Mountain forests are subject to high rates of
physical erosion which can export particulate nitrogen from
ecosystems. However, the impact of geomorphic processes
on nitrogen budgets remains poorly constrained. We have
used the elemental and isotopic composition of soil and plant
organic matter to investigate nitrogen cycling in the mountain
forest of Taiwan, from 24 sites with distinct geomorphic
(topographic slope) and climatic (precipitation, temperature)
characteristics. The organic carbon to nitrogen ratio of soil
organic matter decreased with soil 14C age, providing constraint
on average rates of nitrogen loss using a mass balance
model. Model predictions suggest that present day estimates
of nitrogen deposition exceed contemporary and historic nitrogen
losses. We found ∼ 6 ‰ variability in the stable isotopic
composition (δ
15N) of soil and plants which was not related
to soil 14C age or climatic conditions. Instead, δ
15N was
significantly, negatively correlated with topographic slope.
Using the mass balance model, we demonstrate that the correlation
can be explained by an increase in nitrogen loss by
non-fractioning pathways on steeper slopes, where physical
erosion most effectively removes particulate nitrogen. Published
data from forests on steep slopes are consistent with
the correlation. Based on our dataset and these observations,
we hypothesise that variable physical erosion rates can significantly
influence soil δ
15N, and suggest particulate nitrogen
export is a major, yet underappreciated, loss term in the
nitrogen budget of mountain forests.