Relationships between nitrogen rate, plant nitrogen concentration, yield and residual soil nitrate‐nitrogen in silage corn

Abstract

Abstract Nitrogen (N) fertilizer is a key factor of yield increase but also an environmental pollution hazard. The sustainable agriculture system should have an acceptable level of productivity and profitability and an adequate environmental protection. The objectives of this study were to determine the relationships between N rate, DM yield, plant N concentration (NC) and residual soil nitrate‐nitrogen in order to improve the predicted N rate in corn (Zea mays L.) silage. The experiment was conducted over a period of three years in the province of Quebec on three soil series in a continuous corn crop sequence. Treatments consisted of six rates of N: O, 40, 80, 120, 160, and 200 kg N ha‐1 as ammonium nitrate applied at planting: broadcast and side banded. Four optimum N rates were calculated using different models: (i) economic rate base on fertilizer and corn price using the quadratic model (E); (ii) economic rate based on fertilizer and corn price using the quadratic‐plus‐plateau model (QP); (iii) critical rate based on linear‐plus‐plateau model (P); (iv) lower than maximum rate (L) corresponding to 95% of maximum yield. The optimum plant NC at all growing stages and the N uptake at harvest were calculated depending on these N rates and yields. The NC of whole plant at 8‐leaf stage (25–30 cm plant height) of ear leaf at tasselling and of whole plant at harvest stage, the N rate, the N uptake at harvest and the DM yield were all significantly intercorrelated and affected by soils and years, but not affected by N fertilizer application method. The DM yield was linearly and significantly related to NC of whole plant at 8‐leaf stage (rv = 0.932**). At this stage, the average NC corresponding to the optimum N rate and yield was of 3.71, 3.68, and 3.66% as calculated with E, L, and P model, respectively. Our data suggest that the NC of whole plant at 8‐leaf stage may be used to evaluate the N nutrition status of plant and the required optimum N fertilizer rate. The NC of ear leaf at tassel stage was also significantly correlated to corn yield (r = 0.994**). It may be used as an indicator to evaluate the near‐optimum N rate in the subsequent years. The N uptake by whole above‐ground plant at harvest was quadratically related to corn yield. Data show that at high fertilizer N rate, the N uptake still increased without significantly increasing yield. The N uptake was of 176.5, 163.0, and 155.0 kg N ha‐1 using the E, L and P rates of 146, 126, and 115 kg N applied ha‐1, respectively. The optimum N rate and yield were affected by soil type and year, but not by the method of N fertilizer application. The yield increased rapidly up to a N rate of about 120 kg N ha‐1 and then quite slightly to a maximum N rate of 192 kg N ha‐1. The optimum N rate was of 115 and 126 kg N ha‐1 using the P and L model respectively and as high as 146.8 kg N ha‐1 using the E model. The L model, using a much smaller N rate, gave a reasonably high yield compared to E rate (12.2 and 12.5 Mg ha‐1, respectively). The data show that a relatively much lower N rate than maximum did not proportionally diminish the yield. Thus, for a difference of 40.4% between maximum N rate and P rate a difference of only 7.4% in yield was observed. Using the L model the differences in rate and yield were of 34.4% and 4.7%, respectively. The QP model gave no significant difference compared to E model. At harvest the residual soil NO3‐N increased significantly with increasing N fertilizer rate in whole of the 100 cm soil profile, but mainly in the top 40 cm soil layer. The total NO3‐N found in 0–100 cm profile at rate of 0, 120 and 200 kg applied N ha‐1 at planting was as high as 33.7, 60.5, and 74.5 kg N ha‐1 respectively in a light soil and 37.5, 97.5, and 145.5 kg N ha‐1 in a heavy clay soil. The difference in NO3‐N content in the 60–100 cm layer between different applied N rate suggests that at harvest, part of fertilizer N applied at planting was already leached below the 100 cm soil layer. Results, thus, show that reasonably high corn yields can be obtained using more adequate N fertilizer rates which avoid the overfertilization and are likely to reduce the air and ground water pollution.