Effect of biochar and digestate on microbial respiration and pesticidedegradation

To overcome the problem of on farm point sources of pollution stemming from improper handling, spillages,and leakages of pesticides during filling and cleaning of spraying equipment, environmental friendly and lowcost technology filter systems are currently under development. Based on a laboratory screening approach, wheredifferent biomixtures (soil, with biochar and/or digestate) are tested a full scale outdoor system will be developed.Therefore, different fundamental processes like pesticide mineralization, metabolization, sorption-desorption, andtransport behavior of three radiolabelled pesticides (Bentazone, Boscalid and Pyrimethanil) will be investigated.Biochar and digestate mixtures with two contrasting soils (sandy and silt loam) had been used as a novel biofiltermaterial for respiration study instead of conventional soil and straw mixtures. To analyze the pesticide degradationpotential and to gain information about the temporal evolution of the degradation process of the biochar anddigestate soil mixtures microbial respiration was measured over the course of three month. As expected, digestateacts as an easily available C-source leading to highest release of CO2 compared to other biomixtures used. Incontrast, the addition of even small amounts (1 %) of biochar caused a profound suppression in the CO2 releasefrom digestate based mixtures. The exact driving mechanism for this suppression can be manifold likes negativepriming or chemisorption of CO2 on biochar or NH3 toxicity induced by the large amount of digestate appliedin the experiment (30 %) or can be combination of all effects. Surprisingly, a repeated experiment with same butaged digestate did not show such negative priming.On the other hand, the fate of applied organic contaminants to biomixtures depends on several factors likesoil properties and climatic conditions as well as biological degradation. To analyze the degradation potentialof the different soil/amendment mixtures a degradation study was performed to determine the effects of biocharand digestate in different mixing rates on the metabolization behavior of the studied pesticides, and to identifyand quantify the metabolites derived during the degradation process. The results from the 14C Bentazone studyindicate that 5 % digestate and 5 % biochar mixture showed highest (nearly 15 %) and 1 % biochar lowest rateof mineralization (1 %), whereby highest microbial activity was measured in the soil/digestate mixture