala Field System, Hawai‘i Island

“…Areas with differently spaced kuaiwi would probably feature different optimal zones for ‘uala cultivation. The yields in this transect’s optimum zone (3.8–6.0 kg m –2 ) compared favorably to those reported by Kagawa and Vitousek (2012) (1.8–4.0 kg m –2 ) and Marshall et al (2017) (0.4–1.6 kg m –2 ) even without the use of rock or litter mulches that have proven highly effective at Puanui (Sirabis et al 2022). The high yields we achieved can probably be attributed to higher-than-average rainfall during the growth period.…”

“…The farthest downwind plot was located immediately upwind of the third k o row. Soil in the plots was mixed thoroughly to ~30 cm and mounded before four 30-40 cm 'uala cuttings of the Lanikeha variety were buried in the sides of the mounds (Marshall et al 2017;Sirabis et al 2022). 'Uala tubers were harvested and immediately weighed on May 9, 2019.…”

“…While indigenous Hawaiian agricultural knowledge will never be fully recovered, researchers and land stewards have sought to restore productivity to Hawaii's dryland field systems by combining archaeology, ethnography, and field experimentation to revitalize traditions of agricultural innovation (Kagawa and Vitousek 2012;Marshall et al 2017;Sirabis et al 2022).…”