Soils are estimated to contain more carbon (C) than plants and the atmosphere combined, but anthropogenic activities are changing the rate at which they store C and the amount of C they can store. Grasslands in particular are an important soil C sink because they store two-thirds of their C belowground. Increasing nitrogen (N) deposition causes increases in plant community productivity (and thus C assimilation) globally, but also decreases plant diversity. Understanding how grassland functions, like C storage, are changing in response to decreasing plant diversity is of utmost importance when considering the future of the grassland soil C sink. Here, we quantify changes in soil carbon efflux (autotrophic and heterotrophic respiration) in a temperate grassland (Cedar Creek Ecosystem Science Reserve, East Bethel, MN). In this experiment, N was added as slow-release urea at three rates (1g, 5g, and 10g m-2 year-1) since 2007. These rates represent approximately double, five times, and ten times ambient N deposition rates. We measured in situ soil C fluxes biweekly using an infrared gas analyzer throughout the growing season of 2019. To understand changes in C fluxes in the context of changing plant diversity, we collected aboveground biomass (AGB) at peak biomass in mid-August.
Results/Conclusions
We found a nonlinear relationship between N addition rate and soil respiration, with respiration peaking in our lowest N addition treatment. Soil respiration in this treatment was significantly higher than both the Control and our highest N treatment (p = 0.0061 and 0.0058, respectively), but was not different from our mid-level N treatment (p = 0.5226). There was no significant difference between our mid- or high-level N addition treatments and Control (p > 0.05). Across the gradient of N addition, treatment (p = 0.02) explained 36% of variance in soil efflux, and a significant (p = 0.041) interaction between treatment and C3 grass biomass explained another 29%.
We found significant increases in C losses from the soil at our lowest N addition treatment. Although this could indicate significant shifts in belowground communities, we found that AGB of C3 plants explained a significant proportion of the variance in soil respiration. This may indicate that C3 plants, in addition to being less-efficient photosynthesizers than their C4 counterparts, may also have less efficient belowground structures that result in increased soil respiration. Thus, a doubling of ambient N deposition rates could weaken the grassland soil C sink through changes in plant diversity.