Nutrient Cycling In Tropical Grasses Irrigated With Dairy Effluent in a Tropical Island Environment

Abstract

In Hawaii and other island environments, dairy producers accumulate large quantities of effluent in lagoons. These lagoons can potentially overflow causing the nutrients and other contaminants to pollute the land and associated water bodies. Alternative uses of effluent are urgently needed for a sustainable and environment-friendly dairy production. This study assessed the effects of effluent irrigation on plant and soil (Cumulic Haplustoll) properties. Five tropical grasses—bana {Pennisetum purpureum S.), California (Brachiaria mutica S.), signal (Brachiaria decumbens S.), star (Cynodon nlemfuensis V.) , and suerte (Paspalum atratum S.)—received subsurface drip irrigation of dairy effluent at two rates based upon the potential evapotranspiration (ETp) at the site (Waianae, Hawaii)—2.0 ETp (7 to 44 mm d"‘) and 0.5 ETp (2 to 11 mm d"*).

No excessive increases in extractable soil P (81 to 176 mg kg'') and soil solution total P (3 to 9 mg L'') was observed after two years of effluent irrigation. Soil pH and soil solution pH fluctuated over time due to the high soil buffering capacity. Salinity and sodicity were not observed in this effluent-irrigated soil. Soil electrical conductivity (ECspc) declined from 18.0 dS m'' in July 2003 to 2.7 dS m * in Aug 2006—lower than the U.S. Salinity Laboratory’s critical level for classifying soils as saline (4.0 dS m''). Soil exchangeable sodium percentage (6.4 to 10.2%) remained below 15%—critical value critical value for classifying soils as sodic. Brachiaria mutica and P. purpureum yielded the highest dry matter of 57 and 53 Mg ha'' y ', respectively. Average nutrient removal of grasses was 30 to 187%, 13 to 86% and 2 to 14% of applied effluent N, P and K, respectively. Forage quality was within acceptable levels for feeding to dairy cattle. Modeling results showed that total applied phosphorus determines how many animals may be raised and how much area may be utilized to produce the forage. Results indicated that irrigating high yielding tropical grasses with effluent at 2.0 ETp was acceptable for recycling of nutrients from the effluent. Additional monitoring is needed to determine the longer-term impacts of effluent application on soil and plant properties.