عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Soil erosion is exposed soil caused by the loss of vegetation by water, wind, and ice with the result of reduced soil-water capacity and with the reduced soil productivity due to the losses of sediments, nutrients, etc., by runoff. Soil erosion has been considered as a major form of soil degradation that is a naturally occurring process and that can be accelerated by human activities and it is one of the most serious eco-environmental problems around the world. Soil erosion is often highest during thaw, and it is enhanced by freeze_thaw (FT) cycles. The extreme FT action occurs in the Bojnourd region located in the Northern Khorasan, Iran, due to its unique climate resulting from high elevations and cold temperature. This action causes damage to the surface soil structure, as soil erosion in the Bojnourd region is dominated by FT erosion. Limited by natural and scientific factors, FT erosion, as a typical erosion process along with wind and water erosion, has not been given enough attention. On the other hand, many organic (dairy manure, sewage sludge, compost) and inorganic (polyacrylamide (PAM), polyvinyl alcohol (PVA), fly ash, perlite, vermiculite) amendments have been extensively used for improving the soil structure characteristics. So, in this research, freezing-thawing process of the soil samples collected from the Bojnourd region was carried out by laboratory experiments to determine the runoff volume and soil loss subjected to FT cycle and after PAM application.
The present study aimed to investigate the effects of PAM on runoff volume and soil loss subjected to FT cycle from small experimental plots induced under rainfall simulation condition. A completely randomized design with FT cycle and none use of PAM as control were used to compare the effect of application of different rates of PAM viz. 0.25, 0.5 and 0.75 g.m-2, on runoff generation time, runoff volume and soil loss. A FT cycle was simulated in Soil Erosion and Rainfall Simulation Laboratory of Tarbiat Modares University. The present study was conducted under controlled laboratory conditions with a simulated rainfall. Towards this attempt, air and different soil depth temperatures were analyzed in a natural condition and 10 cm soil depth was targeted for the soil laboratory experiments. The rainfall storm with 72 mm h-1 and 30 min duration was simulated and conducted for the study treatments. The soil was poured in small erosion box with 0.25 m2 surface area in three replicates. A thick filter, draining the lower 20 cm of the soil profile was generated using mineral pumices. The prepared soil sample was evenly packed into the soil plots at a bulk density of 1.3 g cm?3 similar to that measured under natural conditions. The plots were then placed in a saturated pool for 24 h and then left to be drained to achieve an average moisture content of 35% similar to that recorded for the realities in the study area. For each event, the time to runoff initiation was recorded as the elapsed time between the start of rainfall and the time at which surface runoff began entering the runoff collection container located at the end of the plot. Runoff was sampled at different time steps of 2–5 min and its volume was accordingly measured. The collection gutter at the lower end of each box was protected by a shield to prevent rainfall from directly entering the collection container. The amount of soil loss was then measured using a decantation procedure, oven drying at 105 ?C for 24 h and weighing by means of a high-precision scale.
The Analysis of Variance (ANOVA) results showed that the PAM had a significant increasing effect on runoff generation time. Application rates of 0.25 and 0.50 g m-2 decreased runoff and soil loss at the tune of 49, 23 and 77 and 52, respectively, compared to those recorded for the control plot. In contrast, runoff and soil loss increased 1.62 and 4.5 times subjected to 0.75 g m-2 application rate. It was also understood from the results that the PAM with the particle flocculation capability and increasing penetration rate could control the effects of FT cycle, with further effect on soil loss. Whereas, the PAM treatment of 0.75 g m-2 had an increasing effect on runoff yield and soil loss. It can be concluded that more experiments are needed for monitoring and evaluating long-term effects of PAM on soil hydrology and erosion processes with a particular focus on FT cycle.