The effect of changes in soil characteristics with depth because of soil horizonation and macroporosity on fate of pesticides is poorly understood. Soil from the surface A horizons (0–0.65 m), albic E horizons (1.05–1.35 m), argillic Bt horizons (1.4–3.5 m), and surface linings of macropores in the argillic horizons (Bt‐M, 1.4–3.5 m) of a Thatuna silt loam (fine‐silty, mixed, mesic Xeric Argialbolls) were characterized for their physical, chemical, and microbiological effects on adsorption, desorption, and mineralization of 2,4‐D, carbofuran, and metribuzin. Organic carbon (OC) contents decreased for soil materials in the order A (1.22%) > Bt‐M (0.36%) > Bt (0.25%) = E (0.20%), and were correlated with the number of soil microbial colony forming units (log CFU) that decreased in the order A (7.4) > Bt‐M (6.1) > E (5.6) = Bt (5.4). Percent macroporosity (pores 2–5 mm diam.) decreased in the order Bt (1.1%) > A (0.7%) > E (0.6%), which is the same order for decreases in saturated hydraulic conductivity (log m s−1), namely; Bt (−6) > A (−6.2) > E (−7.0). Freundlich adsorption partition coefficients (Kf− L kg−1) for A horizon soil with 2,4‐D (1.1), carbofuran (1.0), and metribuzin (1.6) were significantly greater than in subsurface E or Bt matrix material. Sorption Kf values in macropore linings with carbofuran (1.1) and metribuzin (2.0) were comparable to or greater than their respective values of 1.0 and 1.6 in A horizon material. Percent mineralization in the A horizon after 139 d for [U‐14C] 2,4‐D (81.8%) and carbofuran (14.7%) was significantly greater than in all subsurface soil materials. Mineralization in Bt macropore linings for 2,4‐D (17.0%) and carbofuran (8.4%) was significantly greater than in other subsurface soil materials. Mineralization of [U‐14C] metribuzin was negligible (<3%) in all soil materials after 139 d.
Soil erosion is one of the main forms of land degradation. Erosion contributes to loss of agricultural land productivity and ecological and esthetic values of natural environment, and it impairs the production of safe drinking water and hydroenergy production. Thus, assessment of soil erosion and identifying the lands more prone to erosion are vital for erosion management process. Revised Universal Soil Loss Equation (Rusle) model supported by a GIS system was used to assess the spatial variability of erosion occurring at Kalu Ganga river basin in Sri Lanka. Digital Elevation Model (30 × 30 m), twenty years’ rainfall data measured at 11 rain gauge stations across the basin, land use and soil maps, and published literature were used as inputs to the model. The average annual soil loss in Kalu Ganga river basin varied from 0 to 134 t ha−1 year−1 and mean annual soil loss was estimated at 0.63 t ha−1 year−1. Based on erosion estimates, the basin landscape was divided into four different erosion severity classes: very low, low, moderate, and high. About 1.68% of the areas (4714 ha) in the river basin were identified with moderate to high erosion severity (>5 t ha−1 year−1) class which urgently need measures to control soil erosion. Lands with moderate to high soil erosion classes were mostly found in Bulathsinghala, Kuruwita, and Rathnapura divisional secretarial divisions. Use of the erosion severity information coupled with basin wide individual RUSLE parameters can help to design the appropriate land use management practices and improved management based on the observations to minimize soil erosion in the basin.
The brackish coastal wetlands in the Bundala National Park, the only RAMSAR site of southern Sri Lanka, are an important waterfowl habitat and economic zone. Bundala Lagoon, one of the three key lagoons of the Bundala wetlands, remains largely intact and relatively pristine, but the other two interconnected lagoons, namely, Embilikala and Malala, are impacted by drainage from 25.6 km 2 of upstream agricultural lands. Seasonal variations of water quality of the three lagoons and the key processes affecting water quality and quantity in these lagoons were studied during three agricultural seasons, to better understand the characteristics of the system. Bundala Lagoon, which was not affected by agriculture, recorded the highest ammonia and total nitrogen concentrations and the lowest phosphorus levels. Higher phosphorus levels in Embilikala Lagoon were related to the upstream agricultural activities with 65% of its total phosphorus measured being reactive phosphorous. Phosphorus additions occurred during the early months of the paddy cultivating seasons. Processes affecting the water quality of the Embilikala-Malala lagoon system included agricultural drainage, livestock additions, and breaching of the sand bar between Malala Lagoon and the sea. The salinity level of the Bundala Lagoon was higher than the others due to the connection to the sea, salt farms in the western part, and less dilution of salt from relatively low surface runoff and rainwater. All three lagoons reported pH levels conducive to most aquatic species. The primary production by phytoplankton in the lagoons of the Bundala wetland was phosphorus limited regarding their ratios of nitrogen to phosphorus. This study provides an overview of the present status of the lagoons of the wetland. Further work is needed to evaluate the impact of the external nutrient and water inputs on the flora and fauna of the lagoon environments. Suitable management practices to ensure the sustainability of the lagoon ecosystem can be derived through this increased understanding.
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