Nitrogen availability and fungal‐bacterial responses in successional semiarid steppe soils

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Abstract
Nitrogen availability plays a major role in influencing the microbial community structure as plant‐soil systems develop in succession. In this study, three successional sites, sampled 5, 13, and 39 years after disturbance, were compared with an undisturbed native site. Microbial responses were related to time since disturbance and soil nitrogen (N) availability. Nitrogen was modified by mineral N and carbohydrate additions. Microscopic techniques were used to assess the biomass (total and active) of bacteria and fungi at spring and summer samplings. In the undisturbed native plots, with decreased N availability, increases occurred in microscopically assessed microbial biomass, primarily fungal, in comparison with control plots. This fungal response did not occur on the previously disturbed successional plots that were treated in a similar manner. Ratio analyses of the various parameters provided additional information related to these responses. For both samplings, the ratio of total fungi/total bacteria increased from the 5‐yr to the native plot. Particularly in the spring sampling, where there was more active plant growth, decreased N availability was associated with an increase in this ratio on the late successional and native plots. In contrast, active/total fungal and active/total bacterial ratios were highest in the 5‐yr plot, suggesting more available substrates for microbial use in comparison with the older plots. The active/total bacterial ratio did not show responses; however, an increased active bacteria over active fungal ratio occurred on the earliest successional plot, which was most distinct in the summer sampling. These results indicate the long‐term differential effects of disturbance and N availability on various active/total ratios of fungi versus bacteria, which suggest that a shift from a more active bacterial community in early successional plots to a system dominated by less active fungi occurs later in succession.