Harvard Forest >

Harvard Forest Symposium Abstract 2006

• Title: Microbial Responses to Soil Warming and Nitrogen Additions
• Primary Author: Serita Frey (University of New Hampshire - Main Campus)
• Additional Authors: Alix Contosta (University of New Hampshire - Main Campus); Melissa Knorr (University of New Hampshire - Main Campus)
• Abstract:

  Since soil microbial communities are key regulators of organic matter dynamics, shifts in community structure in response to global change may play an important role in determining nutrient cycling rates. We have begun to examine the links between microbial community structure and function at the Chronic Nitrogen Amendment Plots and Soil Warming Study. To date, we have found that active fungal biomass is significantly lower (27-69%) in the fertilized compared to control plots in the hardwood and pine stands in the Chronic N Amendment Study. This shift in microbial community composition was accompanied by a significant reduction in the activity of phenol oxidase, a lignin-degrading enzyme produced by white-rot fungi. Differences in ectomycorrhizal community structure were also detected between control and fertilized pine plots. Finally, N enrichment has altered the pattern of microbial substrate use, with the relative response to substrate addition being significantly lower in the N treated plots, even after the data were normalized to account for differences in microbial biomass.
  
  
  
  
  
  We have also evaluated the response of the soil microbial community to long-term soil warming. We collected soil cores from the control, disturbance control, and heated plots at the Harvard Forest soil warming experiment where soils in the heated plots have been elevated 5ºC above ambient since 1991. Microbial biomass, measured by substrate induced respiration or as extractable lipid P, was 26-44% and 26-28% lower in heated compared to control plots for the organic and mineral horizons, respectively. Lower biomass in the mineral soil of heated plots was concomitant with significantly less labile C and a trend, though not significant, toward lower total and particulate organic matter C contents. We observed a significant effect of warming on the overall respiratory response (normalized for differences between treatments in microbial biomass) following addition to soil of 25 organic substrates, with the heated soils exhibiting a lower relative utilization compared to the control for the majority of the added substrates. Microbial community composition was determined by fatty acid methyl ester (FAME) analysis, where specific FAMEs are used as biomarkers for fungal and bacterial cell components. Samples collected from the mineral horizon of the heated plots had significantly lower concentrations of the fungal biomarker, the arbuscular mycorrhizal fungal biomarker, and several bacterial biomarkers. In summary, microbial community structure and function has been significantly altered by long-term soil warming. However, the question remains as to whether and how these changes in the microbial community impact nutrient cycling dynamics.
  
  
  
  
  
  We recently initiated a new field experiment at Harvard Forest to examine the interactive effects of soil warming and N additions on microbial community structure and nutrient cycling. The plots were established in fall 2005. There are four treatments (control, heated +N, heated –N, +N only) and six replicates in a completely randomized design (24 3 x 3 m plots). The heated plots will be turned on in spring 2006. Average soil temperature in the heated plots will be elevated 5ºC above ambient by the use of buried heating cables placed at 10 cm depth in the soil and spaced 20 cm apart. The heating cables will be controlled by a data logger that monitors thermistors in each plot every 10 min. Plots will automatically turn on and off to maintain a 5ºC temperature difference between the heated and control plots. The N addition plots (heated +N, +N only) will be fertilized following the protocol of the Chronic N Addition Study. An aqueous solution of NH4NO3 will be applied at a rate equivalent to the low N plots at the chronic N study (5 g m-2 yr-1). Fertilizer will be applied in equal monthly doses during the growing season (Apr-Oct). The control plots and unfertilized, heated plots (heated –N) will receive water only. Routine measurements will include soil respiration, N mineralization, microbial biomass and metabolism, and C and N pools (total C and N, dissolved organic C, inorganic N).
  
  
  

• Research Category: Large Experiments and Permanent Plot Studies