Effects Of Irrigation And Nitrogen Application Rates On Growth, Yield, Water And Nitrogen Efficiencies Of Spring Wheat Under Permanent Raised Bed

The yield of spring wheat is limited by water and nutrient availability in the oasis of Hexi(Gansu) Corridor. The excessive application of irrigation and nitrogen fertilizer had very big influence on agricultural non-point source pollution. Introduction and extension of water saving agricultural practices and reduction of nitrogen fertilizer are very important to realize the high-efficient and high-yield and water-saving cultivation of spring wheat, and obtain the sustainable development of agriculture. In an effort to establish an optimum combination of water and nitrogen for spring wheat under permanent raised bed(PRB) tillage, a field investigation was carried out on a sandy-loam soil in arid northwest China in 2015, the experiment followed a split-plot design, three furrow irrigation(1200, 2400, 3600 m3·hm-2) being in the main plot and four rates of nitrogen(0, 90, 180, 270 kg·hm-2) in the subplot, to assess spring wheat growth, spring wheat root growth, water consumption, production of dry matters, grain yields, distribution and changes in soil NO3-N, and relationships between root systems and grain yields as well as water and nitrogen utilization were also explored. The experimental results are summarized as follows:1. Wheat growth was significantly affected by irrigation and nitrogen application. There was a sharp positive interaction between irrigation and nitrogen application, which contributed to the higher leaf area index, accumulation and distribution of dry matter in grain, the coordination of water and nitrogen was declined when irrigation and nitrogen application were too high or too low. At the same nitrogen application rate, dry matter accumulation, chlorophyll SPAD value, photosynthetic rate, transpiration rate and stomatal conductance were increasing with the increase of irrigation amount(W3>W2>W1), intercellular CO2 was decreasing with the increase of irrigation amount. The ratio of dry matter distribution to grain initially increased and then decreased with the increase of irrigation amount(W2>W3>W1). The effect of nitrogen on spring wheat growth depended on soil water conditions. Chlorophyll SPAD value initially increased and then decreased with the increase of nitrogen rate(N2>N3>N1>N0) under W1 irrigation conditions; under W2 irrigation conditions, nitrogen fertilizer application at a rate of 180 kg.hm-2(N2、N3>N1>N0) obtained the highest values of chlorophyll SPAD value from the jointing of spring wheat to the filling, chlorophyll SPAD value increasing with the increase of nitrogen rate at maturity stage(N3>N2>N1>N0). Chlorophyll SPAD value was increasing with the increase of nitrogen rate under W3 irrigation conditions(N3>N2>N1>N0). Under W1 and W2 irrigation conditions, nitrogen fertilizer application at a rate of 180 kg.hm-2(N2>N3>N1>N0) obtained the highest values of photosynthetic rate, transpiration rate and stomatal conductance; Photosynthetic rate, transpiration rate and stomatal conductance were increasing with the increase of nitrogen rate under W3 irrigation conditions(N3>N2>N1>N0). Intercellular CO2 was decreasing with the increase of nitrogen rate. The irrigation and nitrogen rate had a significant impact on biomass and grain yield of spring wheat, the biomass increased as the nitrogen rate and irrigation amount increased, W2N2 treatment produced the highest the grain dry matter accumulation and distribution ratio, the grain dry matter accumulation and distribution ratio was highest under W2N2 treatment.2. Irrigation and nitrogen input can increase the growth of wheat root(root dry weight density(RWD), root volume density(RVD), root surface area(RSA) and root activity(RA)) significantly and there was a positive interactive term between irrigation and nitrogen fertilizer on the growth of wheat root. However, the overuse of irrigation and/or nitrogen led to poor growth of wheat root, and met the law of diminishing return. The order of irrigation regime and nitrogen rate effect on RWD, RVD, RSA and RA of wheat root was irrigation>nitrogen>irrigation and nitrogen interaction. The order of irrigation amount effect on RWD, RVD, RSA and RA of spring wheat was W2>W3>W1. The application of N2 produced the maximum RWD, RVD, RSA and RA under W2 irrigation, the application of N1 produced the maximum RWD, RVD, RSA and RA under W1 irrigation, and the application of N3 produced the maximum RWD, RVD, RSA and RA under W3 irrigation.The optimum nitrogen application rate and irrigation volume(W2N2) could increase RWD, RVD, RSA and RA. The root system was mainly distributed in the 0~40 cm soil layer, in which the RWD and RLD accounted for 85% and 90% of the total RWD and RLD in 0~80 cm soil depth.W2N2 increased the proportion of root in the deep soil layer(40~60 cm), W2N2 increased root activity. The bigger the border effect of root length density and root surface area was under W2N2 treatment to increasing the root absorption and the interception of water and nutrients. RLD decreased exponentially when the soil layer increased(y=Ae-Bx). The principal component analysis showed that the order of treatment with effect of root control was W2N2.3. There was a sharp positive interaction between irrigation nitrogen application, which contributed to the higher nitrogen accumulation in grain, while it was showed a downward trend under excess irrigation amount and nitrogen application rate. W2N2 had higher nitrogen harvest index dependent on effective improvement in both nitrogen transformation before anthesis and assimilation after anthesis. The content of soil nitrate is highest in 0~20 cm soil layer, and has a trend of decreasing first then increasing and then decreasing in 0~120 cm soil layer. Under the same irrigation rate, with the increasing of nitrogen fertilizer rate, the soil nitrate contents in 0~120 cm soil layers are increased. Under the same nitrogen rate, with the increase of irrigation rate, the soil nitrate contents in 0~80 cm soil layers were decreased and the soil nitrate contents in 80~120 cm soil layers were increased.4.The highest grain yield was attained in W2, W3 under the same nitrogen rate(W2, W3>W1), and there were no significant differences among W2 and W3; the grain yield of N2 was significantly higher than that of N1, N3 and N0(N2>N1>N3>N0) under W1 irrigation, the highest grain yield was attained in N3、N2 that of N0 and N1 under W2 irrigation, no significant difference on grain yield was observed between the N2 and N3 treatments. The grain yield initially increased and then decreased with the increase of nitrogen rates(N2>N3>N1>N0) under W2 irrigation. Biomass increased as the nitrogen rate and water amount increased, W2N2 treatment produced the highest grain yield. The bigger border effect of grain yield was under W2N2 treatment. The highest WUE of spring wheat of 13.71 kg.hm-2.mm-1 was obtained in W2N2 treatment. There was a significantly parabola relationship between RWD in the 0~40 cm and the yield of spring wheat, RWD in the 40~60 cm had higher linearly dependent on the yield of spring wheat. There was no correlation between the yield of spring wheat and the root dry weight of 60~80 cm soil layers. There was a significantly parabola relationship between root length density, root activity in the 0~20 cm and the yield of spring wheat, root length density and root activity in the 20~60 cm had higher linearly dependent on the yield of spring wheat. There was no correlation between the yield of spring wheat and the root dry weight of 60~80 cm soil layers.In summary, considering water resources saving, reducing environmental pollution caused by excessive application of nitrogen, increasing plant yield, water and nitrogen use efficiency. It was concluded that the irrigation level W2(2400 m3·hm-2) and nitrogen level N2(180 kg·hm-2) could be recommended as the best combination of water and nitrogen, which could advanced dry matter and nitrogen accumulation and distribution in grain, promoted the root growth, improved grain yield, reduced contents of nitrate nitrogen 0~120 cm soil layer, increased water and nitrogen use efficiency of spring wheat production under PRB tillage in the experimental area.