The Role of Organic Acids in Manipulating Nutrient Levels in Calcareous Soils.
Zahra Khademi, Soil and Water Research Institiute, Kargar Ave., Jalal Al Ahmad, Tehran, Iran, David L. Jones, Univ of Bangor, SAFS, Bangor, LL57 2UW, United Kingdom, and MJ Malakouti, Tarbiat Modares Univ, Soil and Water Research Institute, PO Box 14115-6185, Tehran, Iran.
Nutrient availability in calcareous soils represents a major constraint to agricultural production in many areas of the world. Calcareous soils cover more than 30% of the earths land surface and represents the dominant soil type in many countries, which rely on continued sustainable agricultural production. It has been speculated that the exudation of Low Molecular Weight Organic Acids (LMWOAs) from plant roots into the soil may be one mechanism for enhancing micronutrient availability to plants. As plants have the potential to release organic acids from their roots either in the acid form (e.g. H3-citrate and H2-oxalate) or non-acid form (e.g. K3-citrate and K2-oxalate), (Using 14C-labeled) we evaluated the sorption properties of both. This study has shown that the sorption reactions of oxalate and citrate in soil are rapid and that the degree of sorption is organic acid specific and to a lesser extent soil type specific. We also demonstrated that CaCO3 is a binding site for oxalate and to lesser extent citrate in soil. In general, oxalate was sorbed more strongly and to a greater extent than citrate in soil and on pure CaCO3. The biodegradation of oxalate and citrate was fast in soil although citrate tended to be mineralized faster, possibly due to greater availability in solution. In order to evaluate the reversibility organic acids sorption, their desorption characteristics were determined in four cycles. The amounts of oxalate released from the three soils (low, medium and high CaCO3) remained low, explaining the limited availability of this anion when adsorbed on soils. The low desorbability of oxalate can be attributed to its high affinity for Ca, implying the formation of oxalate bonds with Ca. The efficacy of organic acids for removing and solubilizing nutrients from three calcareous soils and assess organic acid extractions of three different calcareous soils was also investigated. The results indicate that high concentrations of citrate and oxalate are capable of mobilizing nutrients mostly cations from calcareous soils (Ca, P, Fe, Cu, and Mn). Organic acids such as citrate and oxalate are implicated in rhizosphere processes, including nutrient acquisition. An experiment was conducted to determine the importance of organic acid type and concentration in rhizosphere of wheat plant (Triticum aestivum L.) and its effects on 33P uptake by shoots. A single wheat plant was grown in soil-filled rhizosphere microcosms and allowed to pass through a KH233PO4 labeled of soil. Forty eight-hour after 33P injection citrate and oxalate at concentrations of 1 and 10 mM were injected into the microcosms at the same location every day over a period of 4 days. Oxalate resulted in about several-fold enhancement in plant 33P accumulation, but citrate had no such effect. High mineralization of citrate was observed in comparison with oxalate. It seems that due to more mineralization of citrate, 33P uptake was prevented. As a result organic acids cause an increase in P mobilization and P uptake by plant is enhanced.