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

Abstract

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 2–0.1μm. 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 200–63μm fraction, except for phosphatase and sulfatase, which showed the highest activities in the 2–0.1μm 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 2000–200μm fraction were highest under NPK treatment. PLFA analysis showed that the >63μm fraction contained higher abundance of total PLFAs than that in the 63–0.1μm fraction. Organic treatments significantly enhanced total PLFAs abundance in >2000μm fraction, but decreased PLFAs abundance in the 2000–200μm fraction compared with the NPK treatment. Larger fractions (>2000μm and 2000–200μm) 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 (>2000μm and 2000–2μm) 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 200–63μm fraction, along with abundant and diverse microbial community composition in larger particles.