Effect of low-molecular-weight organic acids on growth and phosphorus uptake via maize (Zea mays L. cv. SC704) in some soils of Golestan province

Introduction The most important constraint in maize crop yield in developing contries worldwide, and especially among resource-poor farmers, is soil infertility. Therefore, maintaining soil quality can reduce the problems of land degradation that decreases soil fertility and rapidly declining production levels that occur in large parts of the world which needing the basic principles of good farming practice. For optimum plant growth, nutrients must be available in sufficient and balanced quantities. After nitrogen, phosphorus is the most limiting nutrient for crop yields, and is essential for maize growth and development. Large quantities of chemical fertilizers are used to replenish soil N and P, resulting in high costs and severe environmental contamination. Maize quantity and quality are increased by utilization of fertilizers, which has become the most important objective of these products worldwide. Phosphorus, is the second most important macronutrient required by the plants, next to nitrogen, and is reported to be a critical factor of many crop production systems due to its limited availability in soluble forms in the soils. The low availability of P to plants is because the vast majority of soil P is found in insoluble forms, and plants can only absorb P in two soluble forms, the monobasic (H2PO4-) and the dibasic (HPO42-) ions. Crop plants can therefore utilize only a fraction of applied phosphorus, which ultimately results in poor crop performance. To rectify this and to maintain soil fertility status, frequent application of chemical fertilizers is needed, though it is found to be a costly affair and also environmentally undesirable. Moreover, phosphorus (P) is an essential nutrientionl element for plant growth. Calcareous soils are frequently characterized by the low availability of P for plant uptake due to the low solubility of P compounds present in soils at high pH and the formation of relatively insoluble complexes, e.g., Ca-P. Many soils in Iran have received large amounts of P fertilizer and consequently contained a high level of available P. On the other hand, the root exudation of organic acids has been suggested to increase P availability in calcareous soils. The most common low-molecularweight organic acids (LMWOAs) identified in soils include oxalic, succinic, tartaric, fumaric, malic, and citric acids and are derived from the decomposition of soil organic matter in the upper soil horizons, microbial metabolites, canopy drip, and root exudation. The concentrations of organic acids in the rhizosphere or in soil solutions vary greatly and range from 10-2μM to over 80 mM. The ability of organic acids to release inorganic anions, such as P, has been reported and has been attributed to desorption of inorganic anions and solubilization of phosphate compounds. LMWOAs and their corresponding anions play a very important role to increase P bioavailability. Many studies have been conducted about the role of organic acids in increasing P availability, but these studies focused on acid soils in which Fe- or Al-bound P is the main P fraction. For calcareous soils where Ca-bound P is the main P fraction, questions that whether organic acids can mobilize P or not still exist. Although, a number of results show that addition of organic acids, especially citric and oxalic acids to soils can solubilize significant quantities of fixed P and reduce the sorption of newly applied fertilizer P. However, there are few studies on the transformations of P fractions induced by organic acids or organic anions, which are important for understanding the mobilization mechanisms of P and for exploring better ways of using different forms of P in soils. The objective of this study is to examine the effects of some organic acids and anions on the solubilization and plant uptake of soil P in some calcareous soils of Golestan province, Iran.
Material and Methods For this purpose, a factorial pot experiment in a completely randomized design with three replications was conducted on maize. The first factor was comprised of 6 soil types from various areas of the province and the second factor was consisted of a combination of phosphorus fertilizer and organic acids (1) control, (2) 50 mg P kg-1, (3 and 4) 50 mmol kg-1 of organic acids (oxalic and malic acids), (5) P oxalic acid and, (6) tomato fruit residue (25% w). After 10 weeks, plants were harvested and the parameters such as plant height, fresh and dry weights, phosphorus concentration and its uptake were determined.
Results and Discussion Results indicated that soil type effect was statistically significant on the plant fresh and dry weights (P≤ 0.05), height, concentration and uptake of P (P≤ 0.01), respectively.
Results also showed that the tomato fruit residue treatment in comparison with P fertilizer and malic acid treatments results in a significant increase in P taken up, and fresh and dry weights (P≤ 0.05). There was a significant difference between P fertilizer oxalic acid and oxalic acid alone treatments in only plant height (P≤ 0.05). Also, no significant differences in terms of measured plant parameters were observed between malic acid and blank treatments (P≤ 0.05).
Conclusion Application of tomato fruit residue rather than P fertilizer can help to take up residual soil P, to grow plants and to decrease of environmental pollution, and to be also affordable economically.