NITRATE AND PHOSPHATE DISTRIBUTION IN SOIL AND RUNOFF WATER UNDER DIFFERENT MANAGEMENT SYSTEMS ON AN ALFISOL IN SOUTH-WESTERN NIGERIA
*D.J. Oyedele1 , O.O. Awotoye2 and J.A. Akwoga2.
1Department of Soil Science, Obafemi Awolowo University, Ile-Ife, Nigeria. 2Institute of Ecology, Obafemi Awolowo University, Ile-Ife, Nigeria.
*Corresponding Author. Tel.: +234-803-4071980. Email address: email@example.com
Accepted in May 2004.
The rate and intensity of inorganic fertilizer usage has increased in recent years in Nigeria, in response to increasing demand for food. There is currently no particular regulation on inorganic fertilizer usage in Nigeria and no efforts are being made to restrict its overuse in order to avoid risk of soil and surface water contamination. This study was therefore carried out to determine the impact of agricultural land management on the distribution of NO3-N and PO4-P in soil and runoff water. The study was done in Ibadan Nigeria, on a ten hectare (ha) experimental farm that had been subjected to long term cultivation with and without inorganic fertilizer application. Four sites, each about 2 ha under different land management were identified. They are (i) continuous cultivation with high fertilizer application at high dosage (CTHF), (ii) continuous cultivation with intermittent fertilizer application at low dosage (CTLF), (iii) continuous cultivation with no fertilizer application (CTNF) and (iv) a regenerated forest (FR). Replicate soil samples collected at depths of 0-15, 15-30, 30-100 and 100-180 cm from different landscape positions under each treatment along with surface runoff water, were collected and analysed for their physical and chemical properties. Bulk density of the surface (0-15 cm) soil was highest under FR (1.59 mg m-3), however among the cultivated treatments, it increased with increased intensity of land use and was least under CTNF (1.42 mg m-3). Soil pH was highest under FR (6.17) and least under CTHF (5.22). Both surface soil NO3-N and PO4-P were highest under FR (8 mgkg-1) but increased with increased inorganic fertilizer usage from 3.5 mg kg-1 under CTNF to about 5 mg kg-1 under CTHF. However, while the soil NO3-N content decreased down the profile under FR (from 14 mgkg-1 at surface level to 6 mgkg-1 at 100-180 cm depth) it increased down the profile under CTHF (from 3 mgkg-1 at surface level to 5.5 at 100-180 cm depth). The concentration of NO3-N and PO4-P in surface runoff water was highest under CTHF and least under CTNF. Landscape position however had no significant effect on the soil distribution of NO3-N and PO4-P.