Nutrient cycling and greenhouse gas emissions from soil amended with biochar-manure mixtures

Integrating biochar into cattle diets has recently emerged as a potential management practice for improving on-farm productivity. Yet, information concerning the cycling of biochar-manure mixtures is scarce. A 70-d incubation experiment was conducted within two surface (0-15 cm) Mollisols with contrasting textures, i.e., sandy clay loam (Raymond) and clayey (Lethbridge), to evaluate the effects of biochar (3 Mg ha(-1)) on cumulative greenhouse gas (GHG) emissions and related fertility attributes in the presence or absence of cattle manure (120 Mg ha(-1)). Five treatments were included: i) non-amended soil (control, CK), ii) soil amended with pinewood biochar (B), iii) soil amended with beef cattle manure (M) (manure from cattle on a control diet), iv) soil amended with biochar-manure (BM) (manure from cattle on a control diet, with pinewood biochar added at 20 g kg(-1) of diet dry matter), and v) soil amended with B and M at the aforementioned rates (B+M). A total of 40 soil columns were prepared and incubated at 21 degrees C and 60%-80% water-holding capacity. On average, total CO2 fluxes increased by 2.2- and 3.8-fold under manure treatments (i.e., M, BM, and B+M), within Raymond and Lethbridge soils, respectively, relative to CK and B. Similarly, total CH4 fluxes were the highest (P < 0.05) in Raymond soil under B+M and BM relative to CK and B, and in Lethbridge soil under M and BM relative to CK and B. In Lethbridge soil, application of BM increased cumulative N2O emissions by 1.8-fold relative to CK. After 70-d incubation, amendment with BM increased (P < 0.05) PO4-P and NO3-N + NH4-N availability in Raymond and Lethbridge soils compared with B. A similar pattern was observed for water-extractable organic carbon in both soils, with BM augmenting (P < 0.05) the occurrence of labile carbon over CK and B. It can be concluded that biochar, manure, and/or biochar-manure have contrasting short-term effects on the biogeochemistry of Mollisols. At relatively low application rates, biochar does not necessarily counterbalance manure-derived inputs. Although BM did not mitigate the flux of GHGs over M, biochar-manure has the potential to recycle soil nutrients in semiarid drylands.