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

Nitrogen (N) fertilization and lime addition may affect soil microbial 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 rotation experiment. 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 0-5cm depth were used to determine the bacterial growth rate by the H-3-thymidine incorporation technique. Incorporation of C-13 into neutral lipids, glycolipids and phospholipid fatty acids (PLFAs) was measured after incubation of soil with C-13-labeled acetate for 24h. Free-living nematodes in fresh soil were extracted using a density sucrose centrifugal flotation method and identified to trophic group level. Liming 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 C-13 incorporation into neutral lipids, glycolipids and phospholipids by 2-15 times. In addition, C-13 incorporation into 16:0, 16:1 omega 9, 18:1 omega 9, 18:1 omega 7 and 18:2 omega 6 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.