Higher biochar rate strongly reduced decomposition of soil organic matter to enhance C and N sequestration in nutrient-poor alkaline calcareous soil

Purpose: Our knowledge of biochar as a tool for soil carbon (C) sequestration and soil quality in semi-arid regions remains limited in recent years. Moreover, there is a lack of understanding on the comparison between effects of biochar rates on soil organic C (SOC) and nitrogen (N) mineralization in organic C poor soils. Materials and methods: Interactive effects of corncob-biochar (B), prepared at 450 °C pyrolysis temperature, and wheat straw (W) on soil native and fresh organic matter were investigated in sandy loam alkaline calcareous soil. The corncob-biochar was applied alone and in combination with wheat straw (WB) at rates equivalent to 0 t ha-1 (unamended control), 22.5 t ha-1 (B1, W1, W1B1), and 45 t ha-1 (B2, W2, W2B2). After treatment applications, soils were incubated under laboratory conditions for 61 days and periodic CO2 evolution rates were measured to calculate the cumulative CO2 efflux and priming effects. Results and discussion: The lowest CO2 efflux was found in B2 treatment, whereas CO2 efflux was lower in W1B1 than in W1 treatment. Biochar alone and with wheat straw significantly reduced net C mineralization (NCM) compared to wheat straw only treatments and the effects became stronger as biochar rate increased. This was reflected in data on the priming effect, which also showed that increased biochar rates induced a stronger negative priming (− 87 to − 67%) on SOC. Biochar with wheat straw at both rates enhanced microbial biomass C (MBC) compared to the control, W1, B1, and B2 treatments. However, the W1B1 and W2B2 treatments decreased water-extractable organic C (WEOC) contents compared to W1 and W2. Microbial metabolic quotient (qCO2) and MBC/TOC ratio increased with biochar rate, suggesting more efficient utilization of labile C by microorganisms at higher biochar rate. Biochar alone and with wheat straw reduced soil mineral nitrogen (N) contents than the control treatment and resulted in net negative N mineralization or net N immobilization. Conclusions: Our findings indicate that biochar can reduce decomposition of soil organic matter to facilitate C and N sequestration in low organic C soils. However, our study also suggests the co-application of biochar and organic amendments/residues as labile C source to induce co-metabolism to improve microbial functions and C utilization efficiency in soils that are deficient in organic C.