2013 Annual Report
The Maui project field team continued to collect data on soil moisture, light interception, biomass/yield, and leaf area for the primary field locations and different crops to monitor datasets for a series of plant and soil parameters as necessary for model simulation. Scheduled harvests for all crops occurred regularly with the field team’s participation. Tetracam images and crop scan of field sites were collected biweekly in concert with the remote sensed satellite data for calibration and routine maintenance and troubleshooting were conducted. The two Eddy flux towers were maintained and cleaned, adjusted, and troubleshot as needed. The field team also provided support for visiting ARS researchers, primarily collected soil and plant samples, and sensor calibration, and provided other services as requested.
A one-year study of the effect of deficit irrigation and grass species (Napier grass and sugarcane) concluded. The objectives of this study were two-fold: to identify the effects of deficit irrigation on greenhouse gas (GHG) production, root biomass, and soil carbon storage on a short-time scale in order to provide management direction for plantation operations on Maui, and to quantify and assess the long-term implications for biofuel production in Hawaii by evaluating different crop scenarios based on their ecosystem carbon budgets and greenhouse gas index.
Average exchanges were 112 and 156 milligrams carbon dioxide per square meter per hour, -7.81 and -9.43 micrograms of methane per square meter per hour, and 1.46 and 2.48 milligrams of nitrous oxide per square meter per hour for sugar cane and Napier grass, respectively. Methane exchanges were predominantly negative during the study period, which suggest that agricultural soils on Maui are a sink for atmospheric methane. From a global-warming potential perspective, this sink was approximately equal to the source of nitrous oxide measured on an annual basis. Application of N based fertilizer caused significant increases in the rate of nitrous oxide emissions from both species, but within five days returned to baseline. There was a significant increase in soil carbon stock in the top 0-40 cm across the field for both species, however no difference in root biomass or change in carbon stock occurred between species. While short-term quantification of these gases suggested reduced soil-based emissions from the sugarcane crop, long-term analysis found all sugarcane scenarios to be a net source of carbon to the atmosphere whereas Napier scenarios were net sinks. These findings will help guide management to adopt irrigation, fertilizer, and cropping practices that benefit long-term sustainability of these bioenergy systems in Hawaii.