Effects of alternate partial root-zone irrigation and nitrogen forms on utilization and movement of nitrate in soil

Abstract: Numerous evidences show that alternate partial root-zone irrigation (APRI) could save large amounts of irrigation water without significant yield reduction. In addition, intensive studies indicated that APRI could reduce nitrate leaching and increase the chance of nitrogen being absorbed by plant. However, all these studies could not discriminate the fate of nitrate in different soil layers under APRI. The fate of residual nitrate in specific soil layers and its regulation by different forms of nitrogen fertilizers (nitrate- or ammonium-nitrogen fertilizer) under APRI were studied in this paper by means of soil column experiment with labeled nitrate-N (K15NO3) applied to 10-20 cm or 40-50 cm layers. The results showed that compared with conventional irrigation, APRI saved 34.31 percent of irrigation water without significant yield reduction. Nitrate-nitrogen fertilizer improved nitrogen absorption by tomato plants to an extent of 27.0 percent, thus increasing plant growth and tomato yield. The rate of 15N absorbed by plant decreased significantly as 15N was labeled to deeper soil layer. The absorption of 15N by plant decreased 33.1 percentage for conventional irrigation and 23.0 percentage for APRI when the 15N-labeled layer decreased from 10-20 cm to 40-50 cm. Nitrate nitrogen supply promoted absorption of residual nitrate in soil profile to an extent of 53.9 percent compared with ammonium nitrogen supply by promoting tomato root growth and biomass accumulation. The leaching of soil residual nitrate from 10-20cm layer was more intensive than from 40-50 cm layer during tomato growth. For the soil columns with 15N labeled to the 10-20cm layer, the remaining rate of 15N in the 10-20cm layer was 2.7 and 23.1 percentage, the 15N accumulation peak moved downward 30cm and 10cm, and 15N accumulated in the 0-100cm soil layer took account of 60.9 and 75.2 percent of total 15N amount introduced by K15NO3, with loss rate 17.0 and 5.17 percentage, respectively, for conventional irrigation and APRI. However, for the soil columns with 15N labeled to the 40-50cm layer, the remaining rate of 15N in the original labeled layer was 38.1 percentage, whereas under APRI, 15N accumulation peak moved upward 10cm with 61.3 percent of 15N moved to 0-40cm layer, leading to 15N accumulation in 0-100cm soil 24.2 percent more than the conventional irrigation and a large extent of reduction of 15N loss. Compared with ammonium nitrogen supply, nitrate fertilizer promoted plant growth and thus 15N uptake, leading to a lower 15N accumulation after plant harvest in the top 100cm soil. However, there was no significant difference in 15N loss between soil columns fertilized with ammonium nitrogen or nitrate nitrogen. These results suggested that APRI could reduce soil residual nitrate leaching, and promote residual nitrate in deeper soil move upward, thus increasing the chance of being utilized by plant. Different forms of nitrogen fertilizer affect the fate of residual nitrate in soil profile through affecting plant growth.