Soil nutrient and microbial activity responses to two years after maize straw biochar application in a calcareous soil

Biochar (BC) addition to soil is a strategy to enhance soil fertility, which may also affect microbial activity. However, little information is available on the responses of soil nutrients and microbial activities to BC in a calcareous soil. This study investigated the changes of soil nutrient contents and microbial activities in a calcareous soil two years after application of biochar at rate of 0, 2.5, 7.5 and 22.5 t/ha. The results showed that the contents of soil organic carbon (SOC), total nitrogen (TN), dissolved organic carbon (DOC), total dissolved nitrogen (TDN), and available phosphorus and potassium increased significantly with increasing BC addition rate, but no significant effect on soil pH. Soil microbial biomass carbon and nitrogen (MBC and MBN) had an increased and then decreased trend. BC amendment increased microbial biomass and promoted soil carbon- and nitrogen-cycling enzyme activities, the ratios of beta-glucosaminidase/phosphomonoesterase, N-acetyl-beta-glucosaminidase plus leucine aminopeptidase/phosphomonoesterase increased significantly with increasing BC addition rate. Redundancy analysis confirmed that DOC and MBN were dominant factors affecting soil microbial biomass, and soil pH, TDN, DOC, MBN and SOC were main factors regulating soil enzyme activities. Besides, principal component analysis revealed that difference in microbial community composition in one year after BC addition was mainly associated with the relative abundance of bacteria and fungi, the relative abundance of bacteria increased, while the ratios of Gram-negative/Gram-positive bacteria and fungi/bacteria, and relative abundance of fungi and arbuscular mycorrhizal fungi decreased in BC-amended soils with control. However, BC had no significant effect on microbial community composition after two years. These results suggest that application of maize BC to calcareous soils may have a great potential for improvements in the soil nutrients and enzyme activity, the changes in soil microbial composition deserve further studies.