Earth System Science

Sugarcane has been a major agronomic crop in Hawaii with an unique, high-yield, two-year production system. However, parameters relevant to advanced, cellulosic biofuel production, such as net ecosystem productivity (NEP) and radiation use efficiency (RUE), have not been evaluated in Hawaii under commercial production. Recent demand potential has rekindled interest in Hawaiian grown biofuels; as such, there is a need to understand productivity under changing climate and agronomic practices. To this end, we established two eddy covariance towers in commercial sugarcane fields in Maui, Hawaii to evaluate the carbon balance and RUE of sugarcane under contrasting elevations and soil types. We combined the tower observations with biometric and satellite data to assess RUE in terms of net biomass accumulation and daily gross primary production. High, sustained net NEP was found in both fields (cumulative NEP 4.23–5.37 103 g C m2 over the course of the measurement period). Biomass RUE was statistically similar for both fields (1.15–1.24 g above ground biomass per MJ intercepted solar irradiance).  Carbon accumulated in both fields at nearly the same rate with differences in cumulative biomass due to differing crop cycle lengths; cumulative gross primary productivity and ecosystem respiration were higher in the lower elevation field. Contrary to previous studies in Hawaiian sugarcane, we did not see a large decrease in NEP or increase in ecosystem respiration in the 2nd year, which we attributed to suppressed decomposition of dead cane stalks and leaves due to drip irrigation and drought. Biomass RUE
also showed little decline in the 2nd year. The results show that Hawaiian sugarcane has a higher productivity than sugarcane grown in other regions of the world and also suggests that a longer (>12 months) growing cycle may be optimal for biomass production.

Sugarcane has been a major agronomic crop in Hawaii with an unique, high-yield, two-year production
system. However, parameters relevant to advanced, cellulosic biofuel production, such as net ecosystem
productivity (NEP) and radiation use efficiency (RUE), have not been evaluated in Hawaii under
commercial production. Recent demand potential has rekindled interest in Hawaiian grown biofuels; as
such, there is a need to understand productivity under changing climate and agronomic practices. To this
end, we established two eddy covariance towers in commercial sugarcane
fields in Maui, Hawaii to
evaluate the carbon balance and RUE of sugarcane under contrasting elevations and soil types. We
combined the tower observations with biometric and satellite data to assess RUE in terms of net biomass
accumulation and daily gross primary production. High, sustained net NEP was found in both
fields
(cumulative NEP 4.23–5.37


103 g C m2 over the course of the measurement period). Biomass RUE was
statistically similar for both
fields (1.15–1.24 g above ground biomass per MJ intercepted solar irradiance).
Carbon accumulated in both
fields at nearly the same rate with differences in cumulative biomass due to
differing crop cycle lengths; cumulative gross primary productivity and ecosystem respiration were
higher in the lower elevation
field. Contrary to previous studies in Hawaiian sugarcane, we did not see a
large decrease in NEP or increase in ecosystem respiration in the 2nd year, which we attributed to
suppressed decomposition of dead cane stalks and leaves due to drip irrigation and drought. Biomass RUE
also showed little decline in the 2nd year. The results show that Hawaiian sugarcane has a higher
productivity than sugarcane grown in other regions of the world and also suggests that a longer
(>12 months) growing cycle may be optimal for biomass production.

Standardized reference evapotranspiration (ET) and ecosystem-specific vegetation coefficients are frequently used to estimate actual ET. However, equations for calculating reference ET have not been well validated in tropical environments. We measured ET (ETEC) using eddy covariance (EC) towers at two irrigated sugarcane fields on the leeward (dry) side of Maui, Hawaii, USA in contrasting climates. We calculated reference ET at the fields using the short (ET0) and tall (ETr) vegetation versions of the American Society for Civil Engineers (ASCE) equation. The ASCE equations were compared to the Priestley–Taylor ET (ETPT) and ETEC. Reference ET from the ASCE approaches exceeded ETEC during the mid-period (when vegetation coefficients suggest ETEC should exceed reference ET). At the windier tower site, cumulative ETr exceeded ETEC by 854 mm over the course of the mid-period (267 days). At the less windy site, mid-period ETr still exceeded ETEC, but the difference was smaller (443 mm). At both sites, ETPT approximated mid-period ETEC more closely than the ASCE equations ((ETPT-ETEC) < 170 mm). Analysis of applied water and precipitation, soil moisture, leaf stomatal resistance, and canopy cover suggest that the lower observed ETEC was not the result of water stress or reduced vegetation cover. Use of a custom-calibrated bulk canopy resistance improved the reference ET estimate and reduced seasonal ET discrepancy relative to ETPT and ETEC in the less windy field and had mixed performance in the windier field. These divergences suggest that modifications to reference ET equations may be warranted in some tropical regions.

The renewed interest in the use of sugarcane (Saccharin officinarum L.) for biofuel could provide a viable market for potential Hawaiian
sugarcane feedstock producers. In Hawaii, sugarcane is grown as an irrigated 2-yr cycle crop. There is however little information
on crop parameter attributes of 2-yr cycle sugarcane. This field study on Maui, Hawaii, analyzed the relationship between sugarcane
biomass accumulation and specific crop parameters. Overall, the high dry biomass yield (80.20 Mg ha–1) was the result of a high leaf
area index (LAI, 7.50) and radiation use efficiency (RUE, 2.06 g MJ–1. The crop growth rate was highly correlated to LAI (R2, 0.86),
and a light extinction coefficient (k) of 0.53 was estimated. Stalk density was estimated at 18 stalks m–2, with a maximum plant height
of 3.6 m, and a rooting depth exceeding 2.0 m. When the crop parameters were incorporated into a biological model of Agricultural
Land Management Alternatives with Numerical Assessment Criteria (ALMANAC) the model accurately simulated sugarcane yields
across seven different soil types and multiple management scenarios of applied irrigation water, N and P fertilizer inputs and various
planting and harvest dates. The mean simulation percent (%) errors ranged from –6.4% to 1.8%, while the calculated Fisher’s paired
t test of 1.41 with 39 degrees of freedom, showed no significant differences (P ≥ 0.05) between measured and simulated yields. The
ALMANAC model should be useful as a decision support tool for evaluating sugarcane management alternatives that maximize yields
while optimizing water, N and P inputs.

Sugarcane has been widely used as a biofuel crop due to its high biological productivity, ease of conversion to ethanol, and its relatively high potential for greenhouse gas reduction and lower environmental impacts relative to other derived biofuels from traditional agronomic crops. In this investigation, we studied four sugarcane cultivars (H-65-7052, H-78-3567, H-86-3792 and H-87-4319) grown on a Hawaiian commercial sugarcane plantation to determine their ability to store and accumulate soil carbon (C) and nitrogen (N) across a 24-month growth cycle on contrasting soil types. The main study objective establish baseline parameters for biofuel production life cycle analyses; sub-objectives included (1) determining which of four main sugarcane cultivars sequestered the most soil C and (2) assessing how soil C sequestration varies among two common Hawaiian soil series (Pulehu-sandy clay loam and Molokai-clay). Soil samples were collected at 20 cm increments to depths of up to 120 cm using hand augers at the three main growth stages (tillering, grand growth, and maturity) from two experimental plots at to observe total carbon (TC), total nitrogen (TN), dissolved organic carbon (DOC) and nitrates (NO−3) using laboratory flash combustion for TC and TN and solution filtering and analysis for DOC and NO−3. Aboveground plant biomass was collected and subsampled to determine lignin and C and N content. This study determined that there was an increase of TC with the advancement of growing stages in the studied four sugarcane cultivars at both soil types (increase in TC of 15–35 kg·m2). Nitrogen accumulation was more variable, and NO−3 (<5 ppm) were insignificant. The C and N accumulation varies in the whole profile based on the ability of the sugarcane cultivar’s roots to explore and grow in the different soil types. For the purpose of storing C in the soil, cultivar H-65-7052 (TC accumulation of ~30 kg·m−2) and H-86-3792 (25 kg·m−2) rather H-78-3567 (15 kg·m−2) and H-87-4319 (20 kg·m−2) appeared to produce more accumulated carbon in both soil types.