Chunxiao WANG, Changsong JIANG, Ke ZHAO, Xiaoping ZHANG, Zhengfeng WU, Yongmei ZHENG, Tianyi YU, Yaping ZHENG
1. Yantai Academy of Agricultural Sciences, Yantai 265500, China; 2. Haiyang City Agricultural Technology Extension Center, Haiyang 265100, China; 3. Sichuan Agricultural University, Ya’an 625014, China; 4. Shandong Peanut Research Institute, Qingdao 266100, China
Abstract Nitrogen plays a very important role in peanut nutrition and fertilization. For peanuts, the nitrogen nutrition comes from root nodules, soil and fertilizer, which are separately referred to as root nodule nitrogen, soil nitrogen and fertilizer nitrogen. The research obtained following findings. (i) The nitrogen supply ratio of the three nitrogen sources for peanut is about 5∶3∶2. There are significant differences in the nitrogen supply capacity of the three nitrogen sources. The root nodules have the largest variation in nitrogen fixation and have a high potential for development. Nitrogen fixation in root nodules is closely related to carbon metabolism indicators such as photosynthesis in peanut leaves. Phosphorus application could increase the accumulation of three nitrogen sources, and the increase in nodule nitrogen accumulation is greater than that of soil nitrogen and fertilizer nitrogen. (ii) Nitrogen fertilizer has a significant effect on nitrogen nutrition of peanuts. Different forms of nitrogen fertilizers, such as amide nitrogen, ammonium nitrogen, nitrate nitrogen and mixed ammonium nitrate nitrogen, have significant effects on nitrogen metabolism and nitrogen accumulation in peanuts. Amide nitrogen fertilizer is beneficial to improving the activity of enzymes related to nitrogen metabolism and nitrogen accumulation. Controlled-release fertilizer can significantly increase the content of soluble protein and improve the activities of NRase, GDH, GS, GPT, etc. in roots and leaves at the pod setting and mature stages of peanuts, which is favorable for delaying the plant senescence and increasing the yield of peanuts. Mixed application of common nitrogen fertilizer and slow-release fertilizer can increase the soil nitrate nitrogen level at the later growth stage, which is beneficial to the development of the root system at the later stage of growth, increasing the distribution ratio of nitrogen in the pods, and also favorable for increasing the yield and nitrogen utilization rate. (iii) Increasing the ploughing depth, improving fertilization methods, selecting nitrogen-efficient varieties, paying attention to foliar topdressing, and adopting fertilizer-water integrated cultivation are conducive to increasing the nitrogen utilization rate of peanuts, reducing the amount of nitrogen fertilizers, accordingly saving costs, increasing efficiency and realizing sustainable development of agricultural production.
Key words Peanuts, Nitrogen, Physiological characteristics, Efficient fertilization
As one of the major nutrient elements for crops, nitrogen plays a very important role in the growth and development of crops. Peanuts belong to leguminous crops, and the nitrogen sources required for growth and development of peanuts mainly come from root nodules, soil and fertilizers. Among them, root nodule nitrogen fixation contributes more than 50% to nitrogen accumulation, so root nodule nitrogen fixation is one of the main ways for peanuts to obtain nitrogen sources[1]. For a long term, in order to increase peanut yield, China has consistently increased the amount of chemical fertilizer input, which has brought about tremendous negative effects, and the effects become increasingly prominent, directly influencing the sustainable development of China’s agricultural production[2].
Saving fertilizers and scientific fertilization have become a major matter related to the development of modern agriculture and a hot spot in China’s agricultural production. Finding and analyzing the characteristics of nitrogen metabolism and utilization in peanuts are helpful for improving the level of nitrogen metabolism in peanuts, guiding the scientific and economic nitrogen application of peanuts, and also promoting the sustainable development of agricultural production. In view of these, our research team has intensified the research on the physiological characteristics of peanut nitrogen nutrition and its regulation in recent years, and established a high-efficiency application technology of peanut nitrogen fertilizer with the core of giving full play to the nitrogen fixation potential of rhizobia and increasing the utilization rate of nitrogen fertilizers. Under the precondition of ensuring a steady increase in yield, the amount of nitrogen fertilizer was reduced by 20%-30%, achieving the synchronization of yield increase, consumption reduction and high efficiency.
2.1 Nitrogen supply characteristics of three nitrogen sources and their influencing factors
2.1.1Nitrogen supply characteristics of three nitrogen sources. There are large differences in the nitrogen supply rates of the three nitrogen sources of root nodule, soil and fertilizer in different growth stages of peanuts. The nitrogen supply ratio of root nodules in the whole growth period showed a unimodal trend, and the peak appeared in the pod setting stage; the soil nitrogen supply ratio took on an irregular V-shape, and the low valley appeared at the pod setting stage; the fertilizer nitrogen supply ratio showed a gradual downward trend. The nitrogen nutrition of peanut mainly came from nitrogen fixation of root nodules, followed by soil nitrogen, and fertilizer nitrogen was the smallest, and the ratio of the three was about 5∶3∶2. The nitrogen fertilizer use efficiency and the contribution ratio of nitrogen fertilizer use efficiency in different growth stages were in the following order: pod setting stage>peg stage>seedling stage>full fruit stage, showing significant differences.
2.1.2Effects of varieties and phosphate fertilizers on nitrogen supply characteristics of three nitrogen sources. Among different peanut varieties, there are significant differences in the nitrogen supply capacity and nitrogen harvest index of soil, root nodules and fertilizers. The root nodules have the largest variation in nitrogen fixation and have a high potential for development. The nitrogen supply levels of soil nitrogen and fertilizer nitrogen were consistent with nitrogen efficiency, but the nitrogen supply levels and nitrogen efficiency of root nodules varied with the yield levels of yield cultivars. When the nitrogen efficiency is similar, the nitrogen supply level of root nodules is high, which is conducive to the formation of yield. Soil nitrogen and root nodule nitrogen have better compensation effect. The pod yield was significantly or extremely significantly positively correlated with nitrogen accumulation of different nitrogen sources, nitrogen harvest index, nitrogen efficiency, nitrogen utilization efficiency, as well as nitrogen partial productivity[4-5].
Phosphorus application can increase the accumulation of peanut soil nitrogen, nodule nitrogen fixation and fertilizer nitrogen. Among them, the increase in the accumulation of nitrogen fixation in root nodules was greater than that in soil nitrogen and fertilizer nitrogen. With the increase in phosphorus application rate, the number of root nodules, fresh weight and the proportion of nitrogen fixation and accumulation in root nodules showed an increasing trend, while the accumulation proportion of soil nitrogen and fertilizer nitrogen showed a decreasing trend. When the phosphorus application rate was in the range of 45-90 kg/ha, the nitrogen use efficiency, pod nitrogen use efficiency and yield showed an increasing trend. When the phosphorus application rate exceeded 90 kg/ha, the three indicators show a decreasing trend or no longer increase. The accumulation of nitrogen fixation in root nodules was significantly positively correlated with pod yield and total nitrogen accumulation in plants, while it was extremely significantly negatively correlated with soil nitrogen, fertilizer nitrogen accumulation ratio and nitrogen pod utilization efficiency. Appropriate phosphorus application can increase the nitrogen fixation and nitrogen supply ratio of peanut nodules, and reduce the dependence on fertilizer nitrogen and soil nitrogen, but excessive phosphorus application is not conducive to increasing the nitrogen, phosphorus efficiency and yield[6].
2.2 Differences in nitrogen fixation in root nodules of different varieties and their physiological mechanismsThere are significant differences in the number of root nodules, fresh weight, content and nitrogen fixation among different varieties. The nitrogenase activity is not only affected by the content of leghemoglobin, but also affected by other factors; when the nitrogen fixation of root nodules is insufficient, other nitrogen sources can automatically compensate for the lack of root nodules to a certain extent[7]. The amount of nitrogen fixed by root nodules of different varieties is closely correlated with carbon metabolism indicators such as photosynthesis in peanut leaves. The order of correlation is sucrose synthase activity>soluble sugar content≈carotenoid content>sucrose phosphate synthase activity>total chlorophyll content≈chlorophyll b content>net photosynthetic rate>chlorophyll a content. Leaf soluble sugar level and sucrose synthase activity can be used as physiological indicators for cultivating peanut varieties with high nodule nitrogen fixation ability. Carbon metabolism provides energy for nitrogen fixation in root nodules, while nitrogen fixation in root nodules provides nitrogen nutrition for peanut leaves, and the two together regulate peanut carbon and nitrogen metabolism and nutritional balance[8].
2.3 Regulatory effect of nitrogen fertilizer on peanut physiology and ecologyDifferent forms of nitrogen fertilizers have significant effects on nitrogen metabolism and nitrogen accumulation in peanuts. The peanut nitrate reductase (NRase), glutamine synthase (GS) and glutamate dehydrogenase (GDH) activities of amide nitrogen are the highest. The promotion effect of nitrate nitrogen on the activity of NRase is second to that of amide nitrogen, and ammonium nitrogen is the lowest. Ammonium nitrate mixed nitrogen is between nitrate nitrogen and ammonium nitrogen. The promoting effect of ammonium nitrogen on the activities of glutamine synthase and glutamate dehydrogenase is second to that of amide nitrogen, and higher than that of ammonium nitrate mixed nitrogen and nitrate nitrogen. The nitrogen accumulation in peanuts with different forms of nitrogen fertilizers is in the order of amide nitrogen>ammonium nitrogen>ammonium nitrate mixed nitrogen>nitrate nitrogen[9].
Nitrogen application can increase the content of soluble protein in roots and leaves at the pod-setting and mature stages of peanuts, and improve the activities of NRase, GDH, GS, GPT,etc.Controlled release fertilizer is more effective than common fertilizers. If the controlled-release fertilizer is reduced by 1/4, the content of soluble protein in roots and leaves and related enzyme activities of peanuts in the early growth period would be lower than those of common nitrogen fertilizers and controlled-release nitrogen fertilizer, but there is no significant difference with common nitrogen fertilizers at the later period of the growth stage. Compared with common nitrogen fertilizers, controlled-release fertilizers are more conducive to delaying plant senescence and increasing the yield[10]. At the same nitrogen application rate (105 kg/ha), the soil nitrate nitrogen content is high in the early growth stage and low in the mature stage under the base application of common nitrogen fertilizers, which is helpful for the development of the root system in the early stage. Topdressing or mixing of common nitrogen fertilizers and slow-release fertilizer are recommended. The soil nitrate nitrogen content is low in the early stage of growth and maintained at a high level in the mature stage, which is helpful for the development of the root system in the later stage of growth. Compared with general nitrogen basal application, peanuts mixed with common slow release fertilizer increases the proportion of nitrogen in pods, decreases the proportion in stems and leaves, and also increases pod yield, nitrogen agronomic efficiency and nitrogen partial factor productivity (PFP)[11]. The amount of nitrogen application has a great influence on the development of peanut nodules. With the increase in nitrogen application level, the number of nodules and the fresh weight of nodules gradually decrease[12].
3.1 Appropriately increasing the ploughing depthAt present, the ploughing depth of peanut fields in China is about 20 cm. Shallow tillage causes the bottom of the plough to move up in the peanut field, which not only affects the normal extension of the peanut root system and plant development, but also the surface soil is prone to eutrophication, aggravating nutrient loss, reducing fertilizer utilization, and causing environmental pollution. Deep ploughing can effectively break the plow sole, reduce soil compaction stress, and improve deep soil ventilation and water permeability, improve the life activities of beneficial microorganisms, thereby promoting the development of peanut roots and root nodules, increasing the absorption and utilization of soil nutrients by peanuts and the nitrogen fixation capacity of root nodules, and also increasing the peanut yield. Multi-point experiments show that deep ploughing can significantly promote the absorption of nitrogen, phosphorus and potassium in peanuts and increase the utilization rate of fertilizers, especially nitrogen. As indicated by the experiment, when soil bulk density decreased by 0.1 g/cm3, the dry weight and yield of peanut roots increased by 7.5% and 4.6%, respectively. Therefore, only shallow ploughing (rotatory tillage) 20-25 cm of land in the past years, it is recommended to increase the ploughing depth to about 30 cm; for ploughing 30 cm of land in the past years, it is recommended to increase the ploughing depth to about 35 cm, to break the plow sole. Deep plowing can be carried out before winter or in the early spring.
3.2 Improving fertilization methods and giving full play to the nitrogen fixation potential of rhizobia
3.2.1Increasing application of organic fertilizers and supplementing application of trace element fertilizers. Organic fertilizer is an efficient medium for a large number of soil microorganisms including rhizobia. Increasing the application of organic fertilizers is an effective measure to increase the number of rhizobia in the soil. In general, it is recommended to apply 7.5-12.0 t/ha of decomposed livestock and poultry manure or 3.0-4.5 t/ha of commercial organic fertilizers or bio-organic fertilizers. Organic fertilizers are applied before ploughing. Molybdenum is an important component of rhizobia nitrogenase, and iron is an important component of ferritin-containing proteins such as rhizobia nitrogenase, leghemoglobin and ferredoxin in bacteroids. The application of appropriate amount of molybdenum fertilizer and iron fertilizer can effectively improve the nitrogen fixation ability of rhizobia. Molybdenum fertilizers including ammonium molybdate and sodium molybdate can be used as base fertilizer, foliar spraying and seed dressing. As a base fertilizer, the application rate is 4-5 kg/ha, and the fertilizer effect can last for 3-4 years. When foliar spraying, the concentration is 0.1%-0.2%, and it is sprayed during the flowering period. In case of dressing seeds, 150-200 g/ha of seeds is generally used to prepare an aqueous solution, and then evenly sprayed on the seeds. Iron fertilizers mainly including ferrous sulfate can be sprayed on the foliage during the flowering period at a concentration of 0.2%.
3.2.2Appropriately controlling the amount of nitrogen fertilizer. The nitrogen level in the soil has a great influence on the nitrogen fixation of peanut rhizobia. Appropriate control of the amount of nitrogen fertilizer is favorable for giving full play to the nitrogen fixation potential of the rhizobia. The appropriate amount of peanut nitrogen fertilizer should take full consideration the peanut yield level and soil nitrogen supply capacity. According to years of experiments and production practices, the specific amount is shown in Table 1.
Table 1 Appropriate nitrogen application amount for different yields and soil fertility levels
3.2.3Application of rhizobial agent. Experiments have shown that the effect of increasing the yield of peanuts with rhizobial agents is not stable, but it can improve the nitrogen fixation ability of rhizobia, partially replace chemical fertilizers, and reduce the amount of nitrogen fertilizers. Under normal circumstances, nitrogen fertilizer application can be reduced by about 10% in fields where rhizobial agents are applied[13].
3.3 Application of slow-release nitrogen fertilizerAs a new type of fertilizer, slow-release nitrogen fertilizer has the characteristics of slow but long effect. Slow-release nitrogen fertilizer can not only meet the nitrogen demand of peanuts in different growth periods, but also realize "supply on demand", and avoid the adverse effects on the nitrogen fixation of rhizobia due to high soil nitrogen concentration in the early stage of growth, and improve the nitrogen fixation capacity of root nodules. In general, the amount of slow release nitrogen fertilizers can be reduced by 20%-25% compared with common nitrogen fertilizers.
3.4 Selecting high efficient peanut varieties for nitrogenThere are large differences in nitrogen absorption and utilization efficiency among different peanut varieties. Utilizing high efficient peanut varieties for nitrogen is an effective way to reduce the amount of nitrogen fertilizers. Experiments have shown that among the peanut varieties currently promoted, Weihua 8, Huayu 22, and Huayu 21 have high efficient utilization rate of nitrogen[4].
3.5 Foliar topdressingFoliar topdressing has the characteristics of fast nutrient absorption, high utilization efficiency and less fertilizer use. Experiments show that the utilization rate of fertilizer can reach more than 55% by foliar topdressing of urea. Thus, foliar topdressing is one of the effective measures for peanut high-yield and nitrogen-saving cultivation. The commonly used foliar fertilizer is urea, and the spray concentration is 0.5%-1%. The specific suitable concentration depends on the deciduous properties of the peanut varieties, and the green varieties such as Huayu 39, Huayu 951, Fenghua 1, Shanhua 9, Qinghua 6,etc., take the high value within the appropriate concentration range; Weihua 8, Huayu 22, Huayu 20 and other premature senescence varieties take the low value within the appropriate concentration range; intermediate cultivars such as Huayu 25, Huayu 36, and Huayu 917 take the median value within the appropriate concentration range. The spraying times depends on the level of peanut yield. In low-yield fields (with yield lower than 4 500 kg/ha) or dry and barren fields with low soil fertility, generally spray 3 times, separately in the full-blooming stage, the middle stage of pod setting and the middle stage of full fruit; in middle yield fields (with yield of 4 500-6 000 kg/ha), generally two times of spraying is conducted separately in pod setting stage and the middle stage of full fruit; in high yield fields (with yield greater than 6 000 kg/ha), generally foliar spraying one time is taken in the middle stage of full fruit.
3.6 Fertilizer and water integrationFertilizer and water integration technology is a new agricultural technology that integrates irrigation and fertilization. This technology can achieve "on-demand fertilizer supply" according to the fertilizer requirements of crops in different growth stages. Drip irrigation under mulching film is a commonly used water and fertilizer integrated fertilization technology for peanuts. Compared with the conventional technology, it can save 30%-50% of fertilizer, and apart from reducing the amount of fertilizer by 40%, the yield of pods can be increased by more than 15%, showing high potential of saving fertilizers and increasing yield[14]. The technology has broad application prospects in China’s dry peanut production areas. For producing every 100 kg of pods, it is necessary to apply 1.0-1.5 kg of pure N, and the drip rate at seedling stage, flower needle stage, pod setting stage and full fruit stage is 12%-14%, 28%-32%, 48%-52%, and 6%-8%, respectively.
Asian Agricultural Research2022年7期