ESTIMATION OF RUNOFF, SOIL AND NUTRIENT LOSSES IN VETIVER GRASS STRIPS UNDER CASSAVA (Manihot esculenta CRANTZ) CULTIVATION IN UYO, NIGERIA

Abstract

Soil erosion is a major threat to sustainable cassava cultivation in Southern Nigeria. Accurate estimation of runoff, soil and nutrient losses is key to successful soil erosion mitigation measures such as the use of Vetiver Grass Strips (VGS). The commonly used Single-slot Fractional Method (SFM) estimates soil loss from a fraction of the runoff discharged, whereas the Multi-slot Method (MM), reported to be more accurate, measures soil loss from total runoff. However, the MM of estimation has not been adequately documented in Nigeria. Therefore, this study was carried out to quantify runoff, soil and nutrients losses in VGS plots under cassava cultivation in Uyo, Nigeria. The study was conducted on a Typic Kandiudult soil with 15% slope at Uyo in two cropping cycles. Two erosion estimation methods: SFM and MM were compared under three VGS: 10 m (VGS10), 20 m (VGS20) and 30 m (VGS30). Plots with No Vetiver grass (NV) served as control. Treatments were laid in a randomised complete block design with three replicates. Cassava variety NR8082 was planted at 10,000 plants/ha. Runoff (mm) and soil loss (kg/ha) were measured using standard methods. Nitrate-N and phosphate-P contents (mg/L) in runoff as well as carbon, nitrogen and phosphorus contents (kg/ha) in eroded soils were determined using standard methods. Cassava storage root yield (t/ha) was measured. Data were analysed using descriptive statistics, t-test and ANOVA at α0.05. Across VGS, runoff using SFM (18.2±3.2) and MM (17.7±3.1) were similar. Runoff under SFM ranged from 11.1±4.3 (VGS10) to 25.2±6.3 (NV) and corresponding values under MM ranged from 12.0±2.4 (VGS10) to 24.9±5.9 (NV). Soil loss of 231.7±13.3 estimated from SFM was significantly lower than 297.6±13.9 obtained from MM. Soil loss under SFM ranged from 145.1±12.2 (VGS10) to 292.2±13.6 (NV), while corresponding soil loss from MM increased from 215.9±16.2 (VGS10) to 396.5±17.6 (NV). Under VGS10, VGS20, VGS30 and NV, soil loss estimates of 145.1±12.2, 220.9±13.2, 268.7±13.4 and 292.2±13.6 obtained from SFM were significantly lower than 215.9±16.2, 245.4±17.0, 332.8±17.2 and 396.5±17.6 obtained from MM, respectively. Nitrate-N content in runoff from SFM (0.5±0.2) and MM (0.4±0.2) were similar. Nitrate-N content under SFM ranged from 0.3±0.1(VGS10) to 0.7±0.2(NV) and from 0.3±0.1 (VGS10) to 0.4±0.2 (NV) under MM. However, phosphate-P content in runoff from SFM (0.5±0.2) was significantly lower than from MM (0.9±0.3). Carbon and nitrogen contents in eroded soils from SFM (1.8±0.3 and 0.1±0.0) and MM (2.2±0.5 and 0.1±0.0) were similar. Carbon under SFM and MM ranged from 1.6±0.3 (VGS10) to 1.9±0.4 (NV) and 1.8±0.3 (VGS10) to 2.7±0.9 (NV), respectively. Phosphorus contents of 2.1±0.3, 2.7±0.4, 3.2±0.9, and 3.7±2.1 from SFM were significantly lower than 5.6±2.4, 4.7±2.1, 6.3±3.0 and 9.6±2.9 from MM under VGS10, VGS20, VGS30 and NV, respectively. Cassava storage root yields from VGS10 (35.1±5.3) and VGS20 (30.4±4.8) were similar and significantly higher than that of VGS30 (18.0±3.6) and NV (13.0±3.1). Single-slot fractional method underestimated soil and nutrient losses from the field compared to Multi-slot method and vetiver grass strips at 10 m intervals improved cassava tuberous yields in Uyo, Nigeria.