Nitrogen dynamics and sweet potato production under indigenous soil moisture conservation practices in dryland field systems of Puanui, Kohala, Hawaiʻi Island

Sirabis, William Constantine Lokei
Lincoln, Noa K.
Tropical Plant and Soil Sciences
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Agriculture productivity in the Puanui dryland field system in Kohala, Hawaiʻi Island, is influenced by the heterogeneity of the climate and specific biogeochemical soil characteristics across the ahupuaʻa. Intensive cultivation supported substantial populations of Native Hawaiians prior to the ahupuaʻa’s abandonment in the 19th century. Development of multiple farming methods and infrastructure were necessary to manage crop production as a result of the variation in inter- and intra- annual rainfall. Mechanisms of sustaining soil nitrogen (N) are unclear; however, a pronounced source for crop N is from soil organic matter decomposition through microbial activity. I therefore investigated in-situ, the effects of indigenous Hawaiian mulching practices on soil moisture, and temperature, dynamics that facilitate mineralization of soil organic matter to improve inorganic N in soil mounds cultivated with a late-maturing heirloom sweet potato (Ipomea batatas) variety - Lanikeha. Treatments included rock mulch, sugarcane leaf mulch and a combination of rock-sugarcane leaf mulch, whilst no mulch and an irrigation application were controls. Field experiments were set in two agricultural restoration māla (garden) having distinct climatic and soil characteristics, and managed by Ulu Mau Puanui; a local non-profit organization that functions to research and educate individuals and groups, the prominence of the ahupuaʻa of Puanui. Data evaluated included soil moisture and temperature, inorganic N, growth and development of sweet potato and real-time weather data. Treatments comprising rock mulch conserved the highest soil moisture among indigenous management practices. Irrigation treatment recorded more moisture overall, though measures of inorganic N were observed to be the least relative to alternate practices. Hourly air temperature dominated soil temperature, nonetheless mulching lowered measures during warmer parts of the day. Despite differences in production of above ground biomass of sweet potato between treatments, storage tuber yields were statistically similar, and negligible. Excessive pencil tuber production was prevalent across the field experiment. The characterized below ground production is believed to be associated to the premature harvest of the crop, and agronomic characteristic. Investigated also was the response of N mineralization under controlled soil moisture and temperature conditions. Increased temperatures observed enhanced inorganic N, compared to enhanced soil moisture.
Agronomy, Agriculture, Soil sciences, Hawaiʻi indigenous practices, Inorganic Nitrogen, Leeward Kohala field systems, Soil moisture conservation, Sweet potato
101 pages
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