Impacts of continuous biochar application on major carbon fractions in soil profile of North China Plain’s cropland: In comparison with straw incorporation

Although the increase of soil organic carbon (SOC) in biochar-amended soil has been well documented, there is a lack of understating on the responses of soil inorganic carbon (SIC) to biochar application in soil profile. Here, we conducted a field study to investigate the effects of 10-year application of biochar at 4.5 Mg ha−1 yr−1 (B4.5) and 9.0 Mg ha−1 yr−1 (B9.0), and straw return (SR) of 15 Mg ha−1 yr−1 on SOC and SIC and their dissolved fractions. Our results showed significant difference in SOC content over 0–20 cm, with an order of B9.0 (8.46 g kg−1) > B4.5 (7.58 g kg−1) > SR (6.81 g kg−1) > control (5.26 g kg−1). However, SIC content was significantly lower under biochar (1.0–2.0 g kg−1) and control (1.0–2.0 g kg−1) than straw incorporation (3.2–3.7 g kg−1) over 0–40 cm. On average, DOC:SOC ratio (an index of SOC desorption) was significantly lower under biochar (~2.0%) and SR (2.6%) treatments than under control (3.1%), implying that both biochar and straw amendments can enhance SOC stability. Over the ten years, 62–81% of biochar-C was converted to SOC in the 0–100 cm layer, which was significantly higher than that (21%) of straw-C. Our estimation suggested that total carbon loss rate was 51–53% under biochar amendment and 70% with straw incorporation. This study highlights that biochar application has great potential for carbon sequestration in cropland of north China. •We assessed effects of continuous use of biochar or straw on soil carbon fractions.•Biochar application caused an increase in SOC but a decrease in SIC over 0–100 cm•Straw incorporation led to an increase in SOC over 0–100 cm and SIC over 0–40 cm.•Conversion rate was higher from biochar-C than from straw-C to SOC or TC.