INFLUENCE OF ORGANIC AMENDMENTS AND SURFACE ASH LAYERS ON HYDRAULIC CHARACTERISTICS OF RHODUDULTS IN RELATION TO WATER-REPELLENT CONDITIONS
Soil water repellency (SWR) is caused by the presence of organic substances, making the soil resistant to spontaneous wetting. Losses, additions, and alterations in organic materials due to various factors alter the water-repellent nature of soil. Application of organic matter (OM) as different amendments improves soil OM content. Conversely, natural or manmade fires can cause significant losses or alterations in OM occurring water-repellent conditions. Comprehensive evidence on the effects of repeated applications of organic amendments on SWR and soil hydraulic characteristics does not exist. Available information on how different levels of temperatures and exposure durations alter the water repellency (WR) of ash and their functional groups is not sufficient. Present study aimed to determine how repeated application of organic amendments and surface ash layers influence hydraulic characteristics of soils, considering WR conditions. A field experiment was conducted to assess the development of SWR and the impacts on soil hydraulic characteristics with repeated application of organic amendments (3% w/w, 3 applications) over a year. Compost (COM), cattle manure (CM), biochar (paddy husk; BCPH, casuarina leaf litter; BCCE) were used as organic amendments. Initial infiltration rate (Ii), sorptivity (Sw), unsaturated hydraulic conductivity (k), and steady-state infiltration rate (ISS) were measured in the field using a mini disk infiltrometer. Laboratory experiments (I) and (II) were conducted to assess the heat-induced changes in WR of litter ash layers of mahogany and casuarina species and to determine heat induced changes in functional groups in litter ash of cedar and cypress species, respectively. Litter biomass was exposed to 100 - 600°C for 20 and 40 min duration of exposure using a muffle furnace. The water drop penetration time (WDPT) and the molarity of ethanol droplet (MED) tests were used to measure WR. Functional groups in ash were determined using FTIR spectroscopy. Soil remained non-repellent (WDPT < 5s) with repeated applications of organic amendments although SWR showed an increasing trend. Organic amendments did not cause significant negative impacts on Ii, Sw, and ISS with repeated applications over a year. The k showed a more pronounced decline in BCCE amended soil (2.27 cm h-1). Considering the increasing trend of SWR and the decreasing trend of k, there is a possibility of developing negative impacts of SWR on soil hydraulic characteristics if these applications are repeated for many years. Ash WR and OM content generally decreased with increasing temperature. In casuarina, ash WR decreased continuously, whereas in mahogany it increased up to 150°C, remained stable up to 300°C, and then decreased. The contact angle of ash remained > 90° with exposure to 100–400℃. FTIR spectra analysis showed that the O–H functional groups in biomass disappeared quickly with exposure to heat, and did not contribute to ash WR. Functional groups C–H, C=O, COO−, C=C, C‒O, and C‒O‒C that disappeared around 300 - 400°C concurrent with low WR levels, were identified as contributing to ash WR. Wettable inorganic carbonate groups started to appear around 300°C indicating structural transformations of functional groups. Findings confirmed that repeated applications of organic amendments over a year would not negatively impacts SWR and hydraulic characteristics. The temperature levels of biomass burning during forest fires can cause significant changes to forest floor hydrology. Further studies on repeated application of organic amendments across different soil textures under actual field conditions over several years would be important. Furthermore, it would be important to study the effects of different thicknesses of ash layers on the soil surface.
