Effects of land use type and tank components on soil properties and sustainability of tank cascade system in the Dry Zone of north central Sri Lanka

Village tank cascade system (VTCS) is a globally important agricultural heritage system in Dry Zone watersheds of Sri Lanka. Lands in these watersheds have been under changing management conditions in the recent past, hence their sustainability has been suspected. Objectives of this study were to determine current status of soil properties in different land uses and tank components of a VTCS with respect to their position in the watershed, and to assess sustainability of this agricultural system using soil organic carbon management index (CMI) as a proxy. Study was conducted in Mahakanumulla VTCS in North Central Dry Zone of Sri Lanka. Soil samples (n = 120) were collected from lands under forest, scrub/shrub, paddy and upland crops representing all elevations in two linear transects located in higher and lower positions along the main valley. Soils from components of tanks in upper and lower positions were also collected. Soil texture and some chemical properties were measured. The CMI in different land uses was calculated using total organic carbon (TOC) and permanganate oxidizable carbon contents. Effect of land use type on soil properties did not vary with position in the watershed (p > 0.05). An increase in available phosphorus and sulphur was observed in agricultural lands. Significant decreases in TOC (33%) and total N (43%) contents were observed in paddy lands compared to those of forest lands (p < 0.01). However, CMI indicated that management practices in paddy lands and upland soils are still sustainable but scrub/shrub lands need improvements to arrest their degradation. Electrical conductivity, pH, available P, K and Zn were significantly higher in lower positions while total N was lower. Thaulla (inundation area) and Perahana (filter) tank components had regulated flow of clay particles while Kattakaduwa (interceptor) has significantly high available P and TOC contents due to high wetness. Principal component analysis revealed factors that control soil erosion, soil organic matter content, water movement, cation availability and P dynamics governed the variability of major soil properties. Accumulation of soluble salts in lands of lower positions and increase of available plant nutrients in agricultural lands suggest that management practices so far adopted have slightly degraded soil fertility in this VTCS. Further degradation of soils in lower positions and scrub/shrub lands in the watershed will negatively affect the livelihood of villagers and the environment. Hence, appropriate land management policies should be introduced to this agricultural system to ensure their sustainability.