Microbial biomass, soil ammonium and nitrate in a 69-year secondary successional Populus grandidentata chronosequence.

Abstract

Over the last two hundred years, logging and fire have caused significant changes in the landscape of northern Lower Michigan. Nutrient cycling is impacted as these disturbances affect the forest ecosystem, causing secondary succession. Soil microbes perform the important function of nitrogen nitrification and mineralization, which causes variation in the amount of ammonium and nitrate available in the soil. The trends in microbial and amino N biomass, nitrate, and ammonium levels across secondary successional chronosequences are currently not well understood. We collected "A" horizon soil samples from three Populus grandidentata forests of varying secondary successional ages. We performed a potassium sulfate (K2S04) extraction on chloroform-fumigated and non-fumigated soil samples. Then, we ran a ninhydrin assay on the extractions to determine soil ammonium, nitrate, microbial N biomass, and amino N biomass levels in the soil. Total microbial N, NH4+, and N03- are significantly larger in the 104- year-old bum plot (Microbial N (F,39, p=.863), NH: (Fv., p=<.000), NO, (Fv., p=.003)). There is not a significant difference in amino N mass between the three stands (F,.39, p=.273). There is a positive relationship between amino N mass and NH4+ mass (R2=0.365, P<0.000, 95% Confidence Level). Lastly, there is not a significant relationship between Amino N mass and nitrate mass (R2=0.081, P<0.061, 95% Confidence Level). Total nitrogen content in the soil is higher in later-successional forest stands, and microbial biomass does not account for large changes in soil nitrate. Higher microbial biomass indicates higher ammonium content in the soil. These results are significant because nitrogen in the soil affects forest productivity, an ecosystem service we need to perform carbon sequestration and emitting of oxygen.