Interactive effect of biochar rates and elevated temperature on organic matter cycling, nutrient availability and extracellular enzyme activity in a silt loam Aridisol

Biochar has largely been proven to be an effective soil carbon storage option. However, the research on effect of climate warming on interaction of biochar with soil organic matter (SOM) cycling is inadequate. We designed this study where a biochar-amended silt-loam Aridisol was incubated at 20 °C (normal temperature) or 30 °C (elevated temperature) for 73 days and various SOM cycling processes were measured. Biochar was derived by pyrolysing sugarcane bagasse at 450 °C for two hours. It was added to soil at 1% (low dose) or 5% (high dose) on weight basis. Biochar increased soil respiration by 12.4% and 21.2% at low and high dose respectivley. Elevated temperature induced 4.7% increase in respiration in the un-amended soil. While 1%BC and elevated temperature did not show any effect, 5% BC at elevated temperature further increased soil respiration by 19%. Biochar at 1% and 5% addition significantly increased microbial biomass by 109.3% and 91.3% respectively. However, elevated temperature significantly mitigated this BC-induced increase in microbial biomass. Similar to soil respiration, BC addition significantly increased activity of C-cycling β-glucosidase. However, unlike respiration, elevated temperature significantly reduced β-glucosidase at both rates of BC addition. This result combined with those of microbial biomass and soil respiration indicate that the elevated temperature shifted microbial biomass more towards maintenance mode thereby leading to mitigated microbial growth and increased soil respiration at 5% BC addition. Chitinase activity was reduced by >50% in response to BC addition, while elevated temperature reduced it by 80% in un-amended soil. Elevated temperature further reduced chitinase activity when BC addition was 1%, whereas, elevated temperature did not change it when BC addition was 5%. Activity of another N-cycling enzyme, leucine aminopeptidase did not change in response to BC or elevated temperature or their combination. On the other hand, nitrate content increased with biochar addition. Moreover, increase in temperature alongside BC addition increased ammonium content while decreasing the nitrate content. These results show that increase in temperature in BC amended soils complicates the dynamics of N availability and N-cycling enzyme in soil. Principal component analysis showed that the microbial biomass is positively linked with biochar addition, only at room temperature whereas microbial biomass is negatively affected by elevated temperature even when BC is added. Moreover, chitinase activity was inversely related to nitrate content in soils, with no clear relationship with ammonium. Overall, the treatments arranged on two opposite axes with respect to temperature while rate of biochar addition had little influence signifying that the effects of biochar are overwhelmingly modulated by elevated temperature.