Seasonal Changes of Soil Water Repellency in Pine and Eucalyptus Forest Soils in Up Country, Sri Lanka

Soil water repellency is a natural consequence which reduces the rate of wetting and leading water to remain on the soil surfaces for prolonged periods. Water repellency accelerates runoff and soil erosion in natural and agricultural lands by lowering infiltration and thereby increasing surface runoff. It is normally caused by the presence of hydrophobic coatings on mineral surfaces or intermixed hydrophobic organic materials in soils. This hydrophobic organic matter is in general added to the soils from different plant leaves, root exudates, and micro-organisms. Water repellency has been found in soils under wide range of vegetation types around the world, especially in conifer forests including pine and eucalyptus. Pine (Pinus caribaea) and eucalypt (Eucalyptus grandis) are exotic plant species introduced to Sri Lanka with the aim of preventing the land degradation in hill slopes and to reduce the pressure on natural forests for timber requirements. This study aimed to examine the water repellency, with its possible seasonal changes, in pine (Haputale) and eucalypt (Diyatalawa) forest soils in Up country, Sri Lanka. The vertical distribution of water repellency in the soil profile was studied using bulk soil samples that were collected from 5 soil layers in each site (0-5, 5-10, 10-15, 15-20, 20-25 cm) during both dry and wet seasons November 2016 to July 2018 Following air drying, potential water repellency was examined using water drop penetration time (WDPT) test and modified sessile drop contact angle methods. Actual onsite water repellency was observed using WDPT test. Top soil layer (0-5 cm) of eucalypt site showed extreme water repellent conditions as measured by Log WDPT (>3,600 s) and soil-water contact angle (>85°) in both dry and wet seasons. Pine forest soils showed severely water-repellent conditions in wet season (1000–1100 s) and strongly water repellent conditions (200-300 s) in dry season showing a significant change in water repellency. In both wet and dry seasons, water repellency (log WDPT and soil-water contact angle) showed strong negative exponential relationships with increasing soil depth in both pine and eucalyptus forest soils. Changes in degree and persistency of soil water repellency were observed under natural conditions with seasonal variation in pine and eucalyptus plantations in Up country. In order to determine the actual changes with seasonal variation of soil water repellency, long term seasonal experiments are recommended.Keywords: Pine, Eucalyptus, Soil water repellency, Seasonal changes, Vertical distributio

Initial water repellency affected organic matter depletion rates of manure amended soils in Sri Lanka

The wetting rate of soil is a measure of water repellency, which is a property of soils that prevents water from wetting or penetrating into dry soil. The objective of the present research was to examine the initial water repellency of organic manure amended soil, and its relation to the soil organic matter (SOM) depletion rates in the laboratory. Soil collected from the Wilpita natural forest, Sri Lanka, was mixed with organic manure to prepare soil samples with 0, 5, 10, 25, and 50% organic manure contents. Locally available cattle manure (CM), goat manure (GM), and Casuarina equisetifolia leaves (CE) were used as the organic manure amendments. Organic matter content of soils was measured in 1, 3, 7, 14, and 30 days intervals under the laboratory conditions with 74±5% relative humidity at 28±1°C. Initial water repellency of soil samples was measured as the wetting rates using the water drop penetration time (WDPT) test. Initial water repellency increased with increasing SOM content showing higher increasing rate for hydrophobic CE amended samples compared with those amended with CM and GM. The relation between water repellency and SOM content was considered to be governed by the original hydrophobicities of added manures. The SOM contents of all the soil samples decreased with the time to reach almost steady level at about 30 d. The initial SOM depletion rates were negatively related with the initial water repellency. However, all the CE amended samples initially showed prominent low SOM depletion rates, which were not significantly differed with the amended manure content or the difference in initial water repellency. It is explicable that the original hydrophobicity of the manure as well has a potentially important effect on initiation of SOM decomposition. In contrast, the overall SOM depletion rate can be attributed to the initial water repellency of the manure amended sample, however, not to the original hydrophobicity of the amended manure. Hydrophobic protection may prevent rapid microbial decomposition of SOM and it is conceivable that hydrophobic substances in appropriate composition may reduce organic matter mineralization in soil. These results suggest the contribution of hydrophobic organic substances in bioresistance of SOM and their long-term accumulation in soil

Influence of organic manure amendments on water repellency, water entry value, and water retention of soil samples from a tropical Ultisol

Lowered stability of soil aggregates governed by insufficient organic matter levels has become a major concern in Sri Lanka. Although the use of organic manure with water repellent properties lowers the wetting rates and improves the stability of soil aggregates, its effects on soil hydrophysical properties are still not characterized. Therefore, the objective of this study was to examine the relation of water repellency induced by organic manure amendments to the water entry value and water retention of a Sri Lankan Ultisol. The soil was mixed with ground powders of cattle manure (CM), goat manure (GM), Gliricidia maculata (GL) and hydrophobic Casuarina equisetifolia (CE) leaves to obtain samples ranging from non-repellent to extremely water repellent, in two series. Series I was prepared by mixing GL and CE with soil (5, 10, 25, 50%). Series II consisted of 5% CM, GM, and GL, with (set A) and without (set B) intermixed 2% CE. Water repellency, water entry value, and water retention of samples were determined in the laboratory. Soil-water contact angle increased with increasing organic matter content in all the samples showing positive linear correlations. Although the samples amended with CE showed high soil-water contact angles in series I, set A (without 2% CE) and set B (with 2% CE) in series II did not show a noticeable difference, where >80% of the samples had soil-water contact angles <90°. Water entry value (R2 = 0.83–0.92) and the water retention at 150 cm suction (R2 = 0.69–0.8) of all the samples increased with increasing soil-water contact angles showing moderate to strong positive linear correlations. However, set A (without 2% CE) and set B (with 2% CE) in series II did not differ noticeably. Water entry value of about 60% the samples was <2.5 cm. Mixing of a small amount (2%) of hydrophobic organic matter with commonly used organic manures slightly increased the water repellency of sample soils, however not up to detrimental levels. It did not generate adverse effects on water entry and increased the water retention. It was clear that intermixing of small quantities of hydrophobic organic manure with organic manures commonly used in Sri Lankan agriculture, would not generate unfavorable impacts on soils

Water stable aggregates of Japanese Andisol as affected by hydrophobicity and drying temperature

Hydrophobicity is a property of soils that reduces their affinity for water, which may help impeding the pressure build-up within aggregates, and reducing aggregate disruption. The purpose of this study was to examine the relation of soil hydrophobicity and drying temperature to water stability of aggregates while preventing the floating of dry aggregates using unhydrophobized and hydrophobized surface Andisol. Soil was hydrophobized using stearic acid into different hydrophobicities. Hydrophobicity was determined using sessile drop contact angle and water drop penetration time (WDPT). Water stability of aggregates (%WSA) was determined using artificially prepared model aggregates. The %WSA increased as the contact angle and WDPT increased. Contact angle and WDPT, which provided maximum %WSA showing less than 1 s of floating, was around 100° and 5 s, respectively. Although the %WSA gradually increased with increasing contact angle and WDPT above this level, high levels of hydrophobicity initiated aggregate floating, which would cause undesirable effects of water repellency. Heating at 50°C for 5 h d-1 significantly affected %WSA and hydrophobicity in hydrophobized samples, but did not in unhydrophobized samples. The results indicate that the contact angle and wetting rate (WDPT) are closely related with the water stability of aggregates. The results further confirm that high levels of hydrophobicities induce aggregate floating, and the drying temperature has differential effects on hydrophobicity and aggregate stability depending on the hydrophobic materials present in the soil

Effects of Heat on Surface Water Repellency and Aggregate Floatıng in Eucalyptus Forest Soil

Soil water repellency (SWR) is the resistance of soil for spontaneous wetting induced by the presence of organic substances. The soils beneath various plant species such as Eucalyptus, Cypress, and Pine are in general naturally water repellent. Wildfires are common in these forests due to the waxy nature of their plant debris. The heat generated during wildfires alters soil parameters, including SWR and stability of aggregates, depending on the heating dynamics of fire. Both heat and heat-induced changes in SWR may influence aggregate wetting and result in the floating of aggregates in the topsoil (crumbs). With runoff in sloppy lands, floating aggregates can cause severe erosion during highintensity rainfall incidents. The present study aimed to evaluate the effects of different heating temperatures and exposure times on SWR and floating of surface crumbs using Eucalyptus forest soil. Water repellent crumbs (diameter:3-5 mm) gathered from the soil surface (0-5 cm) of Eucalyptus grandis forest (soil texture:sandy loam) in upcountry Sri Lanka (06° 47′ 45.2″N 80° 58′ 00.9″E) were heated up to three different heating temperatures (150° C, 200° C, 250° C) with 30 min, 60 min, and 120 min durations of exposure. The SWR and aggregate floating time (AFT) were measured 16 h after heating using the water drop penetration time test (WDPT) and immersing aggregates in water to measure AFT (up to 3 h), respectively, and compared with non-heated aggregates (control). Initially, crumbs were severely water repellent (WDPT: ~2,000 s) and showed an AFT of ~3,500 s. Crumbs were strongly water repellent at 150° C under all durations of exposure (310 s, 110 s, 260 s, respectively). Aggregates showed strong SWR (~80 s) at 30 min exposure, and slight SWR at 60 min and 120 min (~14, ~4 s) exposure under 200° C. SWR was completely eliminated at 250° C. AFT increased to &gt;3 h, with heating up to 150° C and 200° C under all three durations of exposure and started to decrease at 250° C with 30 min exposure to be completely eliminated at 60 and 120 min exposure(0,0 s). SWR and AFT showed a positive correlation before (R²=0.64) and after (R²=0.59) heating. Both SWR and AFT decreased with heating temperature and duration of exposure to heat. Further experiments are required considering soil depths below 5 cm to understand the behaviour of less repellent under-surface aggregates upon exposure to heat. Keywords: Heating temperature, Duration of exposure, Floating of aggregates, Soil water repellenc

Soil hydrophysical properties as affected by solid waste compost amendments: seasonal and short-term effects in an Ultisol

Application of compost is known to improve the hydraulic characteristics of soils. The objective of this study was to examine the seasonal and short-term effects of solid waste compost amendments on selected hydrophysical properties of soil during dry and rainy seasons and to explore any negative impacts of municipal solid waste compost (MSWC) amendments on soil hydrophysical environment concerning Agriculture in low-country wet zone, Sri Lanka. Eight (T1–T8) MSWC and two (T9, T10) agricultural-based waste compost (AWC) samples were separately applied in the field in triplicates at 10 and 20 Mg ha−1 rates, with a control (T0). Field measurements (initial infiltration rate, Ii; steady state infiltration rate, ISS; unsaturated hydraulic conductivity, k; sorptivity, SW) were conducted and samples were collected (0–15 cm depth) for laboratory experiments (water entry value, hwe; potential water repellency: measured with water drop penetration time, WDPT) before starting (Measurement I) and in the middle of (Measurement II) the seasonal rainfall (respectively 5 and 10 weeks after the application of compost). The difference in the soil organic matter (SOM) content was not significant between the dry and rainy periods. All the soils were almost non-repellent (WDPT = <1–5 s). The hwe of all the samples were negative. In the Measurement I, the Ii of the T0 was about 40 cm h−1, while most treatments show comparatively lower values. The ISS, SW, and k of compost amended samples were either statistically similar, or showed significantly lower values compared with T0. It was clear that all the surface hydraulic properties examined in situ (Ii, ISS, SW) were higher in the Measurement I (before rainfall) than those observed in the Measurement II (after rainfall). Water potential differences in soils might have affected the surface hydrological properties such as SW. However, water potential differences would not be the reason for weakened ISS and k in the Measurement II. Disruption of aggregates, and other subsequent processes that would take place on the soil surface as well as in the soil matrix, such as particle rearrangements, clogging of pores, might be the reason for the weakened ISS and k in the Measurement II. Considering the overall results of the present study, compost amendments seemed not to improve or accelerate but tend to suppress hydraulic properties of soil. No significant difference was observed between MSWC and AWC considering their effects on soil hydraulic properties. Application of composts can be considered helpful to slower the rapid leaching by decreasing the water movements into and within the soil