The impact of some vegetative buffer strips on runoff and loss of nitrate and phosphate in rainfed lands of Sari, Iran

  Conservation and optimal use of soil and water resources are considered as the principles of sustainable development. Excessive use of natural resources caused by increasing agricultural lands area has led to some adverse consequences such as soil and water pollution. The use of vegetative buffer strips is considered as an effective strategy to reduce surface water pollutions and soil erosion. The study has been carried out to investigate the effect of runoff rate, plant species and type of contaminants on the efficiency of vegetative buffer strips in qualitative and quantitative conservation of soil and water in Mazandaran, Iran. The field of this research is a part of agricultural lands of Sari city. The width and length of the experimental plots were 1m and 10m respectively. The present study was carried out using 12 experimental plots with dimensions of 1m×10m in randomized complete block design, in Mazandaran province, Northern Iran. The studied treatments include vetiver grass, turf-grass with coverage area of 3m2 and control (bare) treatment with four replications. The experimental plots were installed with a slope of 15% those which were isolated from the surrounding environment. Also, a runoff collecting tank drainage was placed at downslope end of each plot. Vetiver grass and turf grass were the plant species used in the buffer strips those which were planted in the early February of 2015; so that, three meters of each plot contained the studied plant species and seven meters of the plot’ length was left as bare to produce runoff. Artificial runoff was produced with runoff rate equal to the runoff generated by 25 and 100 return period precipitation. Runoff sampling was annually carried out from the collected water at downslope of each plot since a month after planting the species; so that, it was conducted since February 2015 until January 2016. In order to evaluate the efficiency of vegetative buffer strips in water and soil conservation, the performance of these buffers for runoff, sediment and pollutants removal was studied. Means of achieved data were compared and statistically analyzed using Duncan test by SPSS (22) software. According to the results, the maximum efficiency of vetiver buffer strips under the runoff generated by 25-year return period precipitation; so that, the mentioned treatment reduced the runoff volume, sediment, nitrate and phosphate concentration by 85%, 88%, 83% and 87% respectively. Also, a relative proper correlation was found between phosphate and sediment (R2=0.66) as well as nitrate and runoff volume (R2=0.67) Plant growth stage and age played an important role in the efficiency of vegetative buffer strips for sediment control so that, by increasing the height and coverage percentage over time, vetiver grass gave a more appropriate performance in pollutants control and specially suspended sediment compared to the other studied vegetative buffer strips in the present study. The efficiency of vegetative buffer strips in runoff quantity and quality control is highly affected by the flow properties. The characteristics of buffer strips, plant species and phenology are less important when the runoff rate is high. Because, the efficiency of vegetative buffer strips affected by 100-year return period runoff showed a more irregular trend compared to 25-year return period runoff. The two buffer plants also showed different performances to reduce runoff volume and its pollutants. The vegetative buffer strips played a role as nutrients and sediment source that caused soil and surface water pollution over time. So, periodic mowing can be considered as an effective strategy to deal with this issue