Evapotranspiration as a Criterion to Estimate Nitrogen Requirement of Maize Under Salt Stress

We tested the hypothesis that by reducing the application of N, based on the decrease in evapotranspiration (ET) expected due to increase in soil salinity, it is possible to reduce N loss without causing N deficiency or further yield loss in salt-stressed maize plants. We tested four levels of salinity of irrigation water (S1=0.5; S2=2.5; S3=5.0; and S4=7.5dSm(-1)) and four N rates using outdoor soil columns with five replicates. The N rates were as follows: N1: N recommendation for maize (2.6g per column); N2: 0.3 times the N recommendation (0.78g per column); N3: reduction in N1 based on the decrease in ET caused by salinity; and N4: reduction in N2 based on the decrease in ET caused by salinity. The amounts of N for N3 and N4 were reduced (in relation to N1 and N2) by 7%, 15% and 30% for 2.5, 5.0 and 7.5dSm(-1), respectively. Salinity caused NO3- accumulation in the soil, plant growth inhibition and stomatal closure. The low rates of N (N2 and N4) did not meet the N demand of maize plants, especially for low levels of salinity (control and 2.5dSm(-1)). On the other hand, based on the available growth data, physiological responses and nutritional status, one can conclude that plants under N1 and N3 had the same potential for final yield. For these N rates, reduction in N application according to ET (N3 rate) not only allowed plant growth and maize physiological responses, but also increased N-use efficiency and greatly reduced soil nitrate accumulation compared to N1 rate, at the same levels of salinity. We conclude that reduction in N application, based on reductions in ET, is a good strategy to reduce both the risk of ground water contamination by NO3- leaching and fertilization costs, without causing additional damage to plant development under salt stress.