Effect of Irrigation System and Nitrogen Fertilizer Level on Nitrate Distribution in Soil Using Nitrogen-15 Isotope Tracking Technique

Soil contamination is the presence, diffusion or fusion of foreign matter into the soil, altering its physical and chemical quality in a manner that is harmful to humans, plants and the environment. Soil nitrate pollution due to excessive use of nitrogen fertilizer and inappropriate irrigation causes nitrate accumulation under the active root zone and its movement to groundwater and endangers the environment. By labelling the soil with 15N-labelled nitrate or urea it is possible to trace the fate of fertilizer derived nitrate down the soil profile. This can be achieved by taking sequential by using suction cups to sample the nitrate in the soil solution. The purpose of this study was to investigate the effect of irrigation system and nitrogen fertilizer level on the amount and pattern of nitrate distribution in different soil depths.

The experiment was conducted in a randomized complete block design with a split plot in two plots and three replications on tomato plant in Agricultural Research Farm of Nuclear Science and Technology Research Institute. Furrow and drip irrigation systems as the main factor and fertilizer treatment (100 and 200 kg N/ha from urea fertilizer source), soil depths (including 15, 30 and 60 cm) and sampling time (Includes 28, 40, 61 and 80 days after plantin) were first, second and third sub-factors respectively. In order to trace nitrogen, CO(15NH2)2 urea fertilizer with enrichment of 4.634% was used. Three soil solution extractors were installed at depths of 15, 30 and 60 cm in each isotopic plot in each replication and extraction was performed 4 times. Soil solution nitrate and 15N/14N isotope ratio were measured by spectrophotometer and mass spectrometer respectively.

The highest soil nitrate-N (N-NO3) concentration(94.31 mg L-1) in furrow irrigation (Fertilizer level of 200 kg N ha-1, soil depth of 60 cm and third time of soil solution sampling) and its lowest concentration(1.73 mg l-1) in drip fertigation system (fertilizer level of 100 kg N ha-1, soil depth of 60 cm and fourth time of soil solution sampling) was observed. The results showed that the concentration of nitrate-N in the drip fertigation system was higher at a depth of 15 cm (active root depth) than at depths of 30 and 60 cm. The highest concentration of nitrate nitrogen derived from the source of nitrogen-15 (N-15NO3 dff)(88.82 mgl-1) in furrow irrigation (Fertilizer level of 200 kg N ha-1, soil depth of 60 cm and third time of soil solution sampling) and the lowest concentration (0.12 mgl-1) in drip irrigation fertilizer (fertilizer level of 100 kg N ha-1, soil depth of 30 cm and second time of soil solution sampling) was observed. Nitrate-N concentration derived from labeled fertilizer source in furrow irrigation at a depth of 60 cm (below the active root depth in furrow irrigation) was greater than the depths of 15 and 30 cm. the results also showed that The highest concentration(42.25 mgl-1) of nitrate-N derived from soil source in drip fertigation system (fertilizer level of 200 kg N ha-1, soil depth of 15 cm and first time of soil solution sampling) and the lowest concentration (0.29 mgl-1) in drip fertigation system (100 kg N ha-1, soil depth of 60 Cm and the fourth time of soil solution sampling) was observed.

The results showed that of the total nitrate nitrogen in the 0-60 cm depth, the values (62, 29 and 9%) in the drip and (10, 34 and 56%) in the furrow irrigation system in Depths of 15, 30 and 60 cm were observed respectively. Nitrogen-15 data showed that of the total soil nitrate nitrogen, the values of 20 and 80 percent in fertigation system and 77 and 23 percent in furrow irrigation system was observed from labeled fertilizer and soil source, respectively. increasing nitrate accumulation was observed in soil depth of 60 cm with increasing nitrogen application in furrow irrigation. The use of fertigation system was effective to prevent nitrogen loss from the active root zone of the plant. In general, fertigation system and fertilizer level of 100 kg N ha-1 was the best irrigation method and the best fertilizer level to reduce nitrate leaching losses in the conditions of this study.