**2. Material and methods**

*Recent Advances in Numerical Simulations*

treatment is in agriculture [8].

first of theses in mineralization:

normally low in soils.

The sequence of products is:

The sequel to mineralization is nitrification:

There is potential for these nutrients present in recycled water to be used as a fertilizer source when the water is recycled as an irrigation source for agriculture [1]. Nitrogen is a valuable nutrient contained in wastewater [2]. Various studies confirm that municipal wastewater can be useful as an additional water resource for irrigation [3–7]. Some researchers have shown that the best way to use wastewater after

Understanding the behavior of nitrogen in the soil system helps to maximize crop production while reducing the impacts of N fertilization on the environment. Nitrogen applied as fertilizer or wastewater may be: utilized and stored in the plant; stored as organic nitrogen in the soil; volatilized as ammonia, nitrogen gas or nitrous oxide; lost in runoff; or leached to the groundwater as nitrate [9, 10]. The main processes response for nitrogen transport and transformations in the soil are mass transport of inorganic nitrogen forms, commonly described by the general convection–dispersion equations and both chemical and biological reactions [11]. Nitrate is one of the nitrogen compounds most susceptible to leaching. Three kinds of soil transformation of the N contained in wastewater are important. The

Organic N NH4

Mineralization occurs in soil as microorganisms, both aerobic and anaerobic convert organic nitrogen to inorganic forms. After wastewater application to soil, organic N quickly converts to ammonium nitrogen and then to nitrate nitrogen [12].

NH 3 / 2O NO H O 2H 4 2 22

No 1 / 2O NO 2 23

Microbial activity is also responsible for the two steps of nitrification. Nitrosomonas convert ammonium to nitrite. The second step of nitrification occurs through Nitrobacter species, which convert nitrite to nitrate. This step rapidly follows ammonium conversion to nitrite, and consequently, nitrite concentrations are

Another important nitrogen transformation in soils is denitrification.

Nitrate, which is the end product of the nitrification process in aerobic soils, it can undergo reduction to NO2 and finally to N2 when the soil oxygen content is low and decomposable organic materials are present to furnish energy for the process.

NO — — NO — — NO— — N O— — N 3 2 2 2

This process, which is done by a group of bacteria, is called desalination or denitrification. Denitrification occurs under oxygen-limiting conditions when anaerobic bacteria use nitrate in respiration in the presence of carbon sources such as organic matter. NO2 and N2 are both gaseous and emitted from the soil. Factors influencing denitrification control include oxygen restriction and the presence of organic matter. In the case of wastewater irrigation, only wastewater with a high BOD can be a source of organic matter for denitrification [12].

<sup>−</sup> > >> > (4)

<sup>+</sup> − → (1)

+ −+ + → ++ (2)

− − + → (3)

**132**

#### **2.1 Site description and measurements**

The weather data (daily maximum and minimum temperature, wind speed, humidity, sunshine hour and rainfall data) was collected from metrological station installed 2006 and 2007 at the Mashhad research station site, (36° 13′ latitude, 59°38′ longitude) in Northern east Iran. A soil profile pit was excavated to 120 cm depth and soil samples at different soil texture layers were sampled on 20 March 2009 before tomato sowing and basic properties, including soil water retention and saturated hydraulic conductivity were measured. The soil consists of heterogeneous layers with a deep groundwater ground water table (far below 80 m) and is characterized as sandy loam top soil (0–40 cm) over sandy clay loam (40–65) over sandy clay (65-120 cm). From the rooting depth (120 cm) of tomato crop 100 soil samples were collected and analyzed for various physical and chemical parameters before starting the experiment (**Table 1**).

TDR probes and ceramic cup tensiometers were installed at 0–20, 20–40, 40–60 and 60–100 cm soil layers in the investigated area. Water content measurements were taken daily starting in January 2009 and concluded in October 2010. TDR data will be used to assess estimate of shallow soil water content at soil profile. The irrigation scheduling was based on the soil moisture deficit in the root zone at each irrigation event (difference between root zone soil water at field capacity and at irrigation time) with intervals of 10 days. The characteristics of water and wastewater are summarized in **Table 2**. Total irrigation depth during this period was 23.9 cm.


#### **Table 1.**

*Physical and chemical properties of soil at initial condition.*


**135**

**Table 3.**

*The proportions of water and wastewater.*

*Numerical Modeling of Soil Water Flow and Nitrogen Dynamics in a Tomato Field…*

T2 - Alternate irrigation by treated wastewater and well water.

the growing season, (%50 wastewater +%50 well water).

T3 -Irrigation with well water plus application animal manure. T4 - Irrigation with well water plus application of fertilizer.

growing season, (%25 wastewater +%75 well water).

analysis of animal manure has been showed in **Table 4**.

season, (%75 wastewater +%25 well water).

T5 - Irrigation with well water only.

The rainfall at the same period was 11.48 cm and reached 42.84 cm for the whole

Plots were irrigated with either well water or wastewater in a random complete block design (RCBD), with four replications according to the following treatments: T1 - Irrigation by treated wastewater during all growing season, (%100

1.Alternate irrigation of tomato with wastewater and well water during the growing

2.Alternate irrigation of tomato with wastewater and well water during

3.Alternate irrigation of tomato with wastewater and well water during the

To obtain these ratios were used in the operation of the irrigation turn. (**Table 3**). The experiments were carried out on 20 plots, and each experiment included five irrigated furrows 4 m in width and 4.2 m in length (along the crop rows). Each plot consisted of 5 crop rows with a plant row spacing of 75 cm. The plots (T3) were grazing prior sowing with 3000 kg ha−1 or (3 kg m−2) animal manure. The chemical

The plots (T4) were fertilized based on soil sample tests with 300 kg ha−1 or (30 gr m−2)of triple super phosphate, broadcast at seedbed preparation, and 110 kg ha−1 of net nitrogen or (200 kg ha−1) of urea at tillage time (at two equal section) and 6 weeks after panting. In this study, the total manure was applied prior sowing and

First Wastewater Well water Well water Well water Well water Second Wastewater Wastewater Wastewater Well water Well water Third Wastewater Wastewater Well water Well water Well water Fourth Wastewater Wastewater Wastewater Wastewater Well water Fifth Wastewater Well water Well water Well water Well water Sixth Wastewater Wastewater Wastewater Well water Well water Seventh Wastewater Wastewater Well water Well water Well water Eighth Wastewater Wastewater Wastewater Wastewater Well water Ninth Wastewater Wastewater Wastewater Well water Well water Tenth Wastewater Well water Well water Wastewater Well water

**Mixing ratios of wastewater 1% 2% 3% 4% 5% 100s 75 50 25 0**

*DOI: http://dx.doi.org/10.5772/intechopen.98487*

simulation period of one year.

**2.2 Experimental design**

wastewater).

**Irrigation turns**

#### **Table 2.**

*Physico-chemical characteristics of water and treated wastewater.*

The rainfall at the same period was 11.48 cm and reached 42.84 cm for the whole simulation period of one year.
