Increasing soil phosphate availability and phosphate fertilizer efficiency
Active phosphate (P) mobilization in the rhizosphere is a key strategy for P acquisition of plant roots and mycorrhiza. Strategies for rhizosphere manipulation may therefore be promising to increase P availability and P use efficiency. If less mobile soil P fractions can be mobilized, particularly from organic fractions, soils may act as considerable P sources themselves, and the efficiency of P fertilization will be increased. This research subject is driven by the following hypothesis: Rhizosphere and rooting zone manipulations by a combination of (1) P-efficient maize genotypes identified by RS 1.1 and from other sources, (2) legumes in the crop rotation, (3) bio-effectors (microbial products and natural extracts), and (4) smart fertilization strategies will increase soil P availability and P fertilizer efficiency. Legumes are addressed both as a part of the crop rotation (main or cover crop) and as intercropping of maize and beans/lupins. Smart fertilization strategies address both the application technique (e.g. fertilizer placement), foliar application and the fertilizer type (e.g. ammonium-based P fertilizers, manures, and other bio-based (recycling) fertilizers) also in combination with bioeffectors. Manures, as well as processed bio-based (recycling) fertilizers, may differ considerably in total P contents and P fractions, calling for a target-oriented characterization and application based on their composition. Precipitation products from sewage sludge and slurry such as struvite, which is ready to be introduced to the European fertilizer market, and incineration products from sewage sludge will be in focus as recycling fertilizers.
Our vision
The three main aims of this RS are: (1) Understanding soil P fraction sizes, turnover and plant availability, and corresponding phosphate fertilizer efficiencies at different scales in time (short-term and existing long-term field experiments) and space (surface-/subsoil, micro/field) as affected by different maize-based cropping systems including legumes with a high P mobilization potential and by different fertilization strategies. (2) The development of applicable strategies for rhizosphere manipulation to optimize P availability and fertilizer P efficiency using the knowledge derived from aim 1. Derived from pot and rhizobox experiments, promising strategies are applied in separate field experiments. In Germany, the central field experiment in Hirrlingen is adapted based on most promising findings in preceding pot and field trials. (3) The final aim is the development of long-term scenarios for sustainable site-specific phosphate fertilization strategies in maize-based cropping systems, thereby closing phosphate cycles at a regional scale.
One focus will be on the central field experiment in Hirrlingen (with RS 2.1 & 4.2) and the corresponding field experiment in Quzhou (Hebei Province, North China Plain, warm-temperate climate). Besides pot and rhizobox experiments, targeted field experiments at “Oberer Lindenhof” (Research station UHOH, Swabian Alb), in Hessberg (TLLLR Thuringia), in Quzhou (Hebei Province), and in Shilin (Yunnan Province, P.R. China, humid subtropical) are performed to verify the applicability of promising fertilization and cropping strategies including legumes in maize-based cropping systems. Long-term effects are investigated in long-term field experiments where soil turnover processes are close to steady-state (compost experiment at UHOH, and cooperation with different external partners). Relevant and promising P recycling fertilizers are obtained from Subject Area (SA) 3 and in cooperation with external partners.
For investigations at the process level, mycorrhization, microbial biomass, and enzyme activity measurements as well as spectroscopic techniques are used (31P-NMR, XANES, Raman). This is seen as a complement to extraction techniques based on equilibrium solutions (e.g. Olsen, Bray, CAL) which potentially underestimate the contribution of organic P to crop nutrition and to sequential extraction, e.g. based on the Headley procedure. The Chinese partners will contribute with meta-genomics and 16s rRNA sequencing as indicators of rhizosphere manipulation, and with transcriptomics and metabolomics to characterize plant P metabolism.
The biological mechanisms of plant Zn uptake and distribution mediated by P are studied by the Chinese partner. 16s rRNA sequencing, laser scanning, and confocal microscopy using zinc-fluorophore Zinpyr-1, in combination with different maize mutants are used to investigate the interaction of P and Zn in the rhizosphere (in cooperation with RS 2.4 & 3.1). This shall reveal physiological and molecular mechanisms of P mediated root Zn uptake. It may finally support a rooting zone-oriented optimization of P and Zn fertilization.
The final aim is the development and test of scenarios for sustainable site-specific P fertilization strategies in maize-based cropping systems, thereby closing P cycles at a regional scale. Investigated scenarios combine the above-mentioned measures of rhizosphere and root zone manipulation. Recycling fertilizers developed and identified within SA 3 and by cooperating partners are seen as the main external P input. The relationship between P and Zn will be considered to achieve a high food quality concerning Zn bioavailability. Besides balance-based model calculations on-field and farm-scale in cooperation with RS 4.2, selected scenarios are compared in container and field experiments. The latter includes targeted field experiments and adaptations of the central field experiments.
