Differing effects of elevated CO2, N addition, and microbial community structure on soil organic matter protection in a Minnesotan grassland

Background/Question/Methods

Changes in microbial community structure could influence rates of carbon cycling in soils. Environmental changes – like elevated atmospheric CO₂ (eCO₂) and increased nitrogen (N) deposition – that influence soil nutrient availability could also shift microbial communities to be more fungal or bacterially dominated. While often considered for their role in decomposition, bacteria and fungi also affect carbon longevity in soils through physical protection of soil organic matter. Filamentous fungi, in particular, enmesh soil minerals and organic matter into aggregates making that organic matter less accessible to microbes for decomposition. Whether global changes affect aggregation through shifts in microbial community structure or other mechanisms remains unclear. Our goal was to experimentally test the effects of fungal presence on aggregation under eCO₂, N addition, and their interaction. We planted microbial in-growth bags in long-term plots receiving eCO₂ (ambient + 180ppm), N addition (+4g N m²y^-1), eCO₂ and N addition, or ambient conditions in the BioCON grassland experiment at Cedar Creek Ecosystem Science Reserve in Minnesota. Mesh in-growth bags were constructed with two mesh sizes to allow i) only bacteria or ii) bacteria and fungi to enter. Bags were harvested after one growing season of incubation. Aggregation was measured through optimal moisture sieving. 

Results/Conclusions

We observed mixed effects of environmental treatment and mesh size (microbial treatment) within the two largest aggregate classes (>2mm and 1-2mm) after one year. There was a significant effect of mesh size on the percent of total soil sample >2mm (P=0.04875): more large aggregates were found in bags where fungi could penetrate through. However, there was no difference among environmental treatments in percent of sample in >2mm fraction. In contrast, we observed a strong effect of environmental treatment on the portion of soil in the 1-2mm aggregate class (P=0.005): under eCO₂ the 1-2mm fraction was almost twice that under ambient conditions, however there was no effect of N addition, or a N by eCO₂ interaction. We also found a significant effect of mesh size on the portion of total sample in the 1-2mm fraction (P=0.01), however this appears to be driven mainly by the ambient and N addition plots. This work suggests effects of fungi on aggregation may differ depending on environmental conditions, however longer-term studies would be helpful to better understand these patterns. Future work focused on mechanisms will be important for better predicting possible changes in aggregation, and hence soil organic matter protection, under global change.