Nitrogen (N) is known to be the most essential nutrient for plant growth and can greatly limit crop production. In Hawai’i, N is not readily available in soils, and therefore must be ‘fixed’ through either atmospheric or biological N fixation. Pre-contact Hawaiian dryland agroforestry is an example of a highly productive agricultural system that utilized biological N-fixation to supplement N in soils through the addition of mulch. While it is known Hawaiians were using sugarcane leaf litter to maintain soil nutrients, an outstanding question remained: How did leaf litter from the other various plants grown in the agroforestry system contribute to N availability? We carried out a two part experiment to address this question. We gathered leaf litter from three agroforestry crops and deployed them in a field plot in various treatments in mesh bags. Part one of this experiment used the Acetylene Reduction Assay (ARA) method to investigate N-fixation of leaf litter. Part two of the experiment used an Elemental Analyzer to investigate the decomposition rate and elemental composition of leaf litter. This research was conducted over a 13-month timeline, emulating the growing season of taro (Colocasia esculenta), a staple crop in pre-contact Hawaiian agriculture.
Results/Conclusions
Our preliminary results show decomposition rates differed significantly between mulch samples comprised of a single crop leaf litter and mulch samples comprised of a mixture of different crops leaf litters. In general, mulch samples comprised of a mixture of different crops leaf litters had quicker decomposition rates and released more N throughout the course of the experiment. N-fixation peaked at different time periods and was sustained at different rates depending on the type of treatment. Most mulch treatments exhibited relatively high overall rates of N-fixation. Using industry set N requirements for productive taro growth, along with N-fixation rates for all mulch treatments, we calculated that biological nitrogen fixation from mulch was an important contribution to pre-contact Hawaiian agriculture. Moreover, our calculations demonstrate the potential mulch could have in reducing intensive fertilizer regimes used on taro fields today. The implications of our findings would contribute to the understanding of how N was managed sustainably in a large field system, which would ultimately help derive a new N fertilization regime in Hawai'i and globally. Such an advancement could lead to the proliferation of sustainable agroforestry farming practices that support biodiversity, reduce environmental externalities, and support farmers from many different backgrounds.