Comparison to other ecosystem nutrient inputs
Overall, the contribution by insect herbivores at background densities
to internal recycling and external inputs to soils of the key nutrients
was relatively small, as we expected (H1). The annual
canopy litter inputs to soils were estimated to ~20 g C,
~0.5 g N and ~0.1 g P
m-2 on average (Table 2). This is comparable to other
literature estimates in terms of biomass (e.g. Kjelvik and Kärenlampi
1975), but 1-2 orders of magnitude more than the flux of the same
elements through insect herbivores at background densities (Table 1).
Relevant external nutrient inputs to the system are atmospheric N
deposition, which was estimated to ~0.05-0.1 g N
m-2 yr-1 in the area for the period
2013-2015 (Alpfjord and Andersson 2017), biological N fixation, which
has been estimated to be 0.1-0.5 g N m-2yr-1 (Jonasson and Michelsen 1996; Rousk et al. 2016;
Rousk and Michelsen 2017) and P from mineral weathering, which has been
estimated to contribute ~0.01 g P m-2yr-1 (Akselsson et al. 2008). This shows that the
external inputs of N were comparable to our estimates of what was
recycled through the litter, but 1-2 orders of magnitude higher than the
contribution by insect herbivores at background densities. The external
input of P to the soil via weathering was about an order of magnitude
lower than the annual litter input, which reinforces evidence that
internal P-recycling from organic matter is crucial for plant production
in these high-latitude systems (Sundqvist et al. 2014). Yet, the input
of P through weathering was still 1-2 orders of magnitudes higher than
the contribution to the canopy-to-soil-flux through insect herbivores.
It is, however, important to emphasise that only a small fraction of the
nutrients transferred through litter ends up as part of the available
pool of soil nutrients in boreal ecosystems (Jonsson and Wardle 2008,
Metcalfe et al. 2016). So in terms of relieving nutrient limitations to
plant growth, smaller labile inputs from insects may be equally or more
important.