The long term effect of biochar application on soil microbial biomass is not well understood. We measured soil microbial biomass carbon (MBC) and nitrogen (MBN) in a field experiment during a winter wheat growing season after four consecutive years of no (CK), 4.5 (B4.5) and 9.0 t biochar ha−1 yr−1 (B9.0) applied. For comparison, a treatment with wheat straw residue incorporation (SR) was also included. Results showed that biochar application increased soil MBC significantly compared to the CK treatment, and that the effect size increased with biochar application rate. The B9.0 treatment showed the same effect on MBC as the SR treatment. Treatments effects on soil MBN were less strong than for MBC. The microbial biomass C∶N ratio was significantly increased by biochar. Biochar might decrease the fraction of biomass N mineralized (K N), which would make the soil MBN for biochar treatments underestimated, and microbial biomass C∶N ratios overestimated. Seasonal fluctuation in MBC was less for biochar amended soils than for CK and SR treatments, suggesting that biochar induced a less extreme environment for microorganisms throughout the season. There was a significant positive correlation between MBC and soil water content (SWC), but there was no significant correlation between MBC and soil temperature. Biochar amendments may therefore reduce temporal variability in environmental conditions for microbial growth in this system thereby reducing temporal fluctuations in C and N dynamics.

No abstract

No abstract

No abstract

No abstract

No abstract

Core Ideas Biochar may change the surface soil heat balance. Biochar significantly decreased soil thermal conductivity and thermal diffusivity. Changes were mainly attributed to soil bulk density and the biochar itself. Few studies have examined the effects of biochar amendment on soil thermal properties. Soil thermal properties dominate the storage and conduction of heat in soil, affect soil temperature and water and heat movement, and consequently influence plant growth and soil biochemical processes. A 7‐yr field experiment was performed to investigate the effects on soil thermal properties of biochar amendment at three application rates: 0 (control), 4.5 t ha−1 yr−1 (B4.5), and 9.0 t ha−1 yr−1 (B9.0). Soil heat capacity (C), thermal conductivity (λ), and thermal diffusivity (α) of the 0‐ to 5‐cm‐depth topsoil were determined throughout a corn (Zea mays L.)–wheat (Triticum aestivum L.) growing period using a heat pulse method. The results showed that soil bulk density was decreased significantly by biochar (P < 0.05). There was no significant difference in soil water content between the control and biochar treatments. There was no significant difference in the soil volumetric heat capacity between the control and biochar treatments. Soil thermal conductivity and thermal diffusivity were decreased significantly by biochar (P < 0.05). There was a strong positive relationship between soil thermal conductivity and bulk density, and between soil thermal conductivity and water content (P < 0.001, n = 99). In contrast, there was a strong negative relationship between soil thermal conductivity and biochar amount (P < 0.001, n = 99). The reduction in soil thermal diffusivity may be attributed largely to the low thermal diffusivity of biochar and the decrease in soil bulk density caused by biochar.

The effects of biochar on alkaline soils in high-yielding agricultural fields remain poorly understood. Nitrate variation in soils due to biochar application without a change in soil pH, is a great concern relating to both crop yield and nitrate leaching. In this study, we monitored changes in dynamics of soil nitrate accumulation and effects on grain yield due to biochar application in a temperate, high-yielding region. Biochar derived from corncob was applied to an alkaline soil at biochar rates (kg ha–1) of 0 (CK), 2250 (C1), and 4500 (C2) for each of two crop seasons in 2007. A treatment with 750 kg biochar-based fertiliser ha–1 (CN) for each of two crop seasons was also included. Biochar had no significant effect on soil water content to 1 m soil depth. Biochar tended to increase the soil cation exchange capacity (CEC) in the 0–20 cm soil layer and nitrate retention to 1 m soil profile, but there was no significant difference between biochar treatments and CK. Grain yield of C1, C2, and CN was improved by 10.3%, 16.9%, and 15.5% compared with CK, respectively, but only C2 was significantly different from CK. Grain yields of winter wheat with biochar application showed a trend similar to soil CEC and average soil-nitrate retention, suggesting that the increases in grain yield were mainly attributable to improvements in soil CEC and soil nitrate retention due to biochar application in the alkaline soil. In conclusion, the effects of biochar on soil water retention, soil nitrate retention, and grain yield were very limited in alkaline soil in a high-yielding region.