Structural Stability of Three Soils under Oil Palm and Arable Cropping In Successive Dry and Rainy Seasons of Southeastern Nigeria

Abstract

Soil structure and its stability is a very important aspect of soil health that influence crop establishment and performance. Land use and season are critical factors in structural stability of soils but the interaction between these two factors is not well understood for tropical soils. Therefore, this study assessed the structural stability of soils under oil palm plantation (OPP) and arable cropping (ACP) land use types in successive dry and rainy seasons of southeastern Nigeria. Disturbed and undisturbed soil samples were collected from three locations namely Umuomaku, Ishieke and Nsukka. Sampling was done from two depth zones (0-30 and 30-60 cm) in each location. Soil samples were analyzed for structural stability and hydraulic properties as well as for physicochemical properties using standard methods. Data were subjected to analysis of variance setting p at 0.05, the topsoils and subsoils were compared by t-test. The results showed that the ACP recorded higher mass of eroded sediments (MASEDeroded), water stable aggregates corrected for sand on slow basis (WSAcfss) and lower soil organic carbon (SOC) while the OPP recorded higher percolation stability (PS), water stable aggregates corrected for sand (WSAcfs) and higher SOC this translates into better soil structure under OPP relative to ACP. Also, higher values for clay dispersion ratio (CDR) and dispersion ratio (DR) were recorded under ACP implying higher tendency for soils to disperse. Higher PS, wettability and sealing index were recorded during the rainy season compared to the dry season. The dry season recorded higher CDR and DR compared to the rainy season which recorded higher CFI at the subsoil. Considering locations, Nsukka and Ishieke recorded higher MASEDeroded compared to Umuomaku, both locations also recorded higher DR at the topsoils. Interaction effects of location, season and land use were significant for aggregate stability, soil strength, water dispersible clay (WDC) and CDR of the topsoils. Depth significantly influenced location, season and land use. The topsoils during the rainy season recorded higher pH-KCl (4.45), available phosphorus (18.50 mg/kg), hydraulic conductivity (23.24%), aggregate stability (42.19%) and percolation stability (390.32 ml/10min) while the subsoil recorded higher clay (13.48 g/kg), SOC (1.02 g/kg), total nitrogen (0.16%) and CEC (13.31 cmol/kg). During the dry season, topsoils recorded higher SOC (0.50 g/kg), aggregate stability (22.39%), Ks (31.73), and available phosphorus (9.48 mg/kg), while the subsoils recorded higher total nitrogen (0.08%), CEC (9.18 cmol/kg), PS (323.00 ml/10min) and wettability (1.33 cm/sec.). Under land use, the topsoils of OPP recorded higher pH-KCl, SOC, total nitrogen, available phosphorus (Av. P), hydraulic conductivity (Ks) and PS while the subsoils recorded higher cation exchangeable capacity (CEC), aggregate stability (AS) and wettability. ACP topsoils recorded higher AS, Ks, Av. P, and CEC compared to the subsoils. Interaction of location, season and land use on the hydraulic properties significantly influenced Ks, with the highest values under ACP at Umuomaku in the dry season (49.21 cm/hr). Interaction of location, season and land use on physicochemical properties were significant for total sand (894 g/kg), coarse sand (637 g/kg) and silt (241 g/kg) at the topsoils and for pH-KCl (4.60), total nitrogen (4.0 g/kg), available phosphorus (28.44 mg/kg) and CEC (23.33 cmol/kg) at the subsoils. PS correlated positively with wettability, CDR correlated negatively with clay, while MASEDeroded correlated negatively with PS and WSAcfs (r = -0.55* and -0.35*) explaining the importance of soils to have high PS to ensure reduced erosion. Also, MASEDeroded was explained by PS, WSA and WDC (p ≤ 0.001; R2 = 0.916). To ensure stability of soil aggregates and reduced water and wind erosion in soils of southeastern Nigeria, OPP should be encouraged to checkmate erosion. The CFI, CDR and DR usually associated with structural stability were found relevant for checkmating erosion. The OPP showed better soil structure compared to ACP, also the higher the PS and WSAcfs of a soil the more difficult it is for soil detachment and erosion occurrence also the better the structure.