Effects of field-aged biochar on soil organic nitrogen fractions in dry farmland of Loess Plateau

Objectives The effects of field-aged biochar on soil organic nitrogen components and microbial biomass carbon and nitrogen were studied to provide scientific basis for improving soil fertility and nitrogen use efficiency in the Loess Plateau of central Gansu.

Methods  The biochar experiment was carried out in Dry Farming Comprehensive Experimental Station of Gansu Agricultural University in 2015, six levels of corn straw biochar addition treatments were setup, as: 0 t/hm², 10 t/hm², 20 t/hm², 30 t/hm², 40 t/hm², 50 t/hm², and denoted as CK, BC1, BC2, BC3, BC4, and BC5, respectively. After 5 years of continuous planting of wheat harvest in 2020, soil samples were collected in 0–5 cm, 5–10 cm and 10–30 cm soil layers for the determination of the contents of organic nitrogen components, microbial biomass carbon (MBC) and nitrogen (MBN) by Bremner method. The variation in soil organic nitrogen components, and MBC and MBN contents caused by biochar treatment were discussed.

Results All the biochar treatments significantly increased the yields of spring wheat, BC3 was recorded the highest yield which was 24.76% higher than that of CK. Compared with CK, biochar treatments significantly increased soil total N and MBC by 6.55%−10.94% and 68.63%−139.7%, enhanced the MBC to MBN ratio by 10.60%−202.44% consequently. The organic nitrogen was divided ino five components: ammonia acid nitrogen (AAN), non-acidolysable nitrogen (AIN), acidolysable ammonia nitrogen (AMN), unknown-acidolyzable nitrogen (UAN), and amino sugar nitrogen (ASN), accounting for 27.46%−45.13%, 15.47%−31.14%, 19.00%−28.66%, 1.59%−18.54%, and 2.76%−8.86% of soil total nitrogen, respectively. The contents of the three acid hydrolysis nitrogen components in all the biochar treatments were in order of AAN>AMN>ASN. Compared with CK, the AAN and ASN contents in BC2, BC3, BC4 and BC5 treatments were significantly increased by 17.58%−81.51% and 43.60%−107.55%, AMN content in BC2 and BC3 treatments was significantly increased by 15.46%−28.95%. Among the treatments, The highest total acid-hydrolyzed N, AMN and ASN, and the least AIN was obtained in BC3 treatment.

Conclusions Soil total nitrogen and microbial biomass carbon are the main factors affecting organic nitrogen components. Application of 10–50 t/hm² of biochar could significantly increase soil total N and microbiomass carbon. Application of biochar 30 t/hm² is the most effective in increasing soil total acidolyzable nitrogen, amino sugar nitrogen, and ammonium nitrogen, thus the spring yield is the highest as result of the highest soil nitrogen supply capacity. Therefore, suitable biochar application rate could increase the soil nitrogen supply capacity in the long-run in Loess Plateau.