Active phosphate mobilization in the rhizosphere is a key strategy for phosphate acquisition of plant roots and mycorrhiza. The following three strategies for rhizosphere manipulation may therefore be promising for increasing phosphate availability and use efficiency (PUE): (1) Combinations of cereals, incl. maize, with legumes as successful active phosphate mobilizers (e.g. Hassan et al., 2013). (2) Bioeffectors (in this context mainly microbial inoculants) as an additional strategy for increasing phosphate availability in the rhizosphere. Positive effects can be shown in principle but results under field conditions are still rather inconsistent, calling for further investigations. (3) Placed band application of phosphate-fertilizers, mainly di-ammonium phosphate (DAP), below maize seeds as a well-established procedure of fertilizer placement in central Europe. Both ammonium and phosphate in DAP perfectly combine low mobility of the nutrients, avoiding disappearance from the depot, and root attracting properties through chemical signalling. This may also be true for ammonium and phosphate containing recycling fertilizers such as Struvite (magnesium-ammonium-phosphate), semi-liquid manure and biogas digestates as promising alternatives for DAP.

Our vision
The three main aims of this RS are: (1) Understanding soil phosphate 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 phosphate mobilisation potential and by different fertilization strategies is the first aim. (2) The second aim is the development of applicable strategies for rhizosphere manipulation to optimize phosphate availability and fertilizer phosphate efficiency using the knowledge derived from aim 1. Starting with pot and rhizobox experiments, promising strategies will be applied in separate field experiments. Central field experiments will be 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 based on finding of aims 1 and 2 (second phase of this IRTG).

At the process level, phosphate fractions, their turnover and their interaction with other soil components must be investigated to create the basic knowledge necessary for a goal-oriented manipulation of rhizosphere processes. We characterize phosphate fractions in different cropping systems and relate these results to other soil characteristics including organic matter. Chemical soil phosphate extraction and sequential fractionation procedures will be applied. Alcaline extraction in combination with solution 31P-NMR particularly will be used to characterize organic phosphate fractions. Soil microbial biomass phosphate and phosphatase activity will be measured to characterize microbial activity. Phosphate release kinetics will be determined and isotopic exchange kinetics will be applied. Raman spectroscopy and XANES spectroscopy will be established. Roots will be characterised by morphology, exudation and mycorrhization. All findings must be related to the potential bio-availability of phosphate, including spatial effects, which can be characterized in accompanying pot and rhizobox experiments. In addition, diffusive gradients in thin films (DGT) will be applied as a promising alternative for the characterization of soil phosphate bio-availability.