Evaluation of granulated struvite over a crop rotation under semi-controlled mesocosm conditions

Introduction The agronomic performance of phosphorus (P) recycling fertilizers such as struvite (STR) is commonly assessed in pot or field trials, both of which have inherent limitations in balancing experimental control and field relevance. Mesocosm systems offer an intermediate approach to assess fertilizer performance under semi-realistic yet controlled conditions. This study aimed to (i) assess crop-specific responses to P fertilization, (ii) compare the agronomic performance of granulated STR with that of diammonium phosphate (DAP), soft rock phosphate (sRP), and a control without P fertilization (P0), and (iii) evaluate the effect of magnesium (Mg) supplied with STR. Materials and methods A two-year crop rotation consisting of ryegrass and forage rye (cover crops) and silage maize and potatoes (main crops) was cultivated in large-scale mesocosms (1000 L), filled with a homogeneous, P-deficient substrate consisting of quartz sand, bentonite, and bark humus. Crop yield, P concentration, and P uptake were assessed across growth stages, alongside changes in CAL-extractable P and CaCl 2 -extractable Mg concentrations in the substrate. Results Crop responses to P fertilization were strongly species-specific. Autumn-sown cover crops showed little or no yield response, whereas the main crops showed pronounced responses to P supply, particularly during early growth. In the main crops, STR outperformed sRP, with relative dry matter yields (gross rDMY, DAP = 100%) of 91% and 83% und STR compared with 78% and 56% under sRP for maize and potato, respectively. However, STR remained inferior to DAP, particularly during early crop development, when P concentrations remained below optimal ranges. Under repeated P application, STR showed a gradual increase in the CAL-extractable P stock over the crop rotation, increasing by 7.84 g P mesocosm -1 , compared with 4.35 g P mesocosm -1 under DAP, whereas it decreased under sRP (-0.77 g P mesocosm -1 ) and P0 (-1.59 g P mesocosm -1 ). No statistically significant effect of additional Mg supply on DMY was detected under the experimental conditions. Discussion Granulated STR behaved as a slow-release fertilizer under semi-controlled conditions, showing potential for long-term P supply, while limited early P availability constrained its agronomic performance.