Impacts and interactions of biochar and biosolids on agricultural soil microbial communities during dry and wet-dry cycles

Extreme hydrological processes, such as prolonged drought and frequent wet-dry cycles, are major consequences of climate change and can influence agroecosystems by altering soil microbial communities and associated processes. Two increasingly popular soil amendments, biosolids and biochar, may reduce the negative impacts of extreme hydrological processes as well as provide a low-cost carbon sequestration strategy, increase nutrient and soil water retention, and increase nutrient availability. We measured the response of microbial communities to amendments of biochar (walnut shell, 900 °C) and biosolids amendments, separate and mixed, under different water regimes. We used phospholipid fatty acid analysis (PLFA) to monitor microbial communities during a 12-week incubation experiment in which we applied different levels of biochar (0, 0.5 and 1% dry wt. added to dry soil) and biosolids (0 and 0.5% air-dry wt. added to dry soil) to agricultural soils under different soil moisture conditions (wet, drought, or wet-dry cycles). Biochar increased soil pH across the soil moisture and biosolids treatments at both 4-week and 12-week. Biosolids only significantly decreased soil pH in wet treatment and the change in soil pH was within 0.5 units. Biosolid increased soil mineral nitrogen. Biochar only increased microbial biomass at week 4 in the absence of biosolids, while biosolids increased microbial biomass regardless of biochar and water regime by 55% on average over 12 weeks. Drought and wet-dry cycles strongly influenced microbial communities in soil, reducing biomass and altering community composition. Biosolids amendment increased soil nutrient level, helped maintain soil microbial biomass and reduced impacts of soil moisture stress on soil microbial community. The effect of biochar on soil microbial community composition depended on its dose and soil nutrient conditions. The co-amendment of biosolids and biochar helped to reduce the changes in soil pH and mineral nitrogen. Amending soils with biosolids or biochar may provide an effective management tool to reduce the negative impact of drought and wet-dry cycles on microbial communities. •Biosolids reduced impact of soil moisture stress on soil microbial community.•Biosolids maintained soil microbial biomass and increased soil nutrient contents.•Biochar's impact on microbial community depends on soil nutrient and moisture level.•Biochar and biosolids co-amendment mitigated changes in soil pH and mineral nitroge