Biochar applied to soil has been considered as an effective tool for mitigation of atmospheric carbon dioxide emission and enhancement of carbon storage in soil, which may also enhance soil quality. However, the effect of biochar aged for 5 years on the different carbon fractions and enzyme activities as well as its changes, is not well understood in the cropland sandy soil of northern China. Therefore, a field trial was carried out in 2014 with biochar applied once at 0, 5.25, 10.50, 21.00 and 42.00 g·kg⁻¹ (BC0, BC1, BC2, BC3, and BC4, respectively). We evaluated the influence of biochar addition to the changes in soil organic carbon (SOC), labile carbon pools (readily oxidized carbon, dissolved organic carbon, and microbial biomass carbon) and enzyme activities (invertase, urease, and catalase). Biochar significantly increased SOC (122.00%) and altered the content of labile carbon (increased ROC, DOC and MBC by 71.29%, 10.35%, and 900.00%, respectively). Soil urease and invertase activities increased by 55.81% and 46.76%, respectively, with an increase in biochar application rate, but catalase activity significantly decreased by 31.79%. The values of the geometric means of labile carbon (0.88) and enzyme activities (2.39) for the BC3 treatment were higher than others, which indicated that the biochar application rate of 21.00 g·kg⁻¹ is suggested for the sandy soil. Our results suggest that the application of biochar in sandy soil for five years increased soil carbon sequestration, changed enzyme activities and ameliorated soil quality.

In arid regions, decreased soil fertility has adversely affected agricultural sustainability. The effects of different amendments in alleviating these issues and increasing soil fertility remain unclear. Herein, a two-year field experiment was conducted to evaluate the properties of grey desert soil and soil respiration (SR) dynamics under six different treatment groups: biochar (BC), leonardite (LD), anionic polyacrylamide (PAM⁻), cationic polyacrylamide (PAM⁺) powder, anionic polyacrylamide solution in water (PAM⁻W), and control (CK). We observed that the BC and LD amendments significantly altered soil pH, organic matter, available nitrogen, available phosphorus, cation exchange capacity, and SR. PAM amendment increased the SR as compared to the control, except in autumn, but PAM did not affect the soil properties. SR under different amendments showed strong seasonal patterns, the highest and lowest SR rates were observed in June and January, respectively. Amendments and seasonal dynamics significantly affected SR, but no interaction was observed between these factors. Temporal variation of SR was substantially influenced by soil temperature at 15 cm of soil depth. Temperature sensitivity of SR (Q₁₀) increased with soil depth and decreased with amendment addition. SR was significantly affected by soil temperature, moisture, air temperature, and their interactions. The outcomes of this study suggested that the BC and LD amendments improved soil fertility and negated the net carbon accumulation by increasing the SR and Q₁₀ in arid agriculture soil.