PRS Publications

Microbial responses to plant functional types and historical resources additions in the shortgrass steppe

Bontti, E. E. 2008.

Abstract

Nutrient addition in rangelands is an appealing way to increase plant biomass and quality, but little is known about the long-term effects of these additions on soil microbial activity and nutrient cycling. In addition, microbial activity may be affected by plant functional types (PFT) through influence on the levels of inorganic nitrogen (N) and labile carbon in the rhizosphere. This is particularly important in the shortgrass steppe (SGS), where plants with the C3 or C4 photosynthetic pathway differ in phenology, which affects the timing of maximum N uptake and root exudate production. To understand the effect of PFT (C3 and C4 species) and historical nutrient additions on temporal patterns of N partitioning between microbes and plants, I estimated seasonal trends in plant biomass and N content, microbial N) and soil N availability. In addition, I evaluated monthly emissions of the greenhouse gases C0₂ and N₂0, discriminating between fungal and bacterial production through incubations of soils under the influence of different PFTs and historical N additions. Last, I tested the effect of biosolid application on C0₂ and N₂0 emissions from fungi and bacteria in SGS soils. Seasonal trends in plant and microbial N concentration indicated that the two were synchronous during most of the plant growing season and both strongly influenced by precipitation. Plant functional type did not explain differences in microbial N and available soil N, but historical N amendments increased plant N content, decreased microbial N, and had no detectable effect on soil available N. Fungi showed higher emissions of C0₂ and N₂0 compared to bacteria in the SGS, whereas there was no difference in emissions between the two groups in the historically N amended plots. There were no effects of PFT on bacterial and fungal emissions of C0₂ and N₂0 but high historical N fertilization resulted in increased C0₂ and N₂0 emissions from bacteria. Fungal emissions of C0₂ were higher than bacterial emissions in SGS sites compared to biosolid amended sites, but I detected no differences between microbial groups in N₂0 emissions. C0₂ and N₂0 emissions were higher in biosolid treated sites than non-treated SGS sites even 20 years after amendments ceased. Biosolid treated sites dominated by forbs showed higher C0₂ emissions compared to sites dominated by C3 grasses, while C3-dominated sites with high available inorganic N had higher N₂0 emissions than C4-dominated sites. In summary, historical N additions had long lasting effects on SGS by increasing plant biomass and N. Given that N additions to ecosystems are increasing worldwide, it may be important to evaluate the impacts of these changes in processes on ecosystems services that grasslands provide. My results suggest that high levels of nutrient additions have unintended consequences such us increased C0₂ and N₂0 emissions, and in particular carbon additions through biosolids increase fungal activity, which is also conducive to N₂0 production. These additions have a profound impact, since the elevated greenhouse gas emissions and changes in microbial communities last at least 20 years after the amendment was carried out.