Laboratory Study of Vegetation Cover Impact on Runoff Generation in Small Plots under Freezing and Thawing Cycle

Freezing-Thawing (FT) cycle is one of the processes that affects runoff generation. It is therefore necessary to control its negative effects due to specific topographic and climatic situation of Iran. However, there is no documented research about positive effects of vegetation cover on controlling detrimental effects of (FT) cycle yet. The present study aimed to explain the affectability of runoff generation from vegetation cover in small experimental plots under FT cycle.

In order to conduct this study, the soil was collected from Badranlou region located in 10 km west of Bojnourd in north Khorasan province. In this study, iron plots with Plexiglas walls with high ability to withstand lateral stresses and thermal insulation with dimensions of 0.5×0.5 m and a height of 0.3 m and a gradient of 20 percent in accordance with the general slope of the maternal area of the soil were used. The soil was passed from 4 mm sieve after air drying and then was placed in the prepared plots. Thirty plots were prepared for the entire selected plants species and transferred to the greenhouse and three species of A. trichophorum, M. sativa and L. prenne were planted. The species were then allowed until completing the main part of the phenological stages to maximally mimic the natural conditions governing the region. A FT cycle including three days of freezing and two consequent days of thawing and rainfall simulation with 72 mm h-1 in 30 min in three plots with vegetation cover was formulated similar to the study area situation. The entire experiments were conducted in three replicates. Similar control plots were also arranged for comparison purposes. Runoff components were measured in two minutes (three-time steps after runoff), three minutes (three second steps), five minutes until the end of the incident (three last steps) and five minutes after the end of the event. After measuring and recording the amount of runoff generation at the end of each rain event, the total amount of runoff generation in different treatments of rangeland species under FT cycle was recorded.

The result showed that the treated plots of M. sativa, A. trichophorum, and L. prenne significantly (P<0.05) reduced runoff generation by 0.97, 0.72 and 0.32 times compared with control treatments, respectively. The different effects of L. prenne and A. trichophorum species on runoff generation under the freeze-thaw cycles was confirmed. However, M. sativa did not have significant effect in reducing the amount of runoff generation compared with the control. The effect of FT cycle due to performance of ice lenses and formation of active melting layer in the soil surface was controlled owing to the presence of litter and adhesion of the surface layer of soil under treatments of M.sativa, A.trichophorum and L.Prenne. Besides, according to the measurement unlike the M. sativa, the main parts of root in two gramineae species of A. trichophorum and L. prenne were distributed superficially and caused more cohesion and improvement in soil aggregation. The runoff generation reduced in the presence of L. prenne more than the other two species. Based on the different studies, the percentage of fiber in gramineae species is more than broad-leaved species. Therefore, the presence of more fiber in A. trichophorum and L. prenne litters and subsequently their more resilience can be an effective factor in the relative reduction of runoff generation. In addition, the results of the study showed that the presence of vegetation on the soil surface increased the accumulation of fine grains on the soil surface and, if the soil remains intact, increases the stability of the soil structure. The accumulation of fine crops in the soil creates a large network of small roots in the soil and the soil particles become more adhesive. Due to the presence of the small roots in L. prenne species, there is the soil greater adhesion and further reduction of runoff in the presence of this plant.

The results of this study verified that plant cultivation can have significant effects on reducing runoff. In this regard, Lolium prenne had maximum benefit on reduction of runoff and soil loss induced by FT cycle due to better formation of superficial root system.