Data from: Direct and indirect effects of nitrogen enrichment on soil organisms and carbon and nitrogen mineralization in a semi‐arid grassland

1. Semi-arid grasslands on the Mongolian Plateau are expected to
experience high inputs of anthropogenic reactive nitrogen in this century.
It remains unclear, however, how soil organisms and nutrient cycling are
directly affected by N enrichment (i.e., without mediation by plant input
to soil) vs. indirectly affected via changes in plant-related inputs to
soils resulting from N enrichment. 2. To test the direct and indirect
effects of N enrichment on soil organisms (bacteria, fungi, and nematodes)
and their associated C and N mineralization, in 2010 we designated two
subplots (with plants and without plants) in every plot of a six-level
N-enrichment experiment established in 1999 in a semi-arid grassland. 3.
In 2014, 4 years after subplots with and without plant were established, N
enrichment had substantially altered the soil bacterial, fungal, and
nematode community structures due to declines in biomass or abundance
whether plants had been removed or not. N enrichment also reduced the
diversity of these groups (except for fungi) and the soil C mineralization
rate and induced a hump-shaped response of soil N mineralization. As
expected, plant removal decreased the biomass or abundance of soil
organisms and C and N mineralization rates due to declines in soil
substrates or food resources. 4. Analyses of plant removal-induced changes
(ratios of without- to with-plant subplots) showed that microorganisms and
C and N mineralization rates were not enhanced as N enrichment increased
but that nematodes were enhanced as N enrichment increased, indicating
that the effects of plant removal on soil organisms and mineralization
depended on trophic level and nutrient status.5. Surprisingly, there was
no statistical interaction between N enrichment and plant removal for most
variables, indicating that plant-related inputs did not qualitatively
change the effects of N enrichment on soil organisms or mineralization.
Structural equation modeling confirmed that changes in soil communities
and mineralization rates were more affected by the direct effects of N
enrichment (via soil acidification and increased N availability) than by
plant-related indirect effects. Our results provide insight into how
future changes in N-deposition and vegetation may modify below-ground
communities and processes in grassland ecosystems.