As the season continued and plant uptake was reduced, excess water further mobilised nitrate–nitrogen out of the root zone, as is evident from 27/04/07 and beyond . At the end of the crop season, little nitrogen remained in the soil system, and what did remain was well beyond the reach of the plants. This nitrogen is expected to continue leaching downwards over time and become a potential source of nitrate–nitrogen loading to the ground water. Additionally, peak NO3 –N concentrations in the soil profile L 1 ) and in drained water L 1 ) were significantly higher than the Australian environmental standard for protection of 80% NO3 –N L 1 ) and 95% of species NO3 –N L 1 ). The NO3 –N concentrations in the soil solution also occasionally exceeded the level of Australian drinking water quality standard for nitrate NO3 –N L 1 ). High levels of nitrate–nitrogen below the crop root zone are undesirable, as some recharge to groundwater aquifers can occur, in addition to flow into downstream rivers, which are used for drinking water and irrigation. These findings are consistent with other studies , in which high nitrate concentrations in drainage water under drip and furrow fertigated irrigation systems have been reported.The seasonal water balance was computed from cumulative fluxes calculated by HYDRUS-2D. Estimated water balance components above and below the soil surface under a mandarin tree are presented in Table 4. It can be seen that in a highly precise drip irrigation system, a large amount of applied water drained out of the root zone,hydroponic nft system even though the amount of irrigation applied was based on estimated ETC. This drainage corresponded to 33.5% of applied water, and occurred because highly permeable light textured soils, such as those found in this study, are prone to deep drainage whenever the water application exceeds ETC.
The drainage amount in our study falls within the range of recharge fluxes to groundwater reported by Kurtzman et al. under citrus orchards in a semiarid Mediterranean climate. Mandarin root water uptake amounted to 307.3 mm, which constitutes about 49% of applied water. Root water uptake slightly increased when the model was run without considering solute stress , which is not a significant difference. It further substantiates the results obtained for seasonal ECsw in Fig. 6, where salinity remained below threshold over the season. Evaporation accounted for 17.7% of the total water applied through irrigation and rainfall. The modelling study overestimated the sink components of the water balance by 4.79 mm . There were major differences between water input and output from January 2007 onwards . During this period, irrigation and precipitation significantly exceeded tree water uptake , which eventually resulted in deep drainage from March 2007 onwards. Therefore, current irrigation scheduling requires adjustment during this period. This illustrates how simulations were helpful in evaluating the overall water dynamics in soil under the mandarin tree. The nitrogen balance is presented in Table 5. The nitrogen fertilizer was applied either in the form of NH4 + or NO3 , but NH4 + transforms quickly to NO3 through the process of nitrification. Model simulations showed that nitrification of NH4 + was very rapid and most of the NH4 + –N converted to NO3 before it moved to a depth of 20 cm, and no traces of NH4 + were observed below this depth. It is apparent that the nitrification of NH4 + took place in the upper soil layer, which contains organic matter and moisture that supports microorganisms , facilitating the nitrification of NH4 + . Though NH4 + was initially nitrified to NO2 and consequently to NO3 , NO2 was short-lived in the soil and decayed to NO3 quickly. Therefore, the simulated plant NH4 + –N uptake was only 0.71 kg ha 1 . Hence, the NO3 –N form was responsible for most of the plant uptake, corresponding to about 85% of the applied nitrogen.
The monthly N applications were slightly higher than plant uptake during the flowering and fruit growth periods . However, the monthly uptake was slightly higher than the N application between these periods. High frequency of N applications in small doses resulted in similar nitrogen uptake efficiency in citrus as in other studies . Similarly, Scholberg et al. reported doubling of nitrogen use efficiency as a result of frequent application of N in a dilute solution. Slightly higher uptake was recorded when fertigation was applied in second last hour of an irrigation event , as compared to when it was applied early in the irrigation event . Hence, it can be concluded that timing of fertigation does not have a major impact in a normal fertigation schedule with small and frequent N doses within an irrigation event in light textured soils. Similar results were also obtained in our earlier study in a lysimeter planted with an orange tree , which revealed that timing of fertilizer N applications in small doses in an irrigation event with a low emitter rate had little impact on the nitrogen uptake efficiency.Monthly N balance revealed that most of the N leaching happened between March 2007 and August 2007, which was correlated with the extent of deep drainage occurring during this period. NO3 –N losses ranging from 2% to 15% were illustrated by Paramasivam et al. and Alva et al. , attributable in part to an improved management of N, which could be a contributor in the current estimation.In our study, it is evident that there were significant deep drainage and nitrate–nitrogen leaching losses , which could be reduced by appropriate management. Hence, different simulations involving the reduction of irrigation and fertigation applications during the whole or part of the crop season were conducted, to optimize water and nitrogen uptake and to reduce their losses from the soil . Increasing the irrigation frequency with short irrigation events while maintaining the same irrigation volume, had no impact on deep drainage and N leaching .
However, the seasonal salinity increased by 11% compared to the standard practice. This confirms that the current irrigation schedule followed with respect to the irrigation frequency seems to be optimal under the experimental conditions. In S2, Dr_W and Dr_N were reduced by 14.4% and 19%, respectively, but salinity increased by 11%. However, a sustained reduction in irrigation by 20% eventually reduced the Dr_W and Dr_N by 28.1 and 38.3%, respectively, at the expense of a 4.9% decline in plant water uptake, but with a 4% increase in N uptake. However, salinity increased by 25.8% compared to the normal practice, which would likely have a significant impact on plant growth. Scenarios S4 and S5 were based on decreasing the nitrogen application by 10% and 20%,nft channel resulting in a decrease in N leaching by 7.4% and 14.8%, respectively, along with a much higher reduction in plant N uptake , suggesting that the reduction in the fertilizer application alone is not a viable option to control N leaching under standard conditions. A combined reduction in irrigation and fertigation by 10% further reduced N leaching by 5.5%, compared to reducing irrigation alone , but at the same time plant N uptake was reduced by 5% more than in S2. Similarly, reducing irrigation and N application by 20% produced a pronounced reduction in N leaching and water drainage , but it also resulted in a decrease in plant N uptake by 15.8% and water uptake by 4.8%, compared to normal practice. At the same time, salinity increased by 25.8%, which is similar to S3. The reduction in plant water and N uptake would have a major impact on plant growth and yield, and would adversely impact the sustainability of this expensive irrigation system. Hence, reducing fertilizer applications does not seem to be a good proposition under the current experimental conditions, as it results in an appreciable decline in plant N uptake. However, Kurtzman et al. reported that a 25% reduction in the application of N fertilizer is a suitable agro-hydrological strategy to lower the nitrate flux to groundwater by 50% under different environmental conditions. Rather, reducing irrigation alone seems to be a better option to control the deep drainage and N leaching losses under the conditions encountered at the experimental site. Additionally, it is worth noting that in S3 and S7 the salinity during a period between October and December at a depth of 25 cm, and during December at a depth of 50 cm, increased considerably, and was higher than the threshold level , confirming that a sustained reduction in irrigation and fertigation is not a viable agro-hydrological option for controlling water and N leaching under the mandarin orchard.
However, it seems unnecessary to reduce irrigation applications uniformly across the season as suggested by Lido9n et al. . Rather, irrigation could more profitably be reduced only during a particular time period when excess water was applied. The water and N balance data in our study revealed that an imbalance between water applications and uptake happened during the second half of the crop season, i.e., from January till August 2007, resulting in maximum drainage and N leaching , coinciding with the fruit maturation and harvesting stage. Hence, there is a need to reschedule irrigation within this period, rather than reducing water applications throughout the entire season. Keeping this in mind, the following 5 scenarios were executed, in which irrigation was reduced during the second half of the crop season, i.e., between January and August, by 10%, 20%, 30%, 40%, and 50%, respectively. Scenarios S10, S11, and S12 showed an enormous potential for reducing water and N losses. In S10, Dr_W and Dr_N were reduced by 8% and 4% more than in S7, N uptake was increased by 6.9% , and salinity was also 4% less than in S7, which seems quite promising. On the other hand, in S11 and S12, the Dr_W and Dr_N were reduced to a greater extent than in S10, and soil salinity increased substantially , due to a considerable reduction in the leaching fraction. This is also shown in Fig. 12, which shows that monthly soil solution salinity in S11 and S12 at the 25 and 50 cm soil depths increased dramatically between January and August. Although ECsw remained below the threshold level, except at a 50 cm depth in S12 during March 2007, there is a significant likelihood of it increasing further in subsequent seasons, which would ultimately impact the growth and yield of mandarin trees. Hence, under current conditions, Scenario S10 represents the best option to control excessive water and N losses, and high salinity, and to increase the water and N efficiency for mandarin trees. Other permutations and combinations, involving fertilizer reductions along with S10, did not provide further improvements in controlling water and N leaching. It is concluded that simulations of irrigation and fertilizer applications, using HYDRUS, can be helpful in identifying strategies to improve the water and N efficiency for drip irrigation systems of perennial horticultural crops.Climate change, continuing population growth, and urbanization are exerting an unprecedented pressure on fresh water supply, mandating the use of nontraditional water resources , such as treated municipal wastewater, for agricultural irrigation . Treated wastewater irrigation, however, poses potential risks because a multitude of trace contaminants, including numerous pharmaceutical and personal care products , are incompletely removed during wastewater treatment and may enter the soil-plant continuum . Land application of bio-solids and animal manure constitutes yet another route for such trace contaminants to enter agroecosystems . Once in the agroecosystems, PPCPs may be translocated into edible plant parts and thus, enter the terrestrial food chains, including human diet. Consequently, plant accumulation of PPCPs is raising widespread concerns due to potentially deleterious effects on the environment and human health . Studies over the past decade show that various PPCPs can be taken up from soil by plants . For instance, Wu et al. detected 16 PPCPs in edible tissues of eight common vegetables grown with treated wastewater irrigation. Additional studies showed that some PPCPs can pose significant phytotoxicity, leading to inhibition of plant growth . For example, carbamazepine, an antiepileptic drug, displayed phytotoxic effects toward Cucurbita pepo at concentrations N 1 mg kg−1 in soil .