Comparing the Effects of Vetiver and Polyacrylamide on Soil Structural Stability and Erosion Indices

Extended abstract 1- INTRODUCTION Soil erosion is one of the most important problems of the environment, natural resources, and agriculture in the world. Soil structure stability is the key element of soil health, the main factor in the improvement or degradation of soils, and the important point in evaluating the effects of land management and the operations practices to control soil erosion. Bioengineering systems and soil stabilizers such as chemical polymers are applied to increase the soil structural stability and resistance against degradative agents, and to control soil erosion. 2- THEORETICAL FRAMEWORK Although many researches have been carried out to study the effect of PAM on soil strcutursl stability and soil erosion, most of them were conducted in the lab condition, and also the mixture of PAM with the bioengineering techniques, especially in field condition and semi-arid regions such as Khorasan Razavi province, was not studied. Regarding the potential of vetiver in soil conservation, it  seems that the vetiver has been well adapted in most regions of Iran, and can decrease soil erosion. Therefore, this research was performed to investigate the effect of the vetiver cultivation system, as a valuable bioengineering technique, and polyacrylamide (PAM) on soil structure and aggregate stability indices and soil erosion characteristics in a loamy soil under field condition. 3- METHODOLOGY The study was conducted in a loamy soil on slope of 5% in Agriculture Campus, Ferdowsi University of Mashhad. To apply the treatments, experimental plots (1 m × 1 m) were prepared in the given area. Experimental treatments include vetiver cultivation (VP0), PAM (20 (P2) and 40 (P4) kg ha-1), simultaneous application of vetiver and above PAM concentrations (VP2, and VP4. In addition, P0 (no PAM and vetiver) was regarded as control treatment. The undisturbed samples were collected to measure the structure indices before and after simulating the rainfall test. Structural stability indices including wet and dry mean weight diameter of aggregates (MWDwet and MWDdry), aggregate stability (AS), structural stability index (SI), and the percentage of aggregate degradation (PAD) were determined. The simulated rainfall intensity of 30 mm h-1 during 30 minutes was applied on the treated soils, and runoff and sediment volume were collected. This study was performed based on the randomized complete blocks design and a factorial arrangement with 3 replications. The data pertaining to soil structural stability and erosion indices were analyzed using SPSS software. 4- RESULTS & DISCUSION The results showed that vetiver and PAM increased the soil structural stability indices i.e. MWDwet, MWDdry, AS, and SI, and decreased PAD. However, vetiver enhanced the stability indices more than PAM. Also, vetiver and PAM decreased soil erosion indices, and the decrease in soil loss, runoff, and runoff coefficient was more due to the vetiver. The higher aggregation and structural stability, and as a result, the considerable reduction of aggregate degradation in the vetiver plots is due to the effect of high density  of fine roots biomass and microbial activity associated with the rhizosphere of the vetiver grass. On the other hand, when the aggregate stability increases, the water infiltration will increase and runoff will decrease. Consequently, particle detachment and aggregate degradation, and transport of soil particles due to the rain drop impact and runoff decrease. Also, vetiver increases soil resistance against raindrops; soil erosion decreases runoff probably through binding soil particles, aggregation and creating the macro pores. Adsorption of the long chains of PAM polymer on the surfaces of soil particles and aggregates, flocculating the soil particles and binding them by PAM ledad to increase aggregation and aggregate stability, creates continuous and stable macropores in soils, and decreases aggregate degradation. Therefore, PAM increases water permeability and as a results decreases the runoff, coefficient of runoff and soil loss. However, the efficiency of PAM to decrease the runoff and soil loss depends on the concentration and viscosity of the dissolved PAM. 5- CONCLUSIONS AND SUGGESTIONS Generally, the results of this research indicated that the vetiver system can be recommended due to the very low cost and long-term bioengineering technique to improve the soil structure; it increases aggregate stability, and decreases runoff and soil loss in thesemi-arid regions such as Iran.