Soil microbial community structure and activity in a 100-year-old fertilization and crop rotation experiment

Aims Nitrogen(N)fertilization and lime addition may affect soil micro-bial and nematode communities and ecosystem functions through changing environmental conditions,such as soil pH and soil organic carbon.The objectives of this experiment were to examine the impact of N input and liming on soil microbial and nematode communities and to identify the key environmental determinant of community composition in a century-old fertilization and crop rota-tion experiment.Methods The field experiment consisting of a 3-year crop rotation regime was established in 1911 in southeastern USA.Four treatments,(i)no-input control,(ii)NPK with winter legume,(iii)PK with legume and lime and(iv)NPK with legume and lime,were included in this study.soil samples collected at the 0-5 cm depth were used to determine the bacterial growth rate by the 3H-thymidine incorporation technique.Incorporation of 13C into neutral lipids,glycolipids and phospholipid fatty acids(PlFas)was measured after incubation of soil with 13C-labeled acetate for 24 h.Free-living nematodes in fresh soil were extracted using a density sucrose centrifugal flotation method and identified to trophic group level.Important Findingsliming resulted in a 10-fold increase in bacterial growth rates compared with the no-input control,whereas N fertilization had no significant effect.multivariate analysis of PlFa profiles showed that soil microbial community composition was different among the four treatments;the difference was primarily driven by soil pH.PlFas indicative of gram-negative bacteria covaried with soil pH,but not those of fungi and actinobacteria.liming enhanced 13C incorpora-tion into neutral lipids,glycolipids and phospholipids by 2-15 times.In addition,13C incorporation into 16:0,16:1ω9,18:1ω9,18:1ω7 and 18:2ω6 were greater than other PlFas,suggesting that gram-negative bacteria and fungi were more active and sensitive to simple C input.bacterivorous nematodes were the dominant trophic group in the soil,but no significant differences in nematode communities were found among the treatments.our results suggest that soil pH had a greater impact than N fertilization on soil microbial community composition and activity in a crop rotation system including legumes.