Responses Of Soil Microbial Communities To Biochar Amendment Changes Over Time In The Field.

Use of biochar as a soil amendment has received considerable international attention due to its reported beneficial effects on soil fertility and its influence on a wide range of soil biological and chemical processes. We examined the effects of biochar amendments on soil microbial community abundance, activity and diversity at 6 months, 1.5 years and 4.5 years after corn stover biochar (Best Energies, Australia) was applied to field plots at the Cornell University Musgrave Farm in Aurora, NY, in May 2007. The biochar was applied at rates of 0, 1, 3, 12 and 30 t/ha and incorporated to a depth of 15 cm. Treatments were replicated three times in a randomized complete block design and plots were planted to corn. Immediately prior to the corn harvest in October 2007, 2008 and 2011 bulk and rhizosphere soil samples were taken from the 0-15 cm depth in all plots for microbial analyses. Microbial respiratory activity, microbial biomass, and community composition by T-RFLP (terminal restriction fragment length polymorphism) fingerprinting were analyzed. In the first two years post application, respiratory activity was inversely related to biochar application rate, with the highest respiration rates measured in the control soil and the lowest rates measured in soil amended with 30 t/ha biochar. Microbial biomass C, adjusted for adsorption, revealed higher biomass in biochar amended soils. Analysis of T-RFLP DNA fingerprinting of the soil bacterial and fungal communities indicated that soils receiving no or low (1 t/ha) applications of biochar differed considerably from those receiving 12 or 30 t/ha biochar. Differences between these treatments were even more pronounced in the 2008 sampling. Sequencing of the fungal ITS region suggested biochar may have a simplifying effect on these communities. However, in the 2011 samples taken 4.5 years after biochar was applied, differences between treatments had all but disappeared. Functional gene array (FGA) analysis supported these findings. Overall, our results suggest that the mechanisms underlying the short-and longer-term impacts of biochar on soil microbial communities differ.  In the short-term, fresh biochar may have a biofumigating effect on the soil biota, causing significant shifts in microbial activity, diversity and abundance.  In the longer-term, particularly with a less recalcitrant biochar (corn stover), adsorption of organic matter and nutrients to the biochar surface over time may reduce chemical and physical differences between the biochar surface and the bulk soil. Should both short- and longer-term applications improve yield and soil quality, regular inputs up to 1 t/ha could be recommended.