Distribution of soil nutrients, extracellular enzyme activities and microbial communities across particle-size fractions in a long-term fertilizer experiment

• Organic treatments significantly enhanced SOC, total N and most enzymes activities.
• The highest SOC, total N and enzyme activities were observed in 200–63 μm fraction.
• Total PLFAs were higher in >63μm fractions than that in smaller particles.
• The >200 μm fractions showed higher F/B and lower G+/G− ratios.
• Total N, C:N, phosphatase, sulfatase, N-acetyl-glucosaminidase and β-cellobiosidase were significantly correlated with PLFAs.

Soils were particle-size fractionated to evaluate changes in carbon and nitrogen contents, enzyme activities and microbial community composition in response to 33 years of fertilization. This study focused on yellow–brown paddy soil and the particle-size fractions of >2000, 2000–200, 200–63, 63–2 and 2–0.1 μm. Microplate fluorometric assays and phospholipid fatty acid analysis (PLFA) were used to determine soil biological characteristics under no fertilizer (control, CK), fertilizer N (N), fertilizer N and P (NP), fertilizer N, P and K (NPK), organic manure plus fertilizer N, P and K (NPKM) and organic manure (M) treatments. The results showed that fertilizer and soil fraction individually and interactively (P < 0.05) affected soil C, N contents, enzyme activities and microbial communities except for α-glucosidase activity, bacterial relative abundance and the G+:G− ratio. Particularly, organic treatments significantly increased soil organic carbon (SOC) and total nitrogen (N) contents of all five fractions. The highest C and N contents and enzyme activities were observed in the 200–63 μm fraction, except for phosphatase and sulfatase, which showed the highest activities in the 2–0.1 μm fraction. The highest activities of β-glucosidase, β-cellobiosidase, α-glucosidase, aminopeptidase, phenol oxidase and peroxidase in each fraction were obtained in the organic treatments (NPKM and M). Activities of phosphatase, sulfatase, N-acetyl-glucosaminidase and β-xylosidase in the 2000–200 μm fraction were highest under NPK treatment. PLFA analysis showed that the >63 μm fraction contained higher abundance of total PLFAs than that in the 63–0.1 μm fraction. Organic treatments significantly enhanced total PLFAs abundance in >2000 μm fraction, but decreased PLFAs abundance in the 2000–200 μm fraction compared with the NPK treatment. Larger fractions (>2000 μm and 2000–200 μm) held relatively lower G+:G− ratios and higher fungi:bacteria ratios, which indicated better soil conditions in these fractions. Principal component analysis showed a smaller variability of microbial community composition among treatments than particle-size fractions. Most treatments of larger fractions (>2000 μm and 2000–2 μm) were well separated from the other fractions. Redundancy analysis showed total N, C:N ratio, phosphatase, sulfatase, N-acetyl-glucosaminidase and β-cellobiosidase activities significantly affected the composition of the microbial community. Significant correlations were also obtained between enzyme activities with SOC, total N and C:N ratio. We concluded that the long-term application of organic fertilizers contributed to improvements in the soil organic carbon and total nitrogen and most of the enzyme activities, especially for the 200–63 μm fraction, along with abundant and diverse microbial community composition in larger particles.