Biochar amendment enhanced soil nitrogen fractions and wheat yield after four to five years of aging in Loess Plateau, China

Soil fertility has become a major issue in the Loess Plateau, China. The present study explored the effects of maize straw biochar application on soil nitrogen (N) fractions, microbial biomass carbon (C), and wheat yields in a calcareous, sandy loam soil in the Loess Plateau region. Six maize straw biochar (BC) application rates were applied to the soil in July 2015, including control with no biochar (CK), BC1 (10 t ha −1 ), BC2 (20 t ha −1 ), BC3 (30 t ha −1 ), BC4 (40 t ha −1 ), and BC5 (50 t ha −1 ). Wheat was cultivated in the amended soil for 5 years using routine mineral N and P fertilization practices. Four to 5 years after biochar application, the soil contents of total N, microbial biomass C, and amino acid N significantly increased by 9.0–30.9%, 55.1–81.4%, and 64.5–68.2% (in 2019) and 6.5–10.9%, 68.6–139.7%, and 66.9–77.2% (in 2020), respectively, as compared to CK. Moreover, the content of unknown-acidolyzable nitrogen decreased by 45.0–63.1% (in 2019) and 83.5–89.6% (in 2020) compared with CK, respectively. Application of BC3 increased the total acidolyzable nitrogen, acidolyzable ammonium nitrogen, and amino-acid nitrogen contents in 0 to 30-cm soil layer by 6.3–7.8%, 23.0–25.2%, and 62.2–0.9% (in 2019) and 14.7–18.0%, 23.5–29.0%, and 41.9–107.6% (in 2020), respectively, as compared with CK. However, after 4 and 5 years, nonacid hydrolyzed N was the highest in BC5 (50 t ha −1 ) treatment, which increased by 27.0% and 44.8%, respectively, compared to the CK, while after 5 years, it was the lowest in BC3 (30 t ha −1 ) treatment, decreased by 35.4%. After 5 years, all biochar treatments significantly improved wheat yields compared to CK. The highest wheat yield was obtained in the BC3 treatment, which was 21.6% and 24.8% higher than the CK in years 4 and 5, respectively. In conclusion, the application of biochar as a soil conditioner can significantly affect the soil total and organic N fractions and microbial biomass after aging for 4–5 years and has a positive effect on improving soil nutrient supply capacity.