Changes of low molecular weight organic acids, soil organic matter, nitrogen transformation and microbial populations in rhizosphere and bulk soils in a temperate rain forest

The ecosystem research in a temperate rain forest was not fully understood. Thus, the aims of this chapter are illustrated the changes of low molecular weight organic acids (LMWOAs), stable isotope ratios of soil organic matter, forest fire influenced the changes of vegetation communities, nitrogen transformation and microbial populations study in rhizosphere and bulk soils, selecting Ta-Ta-Chia long-term ecological research (LTER) site, Taiwan in a temperate rain forest as an example. Low molecular weight organic acids and the nature of organic matter were determined by gas chromatograph (GC) and CPMAS 13C nuclear magnetic resonance analysis. Changes in the meadowforest boundary at this LTER site detected by stable isotope ratios of soil organic matter of δ13C analysis of plants and soils. The buried bag incubation method was used to study temporal pattern of net N mineralization and nitrification in soils. Several types of microorganisms were counted for their spatial and temporal variations. The recovery percentage of LMWOAs determination by GC shows a high correlation of r2 = 0.99. All recovery of LMWOAs was above 83.6%. The proportion of organic functional groups in the rhizosphere and bulk soils quantified by 13C NMR analysis followed the general order: alkyl-C > O-alkyl-C > N-alkyl-C > acetal-C > aromatic-C > carboxylic-C > phenolic-C. δ13CPDB values in pedons 1 and 2 tended to decrease with increasing with soil depth. The values of EB, E and Bw horizons were close to δ13CPDB values of C3 plants. The data indicated that SOM was a mixture of transmorrisonensis (C4), nittakeyamensis (C3) and Tsuga (C3) plant residues.δ13CPDB values obtained from deeper meadow pedons 1 and 2 of soil layers indicated that the vegetation community has changed over the years. M. transmorrisonensis (C4) plants replaced C3 plants (e.g., Tsuga). Extractable NO3 - concentrations ranged from 0.2 to 2.1 mg N kg-1 soil in the meadow zone and from 0.2 to 13.0 mg N kg-1 soil in the forest zone. No seasonal changes in soil nitrate concentration were present. Extractable NH4 + concentrations ranged from 1.0 to 12.4 mg N kg-1 soil in the meadow zone and from 2.8 to 25.0 mg N kg-1 soil. The NH4 + concentrations showed highly temporal variations. The net of mineralization and nitrification rates showed spatial and temporal variations, depending on microbial activities and populations. Bacteria were most abundant, followed by cellulolytic microbes, phosphate-solubilizing microbes, nitrogen-fixing microbes, fungi and actinomycetes. Microbial populations showed high correlation with microbial biomass carbon and organic acid contents.