Biochar and Green Compost Used in Cascade: The Link Between Circular Horticulture and Soil Improvement

Abstract

Carbon-rich biomass such as compost or biochar can be used for a variety of soil applications including C sequestration or targeted improvement of soil cation exchange capacity, organic carbon (C) and pH. The use of exogenous organic matter rich in C is expected to increase, increasing the risk of biomass shortages. Use of materials in cascade is one strategy to avoid competition for C-rich biomass. Green composts and wood-based biochars are examples of renewable materials, most often applied directly to soils, that can also be used as bulk replacement for peat in horticultural substrates. After use in cultivation, these horticultural substrates can again be used as a soil improver, either directly or after further processing into compost or biochar. Three cases were studied: (1) the sequential use of biochar in a horticultural cascade, (2) comparison of green composts vs. spent horticultural substrates as organic soil improver, and (3) comparison of wood-based biochars with biochars made from either spent horticultural substrates or various sources of biomass as organic soil improver. For Case 1, the use of biochar in cascade, starting with growing media containing 10% v/v wood-based biochar, showed an added value for exogenous organic matter used as soil improvers: the addition of wood-based biochar in the growing medium resulted in a higher C content in the spent horticultural substrates. The high C/P ratio was maintained in the spent growing media, even after composting or conversion into biochar. In Case 2, spent growing media containing green compost had a higher added value as soil improver and source of stable C than the pure green compost: these materials had a higher C content and higher biological stability and thus a lower oxygen uptake rate and/or C mineralization rate than pure green compost. Their microbial biomass remained high, at levels comparable to green compost. For Case 3, biochars produced from spent growing media clearly differed from biochars made from wood, bark, or straw-like fibers in terms of nutrients, organic and inorganic C content, and short-term biological stability. In particular, the biochars made from spent growing media had a higher acid-buffering capacity due to their higher inorganic C content. Biochars from spent growing media may have a lower biological stability when produced at 400 degrees C-450 degrees C in comparison to biochars produced at higher temperatures, as well as biochars produced from wood. A high risk of N immobilization was observed in the biochars produced from spent growing media, similar to the wood-based biochars and the biochars produced from bark or straw-like fiber. In conclusion, this study indicated that (a) using biochar in a horticultural cascade resulted in exogenous organic matter for use as soil improver with a higher C content and biological stability, (b) using green compost in a horticultural cascade resulted in exogenous organic matter with a higher C content and higher C stability than for pure green compost and (c) converting spent growing media with green compost into biochar results in biochars with higher acid-buffering capacity but lower organic C content and biological stability than for biochars produced from wood. This study provided specific insights into the added value of organic soil improvers from the horticultural cascade.