Effects of soil flooding and organic matter addition on plant accessible phosphorus in a tropical paddy soil: an isotope dilution study

• Published in: Journal of plant nutrition and soil science 2016-12, Vol.179 (6), p.765-774
• Main Authors: Rakotoson, Tovohery, Rabeharisoa, Lilia, Smolders, Erik
• Format: Article
• Language: English
• Subjects: flooded rice; isotope dilution; organic fertilization; phosphorus availability; phosphorus deficient soil
• ISSN: 1436-8730; 1522-2624
• DOI: 10.1002/jpln.201500383

Plant growth experiments were conducted to reveal the mechanism by which organic matter (OM) and soil flooding enhance phosphorus (P) bioavailability for rice. It was postulated that reductive dissolution of iron‐(III) [Fe(III)] oxyhydroxides in soil releases occluded phosphate ions (PO4), i.e., PO4 that is not isotopically exchangeable in the original soil prior to flooding. Rice was grown in P‐deficient soil treated with factorial combinations of addition of mineral P (0, 50 mg P kg−1), OM (0, ≈ 20.5 g OM kg−1 as cattle manure +/– rice straw) and water treatments (flooded vs. non‐flooded). The OM was either freshly added just before flooding or incubated moist in soil for 6 months prior to flooding; nitrogen and potassium were added in all treatments. The soil exchangeable P was labeled with 33PO4 prior to flooding. The plant accessible P in soil, the so‐called L‐value, was determined from the 33P/31P ratio in the plants. The L‐values were inconsistently affected by flooding in contrast with the starting hypothesis. The OM and P addition to soil clearly increased the L‐value and, surprisingly, the increase due to OM application was larger than the total P addition to soil. An additional isotope exchange study in a soil extract (E‐value) at the end of the experiment showed that the E‐value increased less than the total P addition with OM. This suggests that plants preferentially take up unlabeled P from the OM in the rhizosphere compared to labeled labile inorganic P. The effects of soil flooding on P bioavailability is unlikely related to an increase of the quantity of bio‐accessible P in soil (L‐value) but is likely explained by differences in P mobility in soil.