Even though soil is an important C reservoir most of its microbes have not been described let alone the effects of anthropogenic influences on them. Mycorrhizae, an important part of the soil microbial community, play a significant role in soil formation and soil C storage. Anthropogenic nitrogen (N) deposition decreases and shifts the composition of mycorrhizae. The rate of reactive N affecting mycorrhizal colonization and effects on soil C in developing habitats is uncharacterized. Top down effects on plants, specifically herbivory, may also shift mycorrhizal colonization and in turn soil C. Here, we test the effect of N-deposition and herbivore exclusion on willow growth, mycorrhizal colonization of roots, and soil C in the first 40 years of soil development on Mount St. Helens (MSH). We measured the response of mycorrhizae through percent colonization of Salix sitchensis roots, the dominant woody species on MSH, to 7 years of herbivore exclusion and simulated N-deposition at two (8kg/ha/yr) and four (16kg/ha/yr) times ambient levels. S. sitchensis is heavily attacked by the invasive stem-boring weevil, Cryptorhynchus lapathi; thus, we included a C. lapathi exclusion treatment in a factorial design with N addition. We collected data on soil C, mycorrhizal root colonization, and willow growth.
Results/Conclusions
After 7 years of herbivore exclusion willow basal area increased 75% relative to year 2 and N addition increased growth an additional 50% but only when weevils were excluded. Ectomycorrhizae were not affected by N-deposition or herbivore exclusion, however our two sampling sites differed in percent colonization suggesting other factors such as spore dispersal or inoculum potential may be in effect. Arbuscular mycorrhizae (AMF) colonization levels were low in all treatments (all less than 10%) but were highest in 0kgN/ha/yr plots where herbivores were excluded (0.06%±0.02). At the highest level of N addition weevils further decreased colonization (0.004%±0.02) compared to weevil exclusion (0.01%±0.009).
We have compelling evidence that herbivores play a significant role on willow growth and subsequent plant community development. Mycorrhizal colonization results suggest that herbivores may also have a strong effect on soil community development. This colonization and willow growth data will be further analyzed with soil C data to infer how mycorrhizae influence soil C.