A matter of time: Recovery of plant species diversity in wild plant communities at declining nitrogen deposition

Aim: High levels of nitrogen deposition have been responsible for
important losses of plant species diversity. It is often assumed that
reduction of ammonia and nitrogen oxide emissions will result in the
recovery of the former biodiversity. In Western Europe N deposition peaked
between 1980 and 1988 and declined thereafter. In a 60-year experiment in
hay meadows we tested the hypothesis that increasing and declining
nitrogen deposition had negative, respectively, positive effects on plant
species diversity. Location: Wageningen, The Netherlands Method:
Duplicated plots received different fertilization treatments from 1958
onwards (control, Ca, K, P, PK, N, NPK). Productivity, soil pH and species
composition were measured at regular intervals. In the control plots the
correlations between N deposition, diversity, production and soil
acidification were analysed. Subsequently, we tested whether the treatment
effects (e.g., N addition and liming) confirmed the hypothesized
interactions. Results: In the control plots soil pH, species diversity and
the abundance of legumes and short forbs declined between 1958 and 1987
when atmospheric N deposition was high but recovered after 1987 when N
deposition decreased. However, also in the N addition plots species
diversity recovered partly after 1987, although the soil pH of the
acidified soils in these plots did not. In addition, also in the limed
plots diversity decreased rapidly during the first 30 years while in this
treatment soil acidification was more than compensated. Main conclusions:
We conclude that declining N deposition resulted in the recovery of plant
species diversity, but not in recovery of the former species composition.
Time appears to be an additional, but crucial factor for the recovery of
diverse, flowering meadows. Species not adapted to the new management
conditions created at the start of the experiment disappeared during the
first decades, while species fit for the new environment needed many years
to establish.