Effects of straw returning on microbial necromass carbon in black soil under long-term fertilization

Objectives To study the effects of long-term fertilization and straw returning on the content of biomarker amino sugars in black soil. The current research seeks to provide some theoretical support for regulating soil carbon cycle.

Methods The long-term trail in the black soil of Jilin Academy of Agricultural Sciences was established in 1990, and the soils were collected from the treatments of no fertilizer control (CK), inorganic fertilizer (NPK), and organic fertilizer with chemical fertilizer treatment (MNPK) in a containerized experiment that was conducted in 2018. Adding 6000 kg/hm² of corn stalks with 1 cm length (CKS, NPKS, MNPKS) and no adding of corn stalk (CK, NPK, MNPK) were set in each soil sample, making a total of 6 treatments, which were placed in PVC frames with 0.9 m in length, 0.6 m in width, and 0.6 m in height after mixing. And the upper end of the PVC frame was 20 cm above the ground. Soil samples were taken from the PVC frame that was buried in the soil for 60 days (in summer) and 150 days (in autumn) to determine the physical and chemical properties as well as the contents of GluN, GalN, and MurN, on this basis, the contents of fungal necromass carbon and bacterial necromass carbon were calculated.

Results Compared with CK, both inorganic fertilizer and organic fertilizer with chemical fertilizer treatment promoted the accumulation of amino sugars in soil. On the 60^th day, GluN content increased significantly by 18.81% and 105.36%, respectively; MurN content increased significantly by 19.62% and 129.30%, respectively. Both fertilization methods increased the content of microbial necromass carbon in soil, moreover, the accumulation of microbial necromass carbon in the combined application of organic and inorganic fertilizer was twice as high as that in CK. The content of fungal necromass carbon was notably higher than that of bacterial necromass carbon, both fertilization treatments reduced the proportion of fungal necromass carbon to microbial necromass carbon, this showed that fertilization increased the contribution of bacteria to soil organic carbon accumulation in this process. After straw incorporation, the percentage of glucosamine in inorganic fertilization treatment in the total amino sugar content increased significantly, and the percentage of fungi derived glucosamine in organic fertilizer with chemical fertilizer treatment also increased gradually on the 60^th and 150^th day.

Conclusions The treatment of organic fertilizer combined with chemical fertilizer after straw addition promotes the accumulation of microbial necromass carbon in black soil. The combination of chemical fertilizer and straw addition improves the ratio of fungal necromass carbon to microbial necromass carbon. Therefore, fertilization and straw returning will change the composition of microbial community in soil, which will affect the accumulation characteristics of microbial residues.