Evaluation the soil water repellency in relation to tree species, soil organic carbon pools and aggregate stability (Case study: Shalman Seed and Seedling of Forest Tree Species Research Station, Guilan Province)

Soil water repellency (SWR) is a soil characteristic that creates strong resistance to water penetration and decreases infiltration time from a few seconds to hours, days or weeks. The undesirable consequences of SWR have received interest from the scientific literature. However, the SWR has some beneficial aspects like increasing aggregate stability. Although several authors have reported the soil organic carbon as the origin of SWR but it seems only some parts of the soil organic carbon are hydrophobic and are involved in SWR. Thus, the aims of this study were: (a) to determine which soil organic carbon pools are linked with SWR, (b) to assess the impact of SWR on aggregate stability and (c) to determine which vegetative cover produce more hydrophobic organic compounds and contribute to SWR.
Soil samples were collected from the 0-5, 5-10 and 10-20 cm layers of the lands under different forest tree species including Populus caspica, Oak (Quercus castaneifolia), Alder (Alnus glutinosa), Bald cypress (Taxodium distichum), Loblolly pine (Pinus taeda) and Juniper (Juniperus polycarpos). The soil samples were analyzed for organic carbon content, pH, water repellency and aggregate stability (MWD) and were also separated to different organic matter pools based on particle size fractionation. The experimental variables were soil depth and kinds of vegetative cover and the data obtained from the soil samples were analyzed statistically by a two-way analysis of variance (ANOVA) using a factorial experiment with completely randomized design and 30 replications. Also Pearson linear correlation was used to indicate the relationships between the measure characteristics.
The vegetative covers and soil depths had significant impacts on SWR and the measured soil properties. The highest water drop penetration time (WDPT) was measured for the surface soil layer (0-5 cm). About 13.33 and 3.33 % (n=30) of the samples collected from the soil surface under P. taeda and T. distichum had the more than one hour WDPT (extremely repellent class). In contrast, the lowest SWR classes were mostly observed under the vegetative covers of P. caspica, Q. castaneifolia and A. glutinosa which were broadleaves tree species. The water repellency decreased with soil depth and 82.22% of the soil samples collected from the 10-20 cm layer of all tree species (n=180) determined as wettable soil (WDPT The strong correlation between SWR and the organic carbon content of the sand fraction indicates that this fraction contains organic compounds that contribute to SWR. Increase in soil pH and extent of decomposition of soil organic matter, however changes the solubility of soil organic matter and decrease SWR. As the soil aggregation depends on the total soil organic carbon content, the stability of aggregates is affected by hydrophobic fraction of soil organic carbon. Despite the extreme SWR in coniferous, Considering the undesirable quality of soil under coniferous trees such as low pH, MWD and organic carbon content, it is recommended to use discretion in the selection of these species for afforestation and reforestation projects.