Impact of growing media components on the mineralization of nitrogen from plant-based and animal-based organic fertilizers

Nitrogen is a primary nutrient for plant growth that must be mineralized to become available when applied as organic fertilizer. It is well known that several factors, such as temperature, C/N ratio, pH, water availability and the nature of fertilizer itself can influence the nitrogen mineralization process in soils. Yet, in soilless organic horticulture, few studies have examined the link between growing media (GM) components and nitrogen availability for plants. The objective of this study was to determine the nitrogen mineralization dynamics of two organic fertilizers in seven GM. A total amount of 400 ppm N from an animal-based (pelletized poultry manure and blood meal), or plant-based (soybean meal) fertilizer were incorporated in peat-based GM, composed of peat alone or mixed with compost, perlite, biochar, wood fiber, coir and/or bark. These mixtures were incubated under aerobic conditions in a growth chamber at 23°C and 90% RH, using one-liter Mason jar type microcosms. Four repetitions of each GM × fertilizer combination resulted in 56 experimental units, each consisting of seven microcosms, allowing seven destructive sampling over the eight-week monitoring period (total of 392 microcosms). During the first two weeks, CO₂ fluxes emitted from the GM were higher for those that received soybean meal, suggesting a greater biological activity compared to GM with poultry manure and blood meal. Despite a faster mineralization of the animal-based nitrogen, both types of fertilizers resulted in similar mineralization rate after 14 days. A significant effect of growing media components on the rate of N mineralization was observed, particularly between GM containing compost, biochar or bark, and those without. Indeed, at day 14, the mineralization rate was 41% of the total N applied for GM with added compost, compared to 16% for the peat alone. Ongoing analysis of the composition and functionality of microbial communities related to GM and fertilizer type will help explain the observed differences.