Macronutrients in Hawaii's Coastal Wetland Pastures and Potential Phosphorus Release to Water

Coastal wetland pastures in Hawaii are based on naturalized hilograss (Paspalum conjugatum Bergius) and paragrass [Brachiaria mutica (Forssk.) Stapf ] growing primarily on moderately acid (pH 5.4) to mildly alkaline (pH 7.8) and mildly calcareous (<50 g kg −1 CaCO 3 equivalent [CCE]) Aquepts and Aquolls. Nutrient concentration, distribution, and P retention dynamics have not been quantifi ed for these ecologically important grazing lands. Soil and forage samples were collected from three pastures at each of six sites, with three sites representing each grass. In addition, pastures at one of the hilograss sites were subjected to zonal sampling with zones determined by distance from shade and drainage ditch borders (animal lounging areas). Soil total N, and exchangeable K and Na accumulated in shaded areas, and forage N, δ 15 N (‰), and K tended to follow a similar distribution. Across sites hilograss had very high and probably toxic S concentrations (5.6 to 11.0 g kg −1 ) for cattle (Bos spp.) compared with paragrass, which had S concentrations (1.9 to 4.0 g kg −1 ) within the expected range for tropical forage grasses. Across sites and the 0- to 15- and 15- to 30-cm soil depths, satisfactory predictive relationships (n = 54) were obtained between dissolved (<0.45-μm pore diameter) molybdate-reactive P (DRP) desorbed from soil in a water extract (DRP WE ) and Olsen extractable P (P OLSEN ) and the oxalate extractable molybdate-reactive P (RP OX ) saturation index (PSI OX RP). Oxalate extractable Al (Al OX ) was a more important component of PSI OX RP than oxalate extractable Fe (Fe OX ), which could be omitted.