S06 - Session O6 - End-of-life of organic growing media: carbon sequestration versus circular use of nutrients.

Authors: Bart Vandecasteele *, Lotte Similon, Julie Moelants, Ewan Gage

The composition of a horticultural substrate for soilless greenhouse cultivation directly affects the sustainability of the cropping system but has also an indirect effect through the end-of-life of the spent media. These spent media can be an important source of stable carbon for C storage in arable soils but the reuse of the enclosed nutrients should be optimized as well. Reducing the nutrient accumulation in spent growing media at the end of soilless cultivation is crucial in the framework of circular horticulture and environmental sustainability. Understanding the impact of the growing medium composition on the C stability and the nutrient accumulation in growing media blends allows to optimize (a) the value of spent media for C storage in soils and (b) the nutrient use efficiency, i.e., minimize the losses by nutrient accumulation in soilless cultivation. Horticultural substrates at the end of tomato cultivation were compared under variable conditions of cultivation length and or fertigation scheme in 5 different full-scale trials at commercial scale. Mineral wool and 100% coir slabs, being the conventional substrates, were compared with 100% peat-free, organic blends with variable doses of biochar. Besides plant growth and yield, the accumulation of nutrients in the growing media was measured to assess the effect of the growing medium on the nutrient use efficiency. The microbiome of the spent rooting media was characterized for microbial activity (N release and C mineralization) and microbial biomass (PLFA). The stability of the C source was measured during a 1-month C mineralization test.