Biochar as a Platform to Increase Manure Carbon Residence Time in Soil

ABSTRACT Manure and biochar (BC) based practices influence soil carbon (C) dynamics. However, manure does not enhance soil carbon (C) as quickly as BC does. Data on BC from different feedstocks and their co‐application with manure in stabilizing labile manure C fractions in soil systems is still inadequate. We hypothesize that manure‐BC co‐application will increase soil total C by influencing the microbial community, likely to increase labile and recalcitrant C than manure alone. This study evaluated several stability parameters of manure (swine and dairy) under four rates of different BC (herbaceous corn stover, woody yellow pine, and willow) following 1 month of aging. These aged mixtures were applied to the soil and incubated for 203 days to fit a two‐pool model, and the soil labile C residence time was determined. A significant ( p < 0.05) positive correlation between ash‐free volatile solids: fixed solids and molar H:C org and O:C org supports that BC addition generally stabilizes manure C by changing the mixture's physicochemical properties. Hot water extracted C of the fresh and aged mixtures revealed that high BC addition rates and BC produced from wood are significantly ( p < 0.05) more efficient in decreasing the labile C pool than untreated manure, low BC application rates, and herbaceous BC. Soil incubation study revealed that BC rate significantly ( p < 0.05) reduced ammonium‐N availability, labile C release, and respirational C loss, but increased soil recalcitrant‐C. This study reports that manure type and BC application rate significantly ( p < 0.0001) influence microbial biomass C, and co‐application was harmless to microbes, which in turn influences the residence time of labile C. This laboratory‐based study suggests that manure‐BC addition to soil builds soil total C more consistently than manure alone, supporting our initial hypothesis. However, a field‐based study is warranted to evaluate manure's C and N stability and nutrient release performances under dynamic soil conditions.