Differences of Some Leguminous and Nonleguminous Crops in Utilization of Soil Phosphorus and Responses to Phosphate Fertilizers
As a vital component of a number of macromolecules and an integral part of energy metabolism and major biological processes in photosynthesis, respiration, and membrane transportation, as well as playing a genetic role through ribonucleic acid and energy transfers via adenosine triphosphate, phosphorous is indispensable for all life forms and cannot be substituted by any other element. Being the life-limiting element in natural ecosystems, regular inputs of P fertilizer to replenish the P removed from the soil by crops are one of the characteristics of modern agriculture. The demand for P resources will outstrip supply in the coming decades because the global commercial phosphate reserves may be depleted in another 60–130 years. In addition, rock phosphate (RP) reserves are under the control of a few countries. The P recovery rate is very low and the surpluses of P in soil have produced variable responses of crops to P fertilizers and environmental pollution. Requirements for direct application of RP and improvement of P fertilizer efficiency have led to adoption of specific plant species. Since leguminous crops in general respond better to P fertilizer than cereals, some scientists have proposed the application of P to leguminous crops as the first priority. Many hypotheses have been proposed to explain the different responses to P fertilizer between the two types of crops, but most of them have not been substantiated. A series of experiments have been conducted by us on different aspects for more than 40 years, and this chapter reviews the current investigation status and reports our viewpoints based on results obtained mainly from wheat [Triticum aestivum (L.) em. Thell], pea (Pisum sativum L.), maize (Zea mays L.), and soybean [Glycine max (L.) Merr.]One reason for the different response to P fertilizer supposes that legumes require more P than nonlegumes. A long-term experiment in a maize–maize–soybean rotation sequence in which maize and soybean were grown in the same season with almost the same duration of growing period showed that the total P uptake by soybean from unit area was similar to that of maize and in some cases uptake by maize was higher than that by soybean. Results of pot and field experiments conducted by us showed that P uptake amount by leguminous crops was not higher than that by cereal crops, and wheat had a higher capacity to use soil P than do pea and vetch. Without application of N fertilizer, P amounts taken up by legumes were equal to or slightly higher than those of nonlegumes, while cereal crops with N application took up much more P than legumes in most cases either with or without application of P fertilizer.In calcareous soil, the P availability in P-containing minerals is roughly equal to that in RPs. Some agricultural scientists have used RPs as substitutes to test the crops' ability of using P from soil. Results show that in addition to the physical and chemical properties of RPs and soil pH, some plant species such as rapeseed (Brassica campestris L. and Brassica napus L.), radish (Raphanus sativus L.), and some legumes possess strong abilities in absorbing P from RPs in acid soils, and good responses of rapeseed to the RPs were also found in calcareous soils. However, there were some debatable issues in these studies: comparisons were made not in the same field but in different locations; excessive rates of RPs were used and the available P in the RPs extracted by 2% citric acid was closely correlated with crops' yield increase; only rapeseed was used for comparison; soils in which yield increase by RPs was several times higher than the control and even higher than superphosphate were particularly unique and had strong responses to calcium carbonate and thus there was no way to separate the effect of P and calcium carbonate contained in RPs; and some results were conflicting. A series of pot and field experiments were conducted by us in two calcareous soils using six crops planted in spring and autumn for both pot and field trials with sufficient N supply. Results showed that crops significantly responded to single superphosphate, while the effect of RPs was seen only at high rates of application. Comprehensive comparisons of the yield increase in absolute amount and in percentage of the control showed that there was almost no difference between legumes and nonlegumes used in our experiments in responses to the RPs. Again, for any crop, the response to RPs was closely related to the citric-acid-soluble P (P2O5) in RPs.Since the P concentration in soil solution is very dilute, the movement of P in soil takes place mainly by diffusion, the diffusion coefficient is very low, and the distance moved is very short, and many scientists hold the view that plant roots play a great role in ensuring sufficient P supply to crops and the different responses are attributed to their root characteristics. Our results indicated that wheat had better developed roots, while pea roots had a higher function in supporting shoots. For a single plant, the total root-absorbing and actively absorbing area of pea was larger than that of wheat, but in one unit volume of soil, root dry weight of wheat was 33% higher than that of pea. The root activities in terms of the 2,3,5-triphenyltetrazolium chloride (TTC) reductive amount and intensity were higher for pea than for wheat. All these root properties could not explain the different responses to P fertilizer between legumes and nonlegumes.Many soil microorganisms are able to transform insoluble forms of P to an accessible soluble form and are regarded as plant growth-promoting microorganisms (PGPM). However, there is no evidence showing their release of P to legumes and nonlegumes, and the very limited knowledge has restrained us from continuing further discussion.Of the hypotheses proposed for explaining the difference of crops in utilizing sparingly soluble P in the soil and their responses to P fertilizer, the most common view is the reduction of rhizosphere pH resulting from the release of protons and organic acids. Our experiments showed that pH in rhizospheric soil was generally one unit lower than that in the bulk soil, but there was no difference in pH between wheat and pea either in rhizospheric or in bulk soil. The available P had the same trend as soil pH. Clearly, the acidification of rhizosphere soil could not differentiate the ability of crops to respond to P fertilizer.The cation exchange capacity (CEC) of plant roots was once considered the basis for crops to exchange cations with those held in soil colloids, and crops with a high root CEC could take up more calcium from soil and thus liberate P bound with calcium for crop use. Our study with wheat and pea showed that the root CEC of pea was several times higher than that of wheat in terms of per kilogram dry root or root weight per pot. However, the P uptake amounts by the two crops did not follow the same pattern as the CEC. In relation with root CEC, crop uptake of the calcium was considered a mechanism for P release, and the ratio of CaO to P2O5 in plant tissue was proposed as an index of the plant's ability for absorbing P from RPs. However, later researchers have rejected this hypothesis.A series of experiments have shown that the sensitive responses of legumes to P fertilizer are related to their N fixation capacity. These have been evidenced by many facts.It was not always the case that legumes had good responses to P fertilizer; it was true only in a soil deficient in both N and P nutrients. In a soil with sufficient N supply, the difference in P fertilizer response vanished, and the response by cereal crops was even better than by legumes.Addition of N fertilizer to cereal crops significantly increased the P fertilizer effect. In a soil extremely deficient in N and P supply, maize with N addition absorbed almost the same amount of P from the soil as soybean. Without application of N, P uptake by pea was 33% higher than that of wheat, but when fertilized with N there was no difference in the amount of P uptake for the two crops. Also, in a soil deficient in N and P supply, application of N fertilizer to cereal crops and no N or a small amount of N to legumes often resulted in much better responses of cereal crops to P fertilizer. This shows that the sensitivity of legume response to P fertilizer is related to its N fixation.Experiments showed that, before pea acquired the ability to fix N, the biomass increase and P uptake amount were much lower than for wheat, but after reaching the stage for fixing N, pea took up much more P than wheat. This reveals that N fixation increased the amount of P uptake of legumes.Application of N fertilizer to pea to depress N fixation and not adding N fertilizer to wheat resulted in almost the same amount of P uptake by the two crops. In contrast, application of N to wheat led to more P uptake than in the case of pea. The same trend was found for maize and soybean.A layer of soil in which soybean had been grown previously had no nodules and such a soil layer was sampled for conducting a pot experiment with maize and soybean. Maize was given two treatments, i.e., with and without P fertilization, whereas soybean was treated with inoculation and without inoculation on both P treatments. Results showed that in both cases with and without P fertilizer, there was almost no difference in P uptake. However, when soybean was inoculated, the P uptake amount and dry matter increase by P addition were much larger than in maize and soybean that was not inoculated. This clearly indicates the importance of N fixation in leguminous crops to respond to P fertilizers.The responses of cereal crops to P fertilizer are mainly determined by the available P in the soil, and this is true also for leguminous crops. Eight plants grown in five soils in a pot experiment showed that organic matter, total N, total P, soil CaCO3 contents, and available N in the soil are not related to the P fertilizer effect, but the P available as determined either by the Olsen (available P extracted by 0.5 mol l− 1 NaHCO3 solution) or the Machigin method (a Russian method of extracting available P by 1% (NH4)2CO3 solution) is closely correlated with the crop response to P fertilizer. Field experiments confirmed these results. Application of N fertilizer is beneficial to P fertilization for cereal crops but detrimental to leguminous crops especially when soil mineral N is abundant. The negative effect of N fertilization on the response of leguminous crops to P fertilizer is mainly caused by the reduction of nodule formation and inhibition of root length, and thereby the elimination of the superiority of leguminous crops, leading to low productivity. Since yields of leguminous crops are lower than cereal crops, the P rate for pea can be reduced at least by 20% as compared to wheat.A series of field experiments have further shown that responses to P fertilizer by legume crops were determined by the soil-available P, as was also the case for cereal crops. In a soil having low available P, P fertilization significantly increased pea yield, and the absolute amount of the increase was almost the same as for wheat, but the percentage of increase was higher than that of wheat due to the low yield in the pea control treatment. At a medium level of soil-available P, pea still had good response to P fertilizer, while wheat had almost no response without application of N fertilizer. Wheat became more and more responsive to P fertilizer with N rate increase, but pea became less responsive. This shows that N was the major constraint limiting wheat response to P fertilizer, and addition of N fertilizer could improve its response to P fertilizer. In a soil with high available P, both pea and wheat had no response to P fertilizer no matter whether N was added or not and no matter what the rate of added N was. Such typical results show that, although leguminous crops need a smaller rate of P fertilizer, their response to soil-available P level is almost the same as for cereal crops. The availability index for the application of P fertilizer to cereal crops could be applicable for leguminous crops as well.To sum up, the different responses of legumes and nonlegumes to P fertilizer are caused neither by the difference of P uptake amount nor by the root characteristics such as root biomass, root surface area, root activity, root exudates, root CEC, and CaO/P2O5 ratio in plant tissues, but by the N fixation of leguminous crops. In a soil deficient in both N and P, the application of P fertilizer alone, the leguminous crops' responses to P fertilizer have not been restricted by N limitation, the P fertilizer can fully play its role and therefore the effect of P fertilizer to leguminous crops is much better than that to cereal crops. However, when N supply is sufficient, it is another story for nonleguminous crops. With sufficient N supplies, cereal crops have better responses to P fertilizer than leguminous crops. Responses to P fertilizers are determined by the available soil P levels for both legumes and nonlegumes.
تفاوتهای برخی از محصولات Leguminous و Nonleguminous در استفاده از فسفر خاک و واکنشهای به کودهای فسفات