The effects of plant biostimulants application on soil microbial activity and nutrient uptake under field conditions in Denmark

Plant biostimulants can play significant roles in organic agriculture by stimulating natural processes that enhance nutrient availability and plant uptake, improve abiotic stress tolerance, and boost overall plant growth and development. Despite their roles, data on the efficacy of biostimulants in organic agriculture is limited, particularly in Scandinavian countries such as Denmark. We evaluated the efficacy of 10 different biostimulants on organic snack carrots and potatoes in Denmark for two years. A factorial experimental design was set up under field conditions for two years at two locations characterized by sandy clay (2021) and sandy (2022) soil textures at the experimental stations of Aarhus University in Denmark. The two factors were nitrogen levels (100% and 50% of recommended nitrogen) and biostimulants of different origins, namely, microbial origin (Proradix and Vesta), plant extracts (Crop-set and Combi-set), seaweed extract (Acadian), humic and fulvic acids (Humifirst). The biostimulants were applied at recommended rates, time, and application methods per the guidelines indicated on the product's label. Soil samples were collected to 0-25 cm depth a week before harvest and after removing the upper ground biomass. Two soil enzymes, dehydrogenase (DHA) and β-glucosidase (BGA), were selected as indicators of soil microbial activities. Macro (N, P, K, S, Ca, Mg) and micro (Fe, Mn, B, Cu and Zn) nutrients in the soil and the potato tubers were analyzed to evaluate the effects of biostimulants on nutrient use efficiency indices such as nutrient uptake. There was no significant interaction effect of the biostimulants and nitrogen levels on both enzymes and most of the plant nutrient uptakes at the two locations. The 100% nitrogen level resulted in a significantly (p<0.05) higher DHA than the 50% nitrogen level only in the sandy soil. The biostimulants did not significantly change the DHA and BGA in sandy clay and sandy soil. Some of the biostimulants significantly increased (p<0.05) nutrient uptake for all the nutrients except Fe in sandy soil. However, significant effects (p<0.10) of biostimulants were observed only in P and Mg uptake in the sandy clay texture soil. The results indicate that the effect of biostimulant application on microbial activities and plant nutrient uptake depends on the type of biostimulant and soil texture used in organic potato production in Denmark.