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.