Data from: Above-ground and below-ground responses to long-term nutrient addition across a retrogressive chronosequence

1. There is much interest in understanding ecosystem responses to
local-scale soil fertility variation, which has often been studied using
retrogressive chronosequences that span thousands of years and show
declining fertility and plant productivity over time. There have been few
attempts to experimentally test how plant nutrient limitation changes
during retrogression. 2. We studied a well-characterized system of 30
forested lake islands in northern Sweden that collectively represent a
5350-year post-fire retrogressive chronosequence, with fertility and
productivity decreasing as time since fire increases. For each island we
set up four plots on understorey vegetation, each subjected to a different
fertilizer treatment over six years: no additions, nitrogen (N) only,
phosphorus (P) only, and N + P. 3. We found that both N and P additions
reduced feather moss and thus total plant biomass. Meanwhile the three
dominant vascular plant species showed contrasting biomass responses, but
similar responses of foliar nutrient concentrations to nutrient additions.
Fertilization reduced most microbial groups and altered CO2 fluxes, most
likely through feather moss reduction. Against expectations, the majority
of interactive effects of N and P were antagonistic. 4. Changes in effects
of nutrient additions during retrogression were usually modest. Empetrum
hermaphroditum biomass was increasingly promoted by P and N + P addition
while vascular plant N to P ratios were increasingly reduced by P
addition, indicating increasing plant limitation by nutrients (notably P)
during retrogression. Below-ground, positive effects of N addition on soil
mineral N increased while negative effects of N addition on soil fungi
decreased during retrogression; no other below-ground effects of
fertilization changed along the gradient. 5. Synthesis. Our results show
that forest understorey communities on islands of different fire history
and thus stages of retrogression show relatively modest differences in how
they respond to nutrient addition despite large changes in ecosystem
productivity and soil fertility, probably because of high species turnover
and adaptation of communities to infertile conditions. While increased
nutrient availability (as expected through global change) may have
important ecological consequences, these effects are likely, especially
below-ground, to be rather similar across ecosystems that differ greatly
in nutrient availability and productivity.