undergraduate Washington State University Vancouver, Washington, United States
Five percent of all soil carbon (C) is contained in Andisol soils formed from volcanogenic deposits such as those from the 1980 eruption of Mount St. Helens (MSH). Anthropogenic impacts caused by invasive species and nitrogen (N) deposition can shift biogeochemical cycling. This increased N is known to alter plant community composition and plant-microbial interactions which may affect the development of these C-rich soils. Invasive species significantly alter ecosystem processes and invasive invertebrate weevils on MSH reduce biomass of the keystone woody species, Sitka willow (Salix sitchensis). Here we seek to determine the effect of herbivory and N-addition on productivity of Sitka willow, soil C and N pools, and the development of a stable soil C pool. We measured soil C, N and willow growth in response to 8 years of invertebrate herbivore exclusion and N-addition at two- and four-times ambient levels (i.e. 8 and 16 kg/ha/yr) across two sites. Sites were established in 2013 on the primary substrate each consisting of 18 8m2 plots. In 2018 we sampled 10 random 10x10cm soil pits at 10cm depth from each plot. We measured total C and N and performed physical fractionation to assess C pools.
Weevil exclusion doubled basal area of willows over 8 years, whereas basal area in control plots decreased by 30%. The addition of N further increased willow basal area in exclusion plots but not in control plots. Effects on willow growth did not directly translate to soil C and N but there were site-specific differences with soil C and N lower in west sites than in east sites. We found a significant interaction between N-addition, weevil exclusion, and site with an increase in soil C at 8kgN when weevils were excluded.
The smallest fraction of soil (< 45µm, i.e. silt) contained greater soil C than the 250 µm and 45 µm fractions and δ13C values were significantly higher in the silt fraction. However, soil δ13C values were not significantly different from that of plant C; thus, it does not appear that our treatments have yet affected the formation of stable soil C pools. Invertebrate herbivores play a significant role in plant biomass thus it is surprising that we do not see effects on soil C and N. This disconnect may be partially explained by the potential underdevelopment of plant-soil-microbial interactions in this very young system.