Abstract
Understanding phosphorus (P) dynamics in the rhizosphere is crucial for sustainable crop production. P mobilization processes in the rhizosphere include the release of plant and microbially-derived protons and extracellular phosphatases. We investigated the effect of root hairs and soil texture on the spatial distribution and intensity of P mobilizing processes in the rhizosphere of Zea mays L. root-hair defective mutant (rth3) and wild-type (WT) grown in two substrates (loam, sand). We applied 2D-chemical imaging methods in custom-designed root windows installed in the field to visualize soil pH (optodes), acid phosphatase activity (zymography), and labile P and Mn fluxes (diffusive gradients in thin films, DGT).The average rhizosphere extent for phosphatase activity and pH was greater in sand than in loam, while the presence of root-hairs had no impact. Acidification was significantly stronger at young root tissue (<2 cm from root cap) than at older root segments (>4 cm from root cap) and stronger in WT than rth3. Accompanied with stronger acidification, higher P flux was observed mainly around young, actively growing root tissues for both genotypes. Our results indicate that acidification was linked to root growth and created a pH optimum for acid phosphatase activity, i.e., mineralization of organic P, especially at young root tissues which are major sites of P uptake. Both genotypes grew better in loam than in sand; however, the presence of root hairs generally resulted in higher shoot P concentrations and greater shoot biomass of WT compared to rth3. We conclude that soil substrate had a larger impact on the extent and intensity of P solubilization processes in the rhizosphere of maize than the presence of root hairs. For the first time, we combined 2D-imaging of soil pH, phosphatase activity, and nutrient gradients in the field and demonstrated a novel approach of stepwise data integration revealing the interplay of various P solubilizing processes in situ.
Highlights
Phosphorus (P) is extremely immobile in soil and often the growth-limiting nutrient
Planar optodes, zymography, and diffusive gradients in thin films (DGT) were suc cessfully combined under field conditions to visualize pH, acid phos phatase activity and labile P and man ganese (Mn) around roots of older maize plants grown in a real agroecosystem environment with uncontrolled weather conditions, which would not have been possible in the laboratory
Despite these diffi culties, we investigated general trends of root surface and rhizosphere pH, phosphatase activity and labile P and Mn driven by soil substrate and the presence of root hairs by averaging over different root types and positions along the root with different root diameters and across as many technical replicates as possible
Summary
Phosphorus (P) is extremely immobile in soil and often the growth-limiting nutrient. Understanding P mobilization mechanisms of roots and related dynamics in the rhizosphere is a prerequisite for exploiting poorly available, recalcitrant soil P, i.e., P sorbed, precipi tated or immobilized in organic forms (Pierzynski et al, 2005; Stutter et al, 2012). Organic P (Po) can only be used by plants after enzyme-mediated conversion to phosphate via mineralization into sol uble compounds (Jones and Oburger, 2011). Soil mineralogy and soil texture strongly influence P availability as P is precipitated as Ca, Fe, Al phosphates, or sorbed on Fe and Al (hydr)oxides, organic matter and clay minerals (Baldovinos and Thomas, 1967). To overcome P deficiency, morphological root traits like higher root density and formation of root hairs increase the root surface to expand the area that is in contact with the exploitable soil and contribute to an up to 50% higher P uptake (Daly et al, 2016; Itoh and Barber, 1983; Ruiz et al, 2020)
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