Ph.D Thesis

Ph.D StudentOuaknin Hanna
SubjectThe Influence of Physical Heterogeneity on
Biochemical Process in Soils Irrigated with
Treated Wastewater
DepartmentDepartment of Civil and Environmental Engineering
Supervisors ASSOCIATE PROF. Alex Furman
PROF. Noam Veisbrod
Full Thesis textFull thesis text - English Version


Nitrogen in soil has great environmental importance. On one hand, Nitrogen is essential nutrient for plant growth, and soil microbes, on the other it’s considered a major surface and ground water contaminant and responsible for nitrous oxide, emission to the atmosphere. Nitrogen in the soil changes its form by various processes, including immobilization, mineralization, nitrification and denitrification. Nitrification is an aerobic process performed by two different chemo-litho-autotrophic bacteria, transferring ammonium to nitrite and then nitrite to nitrate. Denitrification is anoxic process performed by heterotrophic bacteria, responsible for transferring nitrate to its main products- nitrous oxide and nitrogen gas.

Soil is physically heterogeneous medium at all scales. The heterogeneity is mainly as a result of soil formation processes, but physical, biochemical and anthropogenic factors. One type of heterogeneity is due to preferential flow paths or soil layering. Soil heterogeneity may create nutrient heterogeneity, on top of the decreasing concentration of nutrients from the upper layers of the soil (few cm) to its lower layers (few meters). Studies show that in deep layers there is a larger concentration of nutrients close to preferential flow paths as compared to the soil matrix. Mass, activity and diversity of the soil microorganisms was found to be affected by various factors, all of which are caused by soil heterogeneity. 

It is well known that different environmental conditions affect the short term biochemical processes and the long term microbial diversity, but little was done to relate those to soil interfaces and flow conditions. Our main objective in this research is to investigate the biochemical processes around textural interfaces (at the single cm scale), under treated wastewater (TWW) irrigated soils.

The objectives were addressed in two different environments: a field survey and laboratory experiments. Series of experiment were conducted in which I created artificial physical (textural) interfaces. The experimental set ups included a customized Hele-Shaw chamber and column experiments. The chamber experiments allowed following the spatial and temporal changes across the created interfaces in high resolution, while in the column experiments only spatial changes were analyzed.

The main experimental results showed that the order of the layers has great impact on the environmental conditions in the layer i.e. oxic/anoxic and the greatest environmental changes occurred in the upper layer. The layering also isolated the bottom layer from the changing environmental conditions at the upper soil and created stable conditions in this layer regardless of depth. The microbial populations shifted in the long-term due to the TWW irrigation.

Additionally, a mechanistic model of the physical, chemical and biochemical processes was developed using the results of the laboratory experiments for model calibration. The model examined the effect of the various configurations of layering on the ammonium and nitrate concentrations. The sensitivity of the model to different parameter was examined as well.

The model results suggest that layering achieved higher breakdown of ammonium and nitrate compared to homogenous profile, and creation of greater contrast of hydraulic conductivity between the layers intensifies the results- higher mineralization, nitrification and denitrification.