Short-term CO2 and N2O emissions and microbial properties of biochar amended sandy loam soils

Biochar produced during pyrolysis of biomass has the potential to reduce greenhouse gas (GHG) emissions from soils. In order to evaluate the effect of four different biochar additions on the emission of the greenhouse gases CO2 and N2O, two incubation experiments were established in a temperate sandy loam soil. Digestate, a waste-product of the wet fermentation of swine manure, and willow wood was slowly pyrolyzed at 350 °C and 700 °C, yielding four biochar types (DS350, DS700, WS350 and WS700). In the first incubation experiment (117 days), C mineralization was monitored in soil amended with biochar at a quantity of 10 Mg ha−1 on an area-basis (biochar to soil ratio of 1:69 on a mass basis) at 50% water filled pore space (WFPS). CO2 emissions from the 350 °C biochar treatments were significantly higher than the control (no biochar) treatment, while we observed no significantly different net C mineralization in the treatments with the 700 °C biochars compared to the control. After fitting a combined zero- plus first-order model to the cumulative C mineralization data, the parameter for the easily mineralizable C pool (CAf) positively correlated with the volatile matter (VM) contents of the biochars. Microbial biomass carbon consistently increased due to all biochar additions, while the dehydrogenase activity increased in the 350 °C biochar treatments but decreased in the 700 °C biochar treatments. Principal component analysis (PCA) of the extracted phospholipid fatty acids (PLFAs) demonstrated that divergent microbial community structures established after the addition of all biochars. The markers for Gram-positive and Gram-negative bacteria were more abundant in the 350 °C biochar treatments compared to the control and to the other biochar treatments. Net N mineralization was higher in the digestate biochar treatments than in the willow biochar treatments and decreased with increasing pyrolysis temperatures and increasing C:N ratio. In a second incubation experiment (15 days) N2O emissions were measured at WFPS of 70% and the same biochars were added in the same quantity as for C mineralization, with the addition of 40 mg KNO3–N kg−1. The cumulative N2O emission after 15 days was positively correlated with the volatile matter content of the biochars and was significantly lower in the 700 °C biochar treatments compared to the control, while no significant differences were found for the 350 °C biochar treatments. This study suggests that volatile matter content could be an important property of biochars in explaining short-term CO2 and N2O emissions from biochar-amended soils.