Effect of Deficit Irrigation Practice on Nitrogen Mineralization and Nitrate Nitrogen Leaching under Semi-Arid Conditions

Agricultural sector acts as a major consumer of water which accounts for 70 percent of global freshwater use. Water scarcity acts as an imminent threat to agriculture, there is a need to use those irrigation and management practices that could overcome this overwhelming situation of water scarcity. Lab incubation study was designed to evaluate the effect of different moisture levels (50%, 60%, 70%, 80%, 90%, and 100% FC) on nitrogen mineralization rate. Net nitrogen mineralization was shown at 60% and 80% FC levels. Two opti-mized irrigation levels (I 0.6 and I 0.8 ) along with four levels of dairy manure (10, 15, 20, and 25 Mg ha −1 ) were used in a lysimetric trial. Nitrate-nitrogen was measured at four depths (D 1 : 30 cm, D 2 : 60 cm, D 3 : 90 cm, and D 4 : 120 cm). Results showed strong interaction of irrigation and dairy manure at all depths. Mean maximum nitrate-nitrogen concentration was shown under full irrigation at 120 cm soil depth with the application of DM @ 25 Mg ha −1 . Under two levels of deficit irrigation, I 0.8 has shown maximum nitrate-nitrogen concentration at 90 cm soil depth with the application of DM 25 , however, deficit irrigation level I 0.6 restricted nitrate-nitrogen movement up to 60 cm soil depth, and high concentration was found at 30 cm soil depth. We concluded that deficit irrigation practice along with dairy manure resulted in more nitrate-nitrogen in the upper 60 cm layer of soil where it can be more available for the crops.


Introduction
Water scarcity is becoming an imminent threat to the agriculture sector all over the world and the problem is of much concern in developing countries [1]. According to a report by the UN, by 2025, two-thirds of the world population would be under the water stress conditions [2]. Pakistan is running out of fresh water at an alarming rate. This shortfall will be devastating for a country whose economy is based on agriculture.
Water and nitrogen are the key factors affecting crop growth. The fate of nitrogen in the soil is directly affected by the availability of water in soil [3]. Mismanagement of nitrogen fertilizer is another aggravated problem [4]. Farmers are applying more and more fertilizer without knowing its fate in soil and impact on the environment [5]. Furthermore, the farming community is wasting a tremendous amount of freshwater by using outdated flood irrigation methods. About 95 percent of freshwater is utilized for irrigation, and yet the country is achieving a lower per acre crop production when compared to India and China in the region.
Under current circumstances of water scarcity and for efficient management and utilization of applied fertilizer, there is a need to improve soil properties for efficient utilization of water and nutrients. In this regard use of dairy, manure is countable because manure contains organic carbon that acts as s a key indicator for soil health [6] [7]. Some studies supported the fact that dairy manure acts as a slow-release fertilizer and avoids nutrient losses [8] [9]. On contrary, some studies reported that the addition of manure causes excessive nitrate-nitrogen leaching [10] under full irrigation but very little knowledge exists regarding role of dairy manure on nitrate leaching under deficit irrigation conditions [11].
Dairy manure can ameliorate soil conditions for maximum nutrients and water utilization but very little knowledge is present regarding its role in nitrogen productivity in soil under reduced water conditions. The present study was designed with the purpose to evaluate the comparative effect of deficit and full irrigation on nitrate-nitrogen productivity under different levels of dairy manure to improve soil nutrient status under deficit irrigation conditions.

Incubation Study
An incubation study was carried out at Soil Physics Laboratory, ISES, University of Agriculture Faisalabad, Pakistan. The soil used for this experiment was collected manually using a shovel from a depth of 15 cm. Soil belonged to the Hafizabad soil series (Aridisol-loam, mixed) and soil basic characteristics are given below in Table. The soil was air-dried and sieved through a 2 mm sieve and placed on a plastic sheet. Six sets of soil were prepared based on water content (50%, 60%, 70%, 80%, 90% and 100% of field capacity). Soil water retention capacity was determined by the filter paper method [12]. Urea fertilizer was applied equally in all sets of soil to allow mineralization without nitrogen limita-tion. Soil samples were placed in air-tight plastic jars with a vial of water to maintain humidity. Soil samples were incubated at 25˚C for 40 days. The moisture level was maintained two times a week. For mineral nitrogen measurements, 200 g equivalent oven-dry soil was taken in 1/2 L plastic jars. 20 g of soil samples from each jar were taken after 7, 14, 28, and 40 days of incubation for measurement of mineral nitrogen.

Lysimetric Trial
The lysimetric trial was conducted at Research Area Institute of Soil and Environmental Sciences, University of Agriculture Faisalabad. Lysimeter used in this study was made of concrete. The area of each lysimeter was 4.5 m 2 . Leachate collection was done at 30, 60, 90, and 120 cm depth with the help of a solution sampler installed at the required depth. Wheat variety (cv. Galaxy-2013) was sown and seeds were pre-soaked for 12 hours to facilitate germination. Two optimized moisture levels from the incubation study were applied as I 0.6 (60% FC) I 0.8 (80% FC and control I 1.0 (100% FC). Four levels of dairy manure were applied as DM 10 , DM 15 , DM 20 , and DM 25 . The dose of NPK was applied as 100-85-65 kg·ha −1 . Nitrogen was applied in less amount the than recommended rate of 120 kg·ha −1 . Applied manure composition is shown in Table 1.

Irrigation water measurement
Cut throat flumes were used to apply a measured amount of irrigation water to the lysimetric trial. The time requirement for plots irrigation to desire depth was calculated as follow:  (STATISTICAL DESIGN) The data was statistically analyzed through Analysis of Variance technique using CRD for incubation study. For lysimeter trial CRD design was used with split plot arrangements. Further significance of interaction was analyzed using honestly significant difference test (HSD) [13] at 5% probability level.

Results
Interactive effect of different levels of moisture and incubation time on nitrate nitrogen release at constant temperature 25˚C is presented in Figure 1.  Under lysimetric trial, mean maximum nitrate nitrogen concentration (22.83 mg·kg −1 ) was shown under full irrigation with application of dairy manure at the rate of 25 Mg ha −1 ( Table 2). Under two levels of deficit irrigation, I 0.8 has shown mean maximum nitrate nitrogen concentration 16.23 mg·kg −1 with application of dairy manure at the rate of 25 Mg ha −1 at 90 cm soil depth. Mean maximum nitrate nitrogen concentration was shown at a depth of 120, 90 and 30 cm soil depth for I 1.0 , I 0.8 and I 0.6 , respectively (Table 2). Our study results showed that maximum nitrogen mineralization with reduced nitrate leaching was shown under deficit irrigation level I 0.8 followed by I 0.6 . Deficit irrigation practice is not only affective for efficient irrigation water management but also ensures nutrients availability within crop vicinity. Furthermore, dairy manure application showed significant result along with deficit irrigation practice. Future research should be focused on the use of other amendments which could perform better at 50% of recommended irrigation rate with no compromise on yield.

Discussion
Soil moisture acts as an important factor regarding nitrogen availability in soil [14]. Moisture also has strong effect on nitrogen mineralization rate through its use by soil microorganism [14]. The amount of nitrate leach down directly relates with percolation volume. Under incubation study, mineralized nitrate has shown direct relation with moisture level, this may be due to highly connected water filled pore spaces, which results in rapid diffusion of ammonia from mineralization to nitrification site and thus releases more nitrate [15]. Significant increase in nitrogen mineralization was shown when moisture was applied at half of recommend rate of irrigation (50% FC), this may be due to rewetting of dry soil that results in release of nitrogen [16]. Water contents at 60% field capacity also showed high release of nitrate nitrogen and ammonium nitrogen.
Zhou et al. (2001) found similar results that rate of mineralization increased as moisture content increased with range of 46% -54%. Overall, net nitrogen mineralization increased when moisture content increased from 70% FC and maximum was noted at 80% FC.
Our results showed significant effect of moisture on mineralization, as other studies also supported this fact that mineralization strongly depends on moisture level in soil [17]. Maximum mineralization found under low moisture levels may be due to rewetting of dry soil causing more release of nitrogen than saturated soils [17] [18].
In lysimetric trial, high nitrate nitrogen concentration was shown under full irrigation as compared to deficit irrigation. Many studies also supported nitrate nitrogen leaching under full irrigation with application of organic amendments, with very few literature on nitrate nitrogen leaching under deficit irrigation level [19]. Under full irrigation conditions, mean maximum nitrate nitrogen concentration found was 117.74% and 101.5% higher than deficit irrigation level I 0.8 and I 0.6 , respectively. Nitrogen undergoes leaching losses usually under full irrigation [20] as compared to deficit irrigation due to high amount of percolation water, because nitrate nitrogen is negatively charged therefore moves rapidly to deep in soil with water.
Application of dairy manure resulted in significant increase in leaching concentration under all levels of irrigation water, this may be due to the fact that manure application results in improved soil structure and enhanced soil porosity which results in efficient water movement within soil profile [21] [22]. Many studies supported the role of dairy manure in increasing leaching concentration [23] [24]. On the other hand, some studies also shown contrary results that dairy manure application restricts leaching losses because manure act as a slow release fertilizer [25]. Under two levels of deficit irrigation, I 0.8 in combination with high application of dairy manure (25 Mg ha −1 ) resulted in movement of nitrate nitrogen deep to soil profile at 120 cm. Under deficit irrigation level I 0.6 , nitrate nitrogen movement was limited up to 60 cm soil depth and mean maximum concentration was shown at 30 cm sol depth. Overall 60% field capacity level pro-vided best results with maximum nitrate nitrogen within root vicinity, but further work in needed to check long-term effect on crop yield and soil properties.

Conclusion
Water scarcity is an imminent threat to agriculture industry which is a major consumer of fresh water use. There is a need of time to introduce water management practices with emphasis on minimum use of irrigation water. For efficient utilization of water and nutrients, there is need to improve soil physical conditions for efficient movement of water. In our incubation and lysimeteric study, we concluded that deficit irrigation practice not only resulted in high release of mineral nitrogen (mineralization) but also restricted nitrate nitrogen in upper 60 cm soil layer where it will be highly available to plant roots. While, under full irrigation nitrate nitrogen leached down to profile up to 120 cm soil where it becomes unavailable to crops. Furthermore, application of dairy manure showed direct relation with the release of nitrate nitrogen. Future research is needed on long term experimentation of deficit irrigation practice in combination with organic amendments.