MACRONUTRIENT, SOIL ORGANIC CARBON, AND EARTHWORM DISTRIBUTION IN SUBTROPICAL PASTURES ON AN ANDISOL WITH AND WITHOUT LONG-TERM FERTILIZATION

Abstract

Pasture fertilization affects plant growth and animal production, and it may influence the redistribution and cycling of nutrients excreted in dung and urine. Kikuyugrass (Pennisetum clandestinum Hochst. ex Chiov.) pastures on a high organic carbon (C) Typic Hydrudand soil with and without (control) annual nitrogen (N) and periodic phosphorus (P) and potassium (K) fertilizations were stocked with Hereford and crossbred cattle (Bos taurus) for 33 years. The pastures were divided into 1.2-ha paddocks for rotational stocking after the 15th year. Selected fertilized paddocks that were uniformly managed over the years were paired with control paddocks to determine management effects on soil organic C; soil and plant N, P, and K; and earthworm distribution. Responses were assessed using a zonal sampling procedure based on distance from the waterer because the paddocks lacked shade. Soil organic C, organic N, and organic P did not statistically differ between managements or zones in any horizon. Relative to the control, however, an apparent recovery of 25% of the total N applied to the fertilized paddocks was obtained as soil organic N within the 80 cm profile depth examined. Inorganic N (NH₄-N + NO₃-N) was greater in the Ap1 and Bw1 horizons of fertilized paddocks, and there was a trend in the Ap1 horizon toward greater concentrations within 15 m of the waterer. Additionally, substantial inorganic P accumulated within 30 m of the waterers in the Ap1 and Bw1 horizons of fertilized paddocks while minimal P accumulation occurred within 15 m of the waterer in control paddocks. The magnitude of K accumulation near waterers was also considerably greater in the Ap1 horizon of fertilized paddocks. It is suggested that fertilization increases the magnitude of P and K accumulation near waterers due to the combination of increased forage P and K concentrations and pasture carrying capacity. Management and zone effects for forage N, P, and K tended to follow patterns relatively similar to the soil Ap1 horizon data for the inorganic forms of these nutrients. Earthworm populations did not differ among zones, but populations in fertilized paddocks were double those of control paddocks.

ACKNOWLEDGMENTS

Funding for this research was provided in part by the County of Hawaii, Department of Research and Development. Thanks are extended to the agroecology students at the University of Hawaii at Hilo for their assistance in sample collection and analysis. The assistance of Mr. Kirk Rapozo in providing MES records and information on pasture management practices is also acknowledged.