Interaction of biochar with multiple amendment combinations affects plant biomass and soil enzyme activities

Biochar is an important restoration amendment that is often applied together with other amendments, such as microbial inoculum, or organic and inorganic amendments. However, the mechanisms underlying biochar interactions with other restoration amendment combinations on soil functions and plant growth often remain unclear. Here we investigate the interaction of biochar with up to six additional restoration amendments using a novel experimental design. We test 6 distinct restoration amendments (amorphous silica, vermiculite, bentonite, AM fungi, potassium dissolving bacteria, and rhizobium) and use them to form a factor pool. We then create a factor number gradient consisting of 1 (single factors), 2, 4, and 6 amendments. We randomly sample the corresponding number from the factor pool to generate 2 and 4 amendment combinations, with the 6- amendment combination representing all amendments. The same set of treatments was replicated with and without biochar application, creating multiple restoration amendment combinations along the factor gradient, with and without biochar as a focal factor. We found that soil enzyme activities decreased with the addition of biochar irrespective of the number of amendments. We further demonstrate that biochar has synergistic interactions with multiple amendment combinations on N-acetyl-glucosaminidase activity and plant growth, whereas antagonistic interactions are observed on soil water stable aggregates, water holding capacity, β-glucosidase, cellulase and phosphatase activities. Our findings emphasize the complexity of biochar-amendment interactions and highlight the urgent need to understand such interaction mechanisms within the context of high-dimensional combinations of amendments, which is likely of vital importance for developing integrated restoration strategies.