| As the main grain crop in China,rice is of great significance to ensure its stable and high yield to ensure the security of grain quantity.Nitrogen fertilizer input and planting density are two important factors affecting rice yield.How to coordinate these two factors,improve the utilization rate of nitrogen fertilizer and reduce the environmental load and resource waste caused by excessive nitrogen fertilizer application are the current research hotspot.This study was in Hunan typical double cropping rice planting area,set up 3 years（Liuyang City,Yong’an Town）and 4 years（Santang Town,Hengyang City）positioning test.The changes of rice growth and yield formation,nitrogen accumulation and distribution,nitrogen fertilizer utilization rate,soil nitrogen contribution under DPRN cultivation in double cropping rice,the feasibility and mechanism of DPRN cultivation in double cropping rice were explored to provide a basis for high-yield and high-efficiency rice cultivation.The study results are as follows:1.Yield and yield composition factors of double cropping rice under nitrogen reduction and density cultivationThe yield of double cropping rice（20%or 27%nitrogen fertilizer reduction,27%or 55%density addition）did not decrease for 3 or 4 consecutive years.Among them,the average yield of DPRN and SPHN cultivation were 5954 kg ha-1 and 5914 kg ha-1in Yong’an test site,and 6438 kg ha-1 and 6627 kg ha-1 in Santang test site,respectively,with no significant difference.Both the hybrid rice yield was higher than the conventional rice yield,whether it was nitrogen reduction and normal nitrogen cultivation.The results of yield composition factors show that the effective number of ears increase and spike number decrease under DPRN treatment,while the seed set rate,thousand-grain weight and harvest index did not change,which showed that the comprehensive effect of yield composition factors of double cropping rice population under DPRN cultivation did not change significantly,so as to achieve the goal of stable production.2.Population and individual growth of cropping rice under DPRN cultivationUnder DPRN cultivation,the number of tillers in double cropping rice group increased significantly.During the period of this experiment,the number of early rice increased by 6.1%-57.0%and that of late rice by 10.0%-70.4%,the number of tillers in late rice was slightly higher than that of early rice,and that in hybrid rice was also higher than that of conventional rice.While the number of tillers per stump decreased,early rice decreased 1.1%-25.3%,late rice decreased 4.1%-29.7%,and the decrease of late rice was higher than that of early rice;the decrease number of tillers per stump of hybrid rice was higher than that of conventional rice.The cumulative amount of dry material in double cropping rice increased under DPRN cultivation,the accumulation amount of dry material in early and late rice increased by 0.6%-16.4%and 6.4%-25.9%,respectively;the dry material accumulation amount of late rice was higher than that of early rice,and that of conventional rice was higher than that of hybrid rice.However,the accumulation of dry material per stump decreased,and the accumulation of dry material per stump in early and late rice decreased by 3.7%-31.7%and 2.2%-31.4%,respectively.The leaf area index under DPRN cultivation did not change,late rice is slightly higher than early rice,conventional rice is slightly higher than hybrid rice,the difference is not significant;the flag leaf chlorophyll content（SPAD value）was slightly reduced,but the difference between different fertilizer dense treatments was not significant.Under DPRN cultivation,the cell tightness was no difference with SPHN,while the chloroplast matrix granules of early rice mesophyll cells slightly increased,and the starch granules and osmium-tropic granules slightly decreased in the chloroplasts of late rice mesophyll cells.3.Laws of nitrogen accumulation and redistribution in double cropping rice under DPRN cultivationThe nitrogen accumulation in cropping rice did not decrease under DPRN,the maturity period nitrogen accumulation of DPRN and SPHN cultivation were 141.1 kg ha-1 and 134.8 kg ha-1 in Yong’an test site,and 131.3 kg ha-1 and 127.8 kg ha-1 in Santang test site,respectively,that of late rice is slightly higher than that of early rice;the activity of NR enzyme and GS enzyme was improved,while the activity of glutamate synthase（GOGAT）was relatively small.The 15N tracer test showed that,under DPRN cultivation,the proportion of nitrogen absorbed from the vegetative growth period（before heading）to the reproductive organs（ear）increased by 2.0%-9.6%,and the transport volume increased by 6.1%-10.8%,and the proportion of nitrogen absorbed from the nitrogen accumulated in the heading increased by 6.6%-12.7%.It is obviously that DPRN cultivation promotes the redistribution of absorbed nitrogen to the spike.The proportion of nitrogen absorbed by the plant of nitrogen from fertilizer decreased,while the proportion of nitrogen from soil increased;the magnitude of the change in late rice（12.4%-13%）was higher than early rice（1.8%-5.5%）;the risk of nitrogen loss of fertilizer is reduced by 7.6%-21.1%;the utilization rate of nitrogen fertilizer absorption,nitrogen fertilizer partial productivity and nitrogen fertilizer agronomy increased under DPRN cultivation.In short,nitrogen reduction cultivation does not reduce the population nitrogen accumulation amount,but improves the transport redistribution of nutrient nitrogen to reproductive organs and reduces the dependence on fertilizer nitrogen,the above phenomenon is more obvious in late rice.4.Nitrogen supply capacity in double cropping rice field under nitrogen reduction cultivationSoil net mineral nitrogen increased under DPRN cultivation and more significantly in late rice,which showed a positive correlation between soil net mineral nitrogen amount and plant nitrogen accumulation and the correlation coefficient between jointing stage and heading stage was greater than that at maturity stage.The change of soil net mineral nitrogen with different nitrogen supply levels(0,90,120,150 and 180kg N hm-2)and density（dense planting and sparse planting）showed that at the same nitrogen fertilizer level,the amount of soil net mineral nitrogen under dense planting were generally higher than sparse planting,especially at 120 kg N hm-2 and 150 kg N hm-2 nitrogen supply levels.The soil net mineral nitrogen the amount of soil net mineral nitrogen at 120 kg N hm-2 levels were relatively high at the same density,followed by90 kg N hm-2 and 150 kg N hm-2 levels.There was little difference in the main soil fertility indexes between different nitrogen fertilizer density treatments.Compared with that before the experimental arrangement,the changes of soil organic matter,total nitrogen,alkali hydrolyzed nitrogen and available phosphorus in the two experimental sites were small,while the soil available potassium and p H decreased relatively significantly. |
