Phosphorus uptake of upland rice genotypes as affected by organic matter application in phosphorus deficient soils: a physiological and agronomic approach

Phosphorus (P) deficiency is a major yield-limiting factor for upland rice in regions where highly weathered, acid soils prevail. Such soils contain large concentrations of iron- and aluminum hydroxides that strongly immobilize phosphate (PO4). Breeding P-efficient rice genotypes may offer an option to overcome P deficiency. The P uptake capacity of a plant under low P availability, termed P acquisition efficiency (PAE), can be enhanced by (i) increasing P foraging through a more efficient root architecture/morphology/anatomy and by (ii) smarter roots that can mine sparingly available P source by P mobilization in the rhizosphere, including better access to organic P. Recent work has shown that the application of farmyard manure (FYM) in combination with mineral P fertilizer (triple super phosphate, TSP) sharply enhances rice yield. This had been ascribed to a combination of soil chemical reactions and to a role of organic P in maintaining a bioavailable source of P. Crops may differ in their capacity to use organic P, hence it might be speculated that rice genotypes may differ in PAE because of smarter roots that can mine organic P under mixed supplies of TSP+FYM. The objectives of this study are (i) to identify to what extent rice genotypes differ in PAE and (ii) to identify why that is, thereby testing if differences in PAE are affected by the supply of organic P. Multiple field trials were conducted in the upland of Madagascar, with factorial combinations of six genotypes, FYM and TSP applications, with blanket nitrogen (N) & potassium (K) additions. The crops consistently failed under zero P input whereas grain yield reached a maximum of 6 t/ha after three years of TSP+FYM applications, with an average of 3.2 t/ha over the years. Grain yield was about 1.2 t/ha for FYM alone and about 1.5 t/ha for TSP alone, again confirming the TSP-FYM interactions. Genotype (G) effects on grain yield were much smaller than the large effects of FYM, TSP or its combination; statistically significant G × TSP × FYM interactions on P uptake were only found in some years and fields. Moreover, the ranking of genotypes for PAE was inconsistent, without superior genotypes under FYM vs. TSP, thereby rejecting our hypothesis that difference in PAE are related to supplies of organic P. The application of FYM increased soil pH and CaCl2-extractable P while decreasing CaCl2-extractable Al. An additional liming trial indicated that the beneficial effects of FYM over TSP relate to liming effects. The FYM application lowers Al toxicity which overrules potential effects of organic P supply. A series of greenhouse trials were performed to evaluate the relative contribution of P foraging vs. P mining strategies on the PAE as affected by FYM and P application. Five upland rice genotypes were grown for 38 days in an extremely P-deficient ferralsol, amended or unamended with FYM and adjusted to three P levels with mineral P. Plant P uptake responded strongly to the P levels but not to FYM application. Under moderately P-deficient to optimal P conditions, the variation in shoot P content was mainly explained by P levels followed by genotype, suggesting that the P uptake mainly reflects the P uptake potential of the genotypes rather than its PAE. Such difference in P uptake was well-associated with root size (R2=0.53) but not with root P uptake efficiency (RE, mg P uptake per unit root biomass). Our data suggests that P uptake of upland rice plants in highly weathered soils is largely dependent on the ability to maintain a large root size, which supports P foraging under this extremely P-deficient condition of the soils. A small scale pot experiment was further conducted to test the relative contribution of organic P fractions to upland rice PAE. Here we disentangled the effect of soluble organic P (PO) from that of total soluble P (PT) on the RE of rice seedlings in acid soil, using a sample of an acid mineral soil, an acid peat and a mixture thereof. This yielded three substrates with significant differences in total soluble P (0.005-0.41 mg P/L) and in the percentage of organic P in that pool (PO/PT, 0-55%). Plants showed a large growth response to the P addition. There was a significant genotypic variation in P uptake and RE under moderate P deficiency but these traits were unaffected by the PO/PT in the soluble fraction of the substrates. Along the same lines, phosphatase activity in the rhizosphere soil was unaffected by genotypes and did not explain the RE among all data. A multiple regression model suggests limited genotypic effects due to the better use of organic P. A root elongation test in the acid mineral soil that was either or not limed suggests that the differences in acid soil tolerance may play a larger role in the genotypic performance of PAE than the organic P utilization potential. A final study tried to identify root-induced P mobilization with special attention to the role of organic P in soil. We measured 1D P fluxes from soil to a root mat and compared these fluxes with corresponding abiotic fluxes towards diffusive-gradient-in-thin-films (DGT), the premise being that the former may be larger than the latter because of root induced mobilization processes. The soil was fertilized either as inorganic P or FYM prior to the tests and fluxes were measured with the 32P isotope to overcome detection limits. The effect of rice genotypes on the fluxes was never statistically significant and plant roots did not change the specific activity (i.e. 32P/31P ratio) in the available P in soil. It was concluded that the root-induced P mobilization of rice in such conditions is small and inconsistent and that other root traits such as root architecture provoke genotypic variation in P uptake in the field. Overall, all lines of evidence in this work consistently showed that upland rice plants have a limited capacity to use organic P and that differences in RE among genotypes are small and suggesting no or limited capacity to mine soil P. Instead, genotypes significantly affect the PAE because of different foraging traits (different root size). The weight of evidence shows that soil acidity is a major factor controlling root growth and the performance of upland rice genotypes for grain/biomass yield and P uptake in highly weathered soils. The FYM application positively affected rice P uptake by adjusting soil pH above toxicity range, optimizing their root growth. Among six genotypes tested in various conditions, we identified that Chomrong Dhan generally displayed superior PAE and grain yield compared to the other genotypes. Aluminum tolerance should be considered when developing rice genotypes for high P efficiency in highly weathered soils.

Jaar van publicatie:2021

Toegankelijkheid:Open