Influence of Water Management on Iron Phosphorus Interaction in Rice Rhizosphere

The availability of phosphorus (P) to rice plants is influenced by the extent of flooding of the paddy soils because of its strong relationship with iron (Fe) dynamics. The interaction between dissolved Fe(II) and root oxygen loss causes the formation of Fe-plaque on and near rice root surfaces. The newly formed Fe-plaque can retain porewater components, such as P, which can be subsequently released following the partial dissolution of the plaque. Such Fe-P dynamics have been described in paddy soils managed with the traditional continuous flooding technique, adopted in Northern Italy up to present. However, climate change is resulting in increasing water scarcity, thus imposing water saving strategies. The new water management are characterized by shorter flooding periods, with a possible consequence on Fe dynamics in the rhizosphere. Particularly, the Fe plaque formation and composition could be affected by the dry periods, with an unknown consequence on P retention/release mechanisms. In order to assess the impacts of the emerging water-saving techniques on the rhizospheric Fe-P dynamics and P availability to rice, a macrocosm experiment was conducted to compare the effects of three different water management practices and P fertilization. The compared water management practices were continuous water flooding (WFL), alternated wet and dry (AWD), and delayed flooding (DFL). In WFL and AWD treatments the soil was submerged just before rice sowing. Subsequently, WFL macrocosms were maintained flooded until rice maturity, while AWD macrocosms were dried at the beginning of tillering until the redox potential (Eh) rose above 105 mV. In the DFL macrocosms rice was dry seeded, then the soil was flooded at the beginning of tillering and maintained under continuous flooding up to maturity. Three P fertilization levels were tested per each water management strategy: natural soil supply (no P), 20 kg P ha-1 fertilization (medium P), and 40 kg P ha-1 fertilization (high P). The concentrations of Fe and P in porewater were monitored until rice harvesting. The plant tissues were analyzed for P concentration, and the contents of amorphous and crystalline Fe (hydr)oxide in root plaque was estimated via oxalate and dithionite extractions at 30 (mid-tillering stage), 60 (stem elongation), 90 (heading) and 120 (harvesting) days after sowing (DAS). The preliminary results showed a different molar P/Fe ratio in porewater as a result of the combination between water management and P fertilization. Because of the presence of phosphate during Fe(III) precipitation is reported to affect characteristics of the newly formed Fe-P association, a different trend in Fe plaque formation and crystalline ratio was observed. With all the water management strategies, the Fe precipitation on rice roots was higher in “no P” treatment than in the two fertilized ones. During the earlier development stages, Fe plaques were mainly composed of amorphous Fe (hydr)oxide, while the proportion of crystalline Fe (hydr)oxide increased with plant development. Despite this, the total amount of Fe plaque decreased in the last development stages. Thus, indicating a consumption of the poorly ordered fraction of Fe plaque. Such an effect was particularly evident in “no P” treatment, as a possible consequence of the plant responses to P-limiting condition on Fe plaque dissolution. Indeed, rice plants are able to release H+ and organic acids to enhance the solubility of sparkling available P (i.e. the P bounded to metal oxides). The principal component analysis (PCA) applied to investigate the relationship between relevant variables, underlined the positive correlation between plant P uptake and P concentration in porewater. Additionally, a negative correlation was observed between porewater P concentration and the crystalline ratio of Fe plaque, meaning that decreasing value of poorly ordered fraction of Fe plaque correspond to higher P concentration in porewater. Thus, corroborating the hypothesis of enhanced P release after Fe plaque dissolution promoted by the plant responses to P-limiting conditions. The relationship between P and Fe dynamics in porewater with P uptake by plants over time will provide insights into the mechanisms controlling P availability under different water management. The amount of Fe plaques formed on the root surface and their crystallinity degree can help to estimate the mechanism that regulate their potential in P retention/release and the consequent effects on plant uptake. However, only synchrotron radiation techniques (bulk X-ray diffraction and bulk X-ray absorption spectroscopy, micro x-ray fluorescence imaging and spectroscopy) will allow to fully elucidate Fe plaque mineralogy and P retention mechanisms in the different water management scenarios, and the subsequent impacts on P availability to rice plants. Acknowledgement: this study was funded by the “P-rice Fosforo in risaia: equilibrio tra produttività e ambiente nell'ottica delle nuove pratiche agronomiche” project, included in the PSR Lombardia 2014-2020