The total quantity of P and plant‐available P often differ greatly in soils of the tropics, which typically range in weathering intensity. Assessing available P is fundamental to managing P in many of these soils. Phosphorus availability in some soils has been inferred from the Hedley sequential extraction assuming that each P fraction reflects similar plant availability in different soils. However, experimental measurements of plant P availability were either of short duration or involved multiple P applications, which complicates assessment of the plant availability of P fractions. The objectives of this study were to examine the changes in P fractions under exhaustive cropping on diverse soils and to discern the differences in plant availability among P fractions. Eight soils ranging in weathering from Vertisols and Mollisols to Ultisols and Oxisols were amended with Ca(H2PO4)·H2O to raise soil solution P to 0.2 mg L−1 and planted for 14 crops to remove available P. The results indicated that the Fe‐impregnated strip–P and inorganic NaHCO3–P (NaHCO3–Pi) decreased the most in response to plant P withdrawal in all soils. The inorganic NaOH‐P (NaOH‐Pi) also declined with plant P uptake in all soils. The HCl‐P and residual P seemed to act as a buffer for the strip‐P and the NaHCO3–Pi in the slightly weathered soils, whereas NaOH‐Pi seemed to act as a buffering pool for strip‐P and NaHCO3–Pi in the highly weathered soils. Residual P in the slightly weathered soils was plant‐available on a relatively short time scale. In contrast, residual P in the highly weathered soils accumulated in the presence of intensive plant P removal, indicating that it was unavailable to plants. Organic P (NaHCO3‐ and NaOH‐Po) fractions were not significant contributors to available P in these soils that received high levels of inorganic P. Phosphorus fractions separated by the same sequential method were not of equal availability to plants in all soils.

Because the duration of growth for commercial sugarcane (Saccharum spp. hybrids) production can vary from 9 to 36 mo, determining the optimum age at crop harvest is important to profitability. To account for variable climate across seasons and locations, there is a need to understand the physiology of yield accumulation and quantitatively describe the effects of crop age on productivity. Few field studies have been conducted to determine the factors responsible for yield variation in different cultivars of sugarcane at different crop ages, and the physiology of yield accumulation has rarely been examined on a dry matter basis with all yield components, including tops, millable stalk, trash, and roots. This paper compares above‐ and belowground biomass accumulation with crop age in two current cultivars grown in field experiments under drip irrigation in Hawaii from 1991 to 1993, and reanalyzes earlier experiments conducted in Hawaii in the 1930s and 1940s to examine historical changes in the pattern of yield accumulation in sugarcane. The key findings from this analysis are that (i) differences in yield accumulation during the first 12 mo of growth were not necessarily reflected in final yields at harvest at 18 to 24 mo; (ii) yield accumulation was less efficient in the second year of growth for current cultivars, but not necessarily so for older cultivars; (iii) belowground biomass decreased from 17% of total biomass at 6 mo to 11% of total biomass from 12 to 24 mo; (iv) there was no indication that older cultivars were less productive than current cultivars; and (v) yields rarely increased beyond 18 mo of age.

Intercepted radiation is a major driving variable of crop production under high‐input irrigated conditions. Quantitative information on the utilization of radiation in yield accumulation allows extrapolation beyond the current season and location, and when this information is incorporated into crop growth simulation models, the effect of crop age on the productivity of different cultivars can be examined under different climatic conditions. This paper examines the differential performance of high‐yielding sugarcane (Saccharum spp. hybrids) crops in terms of the amount of short‐wave solar radiation intercepted (Si) and the efficiency of use of intercepted radiation (RUE) in biomass production. Biomass accumulation during the 12‐ to 24‐mo crop cycle was examined for two experiments conducted in Hawaii, and three experiments conducted in tropical Australia from 1991 to 1993. The analysis showed that (i) RUE was much less for growth after 12 mo than in the first 12 mo; (ii) maximum RUE of sugarcane approaches 2.0 g MJ−1; (iii) biomass accumulation beyond 12 mo was not related directly to radiation utilization; and (iv) cultivars differed in S19 but differences in RUE could not be unequivocally assessed due to the confounding effect of variable recovery of trash in biomass estimates. It is concluded that stalk death and consequent biomass loss are important factors contributing to yield variation in sugarcane crops growing for 12 to 24 mo, with a yield plateau occurring at variable crop ages during the second year of growth.

We examined associations between riparian canopy cover, presence or absence of cattle, rainfall, solar radiation, month of year, dissolved oxygen, turbidity, salinity, and Enterococcus concentrations in riparian surface soils with Enterococcus geometric mean in-stream water concentrations at Waipā watershed on the north side of the Hawaiian island Kaua’i. Each 1% decrease in riparian canopy cover was associated with a 4.6 most probable number (MPN)/100 ml increase of the geometric mean of Enterococcus in stream water (P < 0.05). Each unit decrease in salinity (ppt) was associated with an increase of Enterococcus by 68.2 MPN/100 ml in-stream water geometric mean concentrations (P < 0.05). Month of year was also associated with increases in stream water Enterococcus geometric mean concentrations (P < 0.05). Reducing riparian canopy cover is associated with Enterococcus increases in stream water, suggesting that decreasing riparian vegetation density could increase fecal bacteria surface runoff.

The role feral pigs (Sus scrofa) as a source of fecal contamination in Pacific Island ecosystems is not well understood. This study investigated the effects of feral pigs on enterococci (ENT) in runoff and soils of a Hawaiian forest. Seven sites were established with paired fenced/unfenced runoff plots in the Manoa watershed. Runoff was collected monthly from these plots after rain events from June 2008 to April 2009; soil ENT at each plot were also quantified. ENT in runoff were highly variable ranging from below the detection limit to >4.38 log 10 most probable number (MPN) 100 mL −1 . A repeated measures ANOVA found no overall fencing effects. This ANOVA did reveal a month by site interaction, indicating that while ENT in runoff were the highest in the wet season, this was not consistent across all sites. Soil ENT ranged from 14 to 511 MPN g −1 and differed among sites but not between fencing treatments. The only variables that were significantly correlated to ENT in runoff were runoff volume and soil ENT; slope, throughfall, soil moisture, bare soil cover, and total suspended solids in runoff were not correlated with ENT. While concentrations of ENT in runoff were highly variable across the months and sites, these forested headwaters did serve as sources of ENT to downstream ecosystems throughout the year. To minimize effects on human health, we recommend that public authorities employ greater warnings (i.e., signage) at streams and beaches in the lower reaches of this and other forested Hawaiian watersheds that are frequently used by both residents and tourists.