| Summary: | The study was conducted at the fertilization demonstration plots of the Salal Cedar Hemlock Integrated Research Program. PH, moisture, N availability (NO₃⁻ and NH₄⁺), microbial biomass C,N and P, phospholipid fatty acids (PLFA), and enzyme activities were measured in the forest floor and mineral soil of western red cedar stands and western hemlock stands ten years following fertilization with N or P, or both.
Results showed that forest floor had the largest effect on microbial and soil chemical variables, followed by forest type, and fertilization. N fertilization significantly increased overall bacterial PLFA abundance and reduced fungal PLFA abundance, while P fertilization significantly reduced AM fungal abundance in the organic layer of the hemlock stands. In addition, the stimulatory effect of N fertilization and inhibitory effect of P fertilization on phosphatase activity was still apparent 10 years after fertilization. Moreover, the effect of fertilization on microbial communities was more pronounced in the forest floor than at depth in the soil. Correlations between microbial community structure and function were weak.
After 10 years, fertilization had not inhibited enzyme activities related to lignin decomposition, but had a significant effect on microbial community composition. Future effort should be directed to long-term and in situ research to understand microbial processes at a fundamental level, as well as linking this research with external factors, such as C costs related to fertilization treatment and shorted rotation length, to understand how microbial processes contribute to the bigger picture of C cycle related with forestry.
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