Analysis of the bacterial pollution breakthrough curve in the soil column with different sizes of cow manure in the conditions of grass cultivation

Introduction
Recently, the presence of pathogenic bacteria in the municipal water network has been observed and proven. Applying animal manure in agricultural lands with improper drainage is the main cause of this pollution. Identifying and investigating the movement of intestinal bacteria, especially E. Coli, which is the source of their distribution in most waters, agricultural activities, and urban sewage, is considered one of the appropriate and necessary ways to preserve drinking water resources Some of the soil characteristics that affect the movement of bacteria are: particle size distribution, structure, porosity and apparent density of the soil, in addition, plant roots and pores and cracks are caused by root activity. Plants and animals in the soil create fast water passages to facilitate the transport of pollutants. These routes are called preferential pathways and the flow is named preferential flow. Therefore, considering the environmental importance of the movement of E. Coli bacteria as a pathogen in the soil, so far, most of the studies on the transfer of bacteria without the presence of plants and its effect on the release of bacteria have been investigated. Therefore, this research aims to investigate bacteria transport from cow manure in four granulation levels in the presence of grass plants.
 
Materials and Methods
This study was conducted in the greenhouse of Shahrekord University to investigate the transport of E. Coli bacteria caused by the addition of cow manure in four levels of granular size in the soil profile with/without grass cultivation. Some physical and chemical characteristics of the soil were measured by usual methods. In this research, cow manure with a scale of 36 tons per hectare with four granulation levels of 0.25, 0.5, one, and two mm was used as a source of bacteria. The grass was prepared at a height of five cm and was placed on the surface of the soil columns for 14 days to stabilize the roots. The used columns were 24, made of polyethylene and in the form of a cylinder with an external diameter of 160 and a height of 350 mm. First, the soil was passed through a two mm sieve and then the columns were filled with soil up to a height of 300 mm. The treatments included grass cultivation in two levels (without cultivation and with grass cultivation) and the size of manure particles in four levels (0.25, 0.5, one, and two mm). The columns were irrigated with the usual irrigation schedule (once every two days) with the same volume and flow in the surface method until the field capacity was reached. After seven irrigations, the transfer test was performed. The transfer test with municipal water in the columns continued up to seven pore volumes (PV) and sampling was carried out in pore water volumes of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7,0.8,0.9, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.5 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5 and 7 were done for each treatment. After the end of the transfer test, to measure the population of bacteria in the soil profile, samples were taken from every five cm of soil depth. In this study, the live count method was used to measure the bacterial population.
 
Results and Discussion
There is no significant difference in the relative concentration curve of bacteria in the state of cultivation and without the cultivation of grass. It can be said that the effect of the cultivation of grass in the transfer of bacteria was not observed for 2 mm fertilizer particles, but the shape of the curves has changed in diameters less than 2 mm. It can be stated that in all treatments, the larger the amount of fertilizer, the higher the relative concentration of bacteria in low PVs. In other words, by washing the bacteria from the surface of the fertilizer particles, they are freed and enter the soil, and by continuing the washing, the maximum relative concentration of bacteria in the treatments without grass cultivation and in the diameters of 2.0, 1.0, 0.5 and 0.25, respectively, is 0.6. 0.7, 0.6, and 0.9 times the pore volume occurred. These values were equal to 0.7, 1.0, 0.9, and 1.0 times the pore volume in the treatments with grass cultivation, respectively. After this period, the concentration of released bacteria decreased sharply. The results showed that the presence of grass in the soil for all diameters of fertilizer, except the diameter of 0.25 mm, caused the peak of the breakthrough curve to be delayed. In addition, it is observed that the relative concentration of bacteria in the treatments with grass cultivation has decreased with a gentler slope compared to the treatments without grass cultivation. The amount of zero torque in the treatments with grass cultivation in all fertilizer sizes was more than the same treatment as compared to the conditions without cultivation, and this indicates that the presence of grass caused more bacteria to escape from the drainage of the columns. For fertilizers with particle sizes of 0.25, 0.5, and 2.0 mm in the condition of no cultivation, there is not much difference in the delay factor with the similar treatment in the condition of grass cultivation, but in the treatment with the particle size of one mm in the condition of grass cultivation, the rate of fertilization is delayed. has had a significant increase.
 
Conclusions
The results showed that for two mm fertilizer particles, the amount of bacteria transfer increased in the case of no grass cultivation compared to the one, 0.5, and 0.25 mm treatments. The maximum relative concentration of bacteria in the breakthrough curve for 0.25 mm fertilizer particles was lower between one and two mm compared to larger fertilizer particles and was observed with a delay compared to coarser fertilizer particles. In the treatment without grass cultivation, the maximum concentration per fertilizer with the particle size of 0.25 mm was observed at PV 0.9, while in the treatment with the particle size of two, one, and 0.5 mm, the maximum relative concentration of bacteria was 6.6, respectively. About 0.0, 0.7, and 0.6 times the pore volume occurred. In the presence of grass in the soil, the bacteria reached the bottom of the soil column at a faster rate. One of the causes of this phenomenon is the role of plant roots in accelerating the transfer of bacteria in the soil in such a way that the preferential flow paths created by grassroots have moved the bacteria down.