CAN COMPOST FROM A NITROGEN-FIXING TREE, FALCATARIA MOLUCCANA, REPLACE CHEMICAL FERTILIZER AND STORE CARBON IN AGRICULTURAL SYSTEMS?

Abstract

The challenges of food production, invasive species control, and climate change are intersecting, as they all stem from our ongoing use of land and energy on a global scale. In East Hawai‘i, two problems involving these issues are reflective of global trends. First, an expansion of agriculture is needed here, yet upland agricultural tracts are typically troubled by inherent low fertility, physically degraded and depleted of soil carbon from tillage, and require fertilizer inputs that are environmentally costly. Second, the invasive, nitrogen-fixing tree Falcataria moluccana (albizia) is dominating landscapes and altering ecosystems with rapid-cycling carbon and nitrogen inputs. These two problems are predicted to intensify with climate change, as growing conditions in each region shift and higher temperatures and carbon dioxide levels favor fast-growing, N-fixing species. Yet each of these problems could hold a remedy for the other, using practices described in the new field of climate-smart agriculture (CSA). Hawai‘i Island presents a unique opportunity to test whether or not accumulated nutrients from F. moluccana growth can benefit agricultural systems lacking in fertility, due to the intensity and grave consequences of the F. moluccana invasion, as well as the underutilization of agricultural land and lack of food self-sufficiency in Hawai‘i. This study examined whether compost from F. moluccana can replace chemical fertilizer and store carbon in agricultural lands in East Hawai‘i. Trials were conducted over one growing season and included two crops, Zea mays (corn) and Manihot esculenta (cassava), and 4 replicates across a spectrum of East Hawai‘i farmland sites representative of varying soil conditions and land use history. Treatments included a control, a typical application of chemical fertilizer (1N nitrogen applied), two levels of F. moluccana compost (1N and 2N levels of nitrogen applied), and two levels of combinations of chemical fertilizer and compost (1N and 2N). Harvest yield results showed that the F. moluccana compost was not an adequate replacement for chemical fertilizer in the corn crops. In contrast, F. moluccana compost produced cassava yields equal to chemical fertilizer, and economic and carbon costs were also similar across treatments. Variation among locations and within locations was larger than variation due to treatment in the cassava trial, but results suggest that the compost application was more effective on more degraded farm sites. Economic and carbon costs associated with the chemical fertilizer and F. moluccana compost applications were generally not different across treatments, and a partial analysis of carbon gained or lost was also similar across treatments. The results of these field trials show that CSA using F. moluccana compost is a viable alternative to chemical fertilizer, when the site is in need of organic matter (OM), and when the crop has favorable characteristics (long-season, rooting, and/or able to grow in low fertility areas). This research was designed and completed with the partnership of invasive species managers and farmers to maximize the usefulness of the research to the local community.