Changes in soil microbial community, enzyme activities and organic matter fractions under long-term straw return in north-central China

•High rates of straw return changed microbial community structure under fertilization.•High straw return rates increased most hydrolytic enzymes activities.•High straw rates increased soil light fraction, light and heavy fraction organic C.•Heavy fraction organic C dominated soil organic C storage in all treatments.

Field nutrients management practices influence soil biological properties and organic matter fractions. A long-term experiment (30-year) was conducted to investigate changes in soil microbial community, enzyme activities and organic carbon fractions under straw return in north-central China. Treatments included no-fertilizer control (CK) and maize straw return at rates of 0 (S0), 2250 (S1), 4500 (S2), and 9000 kg ha−1 (S3) under combined nitrogen and phosphorus fertilization. All fertilization treatments increased total phospholipid fatty acid (PLFA) and the abundances of Gram-negative (Gm–) bacteria and fungi over the CK treatment. The S3 treatment increased total PLFA compared with the S0 treatment. The S2 and S3 treatments increased Gm− bacterial abundance by 11.6 and 9.3%, respectively, and increased fungal abundance by 68.2 and 113.6%, respectively, compared with the S0. Fertilization increased the activities of β-glucosidase (BG), β-xylosidase (XYL), and N-acetyl-glucosaminidase (NAG) over the CK. The S2–S3 increased the activities of BG, XYL, and NAG by 10.5–20.7, 19.0–32.5, and 21.6–32.8% compared with the S0, respectively. Although the S1 and S3 had lower activities of phenol oxidase than the CK, the activities did not differ among the S0–S3 treatments. The S0–S3 treatments increased the concentration of total organic C (TOC) than the CK, and the S2–S3 increased TOC than the S0. There were no differences in soil light fraction (LF) and the light fraction organic C (LFOC) among the CK, S0, and S1. The LF and LFOC in the S2 increased by 14.7 and 33.9%, respectively, and these values in the S3 increased by 48.0 and 81.3%, respectively, relative to the S0. The S0–S3 treatments increased the heavy fraction organic C (HFOC) over the CK and the HFOC in the S2–S3 increased by 39.2–43.1% compared with the S0. The LFOC/TOC ratio was lower than the HFOC/TOC ratio for each treatment. Overall, low rates of straw return did not affect, while high rates of straw changed microbial community structure and increased the activities of most hydrolytic enzymes and the concentration of LFOC and HFOC under chemical fertilizer application.