Effect of cow manure’s particle size on the distribution of Coliform bacterial contamination in a soil column under grass cultivation

Introduction
The cow manure application is a common way to improve soil properties, however, the uses may cause bacteria contamination in the soil profile as well as the surface water and groundwater. Thus it is crucial to monitor the amount of bacterial transport and retention on the soil surface and deeper under cultivation conditions. The literature review shows that the structure of manure, soil structure, cultivation, soil moisture, soil salinity, and irrigation methods are important in the transport and retention of bacteria in the porous media. The plant’s roots cause cracks, creating preferential flow in the soil profile. Preferential flow accelerates pollutant transportation in the soil. Therefore It is essential to survey the transport of bacteria in the presence of plants. In this research, it has been investigated the effect of grass cultivation conditions and the size of the particles on the retention of Escherichia coli bacteria in the soil profile.
 
Materials and Methods
This research was conducted in the greenhouse and column experiments condition with grass cultivation. Cow manure was dried for 72 hours and passed through 0.25, 0.5, 1.0, and 2.0 mm sieves. The grass was prepared and placed uniformly on the surface of the soil columns. Then 14 days were given for the stabilization and settlement of the grass roots into the soil columns. The columns were irrigated once every two days by custom surface method until the soil moisture to the field capacity. The column’s height was divided into six equal layers, in which each layer was sampled to measure the remaining E. coli bacteria. The live counting method was used to determine the bacteria accumulation on the soil profile. For the measurement, a nine cubic centimeter of distilled water was added to one gr of each soil sample and they were put and kept in a shaker for 30 minutes. After preparing the soil samples’ dilutions, 100 µl of each dilution was cultured on the Eosin Methylene Blue (EMB) culture medium. The plates were retained at 37 °C, the temperature suitable for E. coli growth. After 24 hours, the bacteria colonies were counted and reported per ml. Due to the difference in the initial concentration of bacteria in the cow manure samples, the relative concentration index of bacteria was used to compare the concentration of bacteria in the effluent and the soil profile. This is the bacteria concentration ratio to the concentration of input bacteria, at each depth of the soil with the S/C0 parameter, it was shown that C0 and S are the bacteria concentration in the manure columns and the bacterial growth rate in the soil depth respectively.
 
Results and Discussion
In this research, the distribution of the soil bacteria is studied at the end of the test period. The results show that in treatments without grass cultivation, it can be seen that the maximum S/C0 for all cow’s manure sizes in the surface layer was 10 cm. With increasing depth, the relative concentration of the bacteria in all treatments has decreased. In the conditions of grass cultivation on the soil surface, the relative concentration of bacteria for the manure particle size of 0.25 mm at lower depths was more than the treatment with a larger particle size. The smaller size of the manure particles of 0.25 mm due to their transfer to the lower layers has led to the observation of a higher amount of bacteria compared to other sizes at lower depths. In other words, due to their small size, cow manure particles with a diameter of 0.25 were more easily transported to the lower layers of the soil column by flowing through the pores created by the plant roots. The reason for the decrease of the bacteria absorption in the soil surface layer compared to the treatments without cultivation can be attributed to the soil structure. The root growth causes the creation of macropores and large pores on the soil surface and increases the transfer of bacteria to the lower layers.

Conclusion
The effect of grass cultivation and different sizes of cow manure particle size (2.0, 1.0, 0.5, and 0.25 mm) were evaluated on the distribution of Escherichia coli bacteria in the soil profile. It shows that the bacteria retention rate decreases with increasing depth in the soil profile. Manure particles with a size of 0.25 mm cause more contaminant transport to the lower depth of the soil profile. Preferential flow in the conditions of grass cultivation caused the transport of bacteria to the lower depths. Grass cultivation causes bacteria retention at lower depths compared to the conditions without cultivation. The average bacterial retention index was 68% for a manure size of 2.0 mm at a depth of 10 cm and 48% in the treatment with a manure size of 0.25 mm. It concluded that the possibility of deep soil contamination with the application of manure with finer particles and grass cultivation is higher than with coarser particles and without grass cultivation.