Ze Ren

Year of Award


Document Type


Degree Type

Doctor of Philosophy (PhD)

Degree Name

Organismal Biology, Ecology, and Evolution

Department or School/College

Division of Biological Sciences

Committee Chair

James J. Elser

Commitee Members

Scott R. Miller, Matthew J. Church, Benjamin P. Colman


Bacterial Community, Ecological Stoichiometry, Grassland Degradation, Nutrient Limitation, Qinghai-Tibet Plateau


University of Montana


We examined the influences of grassland status (as indexed by normalized difference vegetation index, NDVI) on carbon (C), nitrogen (N), and phosphorus (P) concentrations and stoichiometry, nutrient limitation, as well as microbial community structure in soil, stream, and/or lake ecosystems in the Qinghai Lake watershed, where grassland is the dominant landcover and more than half of the grassland is degraded.

Chapter 2 showed that grassland degradation decreased C and N concentrations as well as C:N, C:P, and N:P ratios in soil. Moreover, grassland degradation decreased C, N, and P concentrations and influenced C:N and N:P ratios in soil microbial biomass. Soil microorganisms exhibited strong homeostatic behavior while variations of microbial biomass C:N and N:P ratios suggest changes in microbial activities and community structure. The soil became relatively more P rich and thus N limitation is anticipated to be more apparent with grassland degradation. Chapter 3 provided a picture of potentially differential influences of grassland degradation on DOC, TN, and TP in streamwater. The imbalances of C:N:P stoichiometry between streamwater and biofilms and the non-isometric relationships between biofilm C and P suggest that stream biofilms might be limited by P and sensitive to P variation. Chapter 4 indicated that grassland degradation has the potential to differentially influence the nutrients delivered to streams with substantial increases in P but decreases in N and N:P, alleviating P limitation of stream periphyton and, ultimately, stimulating P-limited phytoplankton growth in the lake. Chapter 5 revealed that grassland degradation shifted bacterial diversity and communities in soil, likely by changing soil moisture, soil organic carbon, total nitrogen, and total phosphorus. Chapter 6 showed that the variation of bacterial communities in stream biofilms was closely associated with rate of change in NDVI, pH, conductivity, as well as C, N, P contents and C:N ratio in biofilms per se. Alpha diversity was positively correlated with C, N, and P in biofilms. Abundant subcommunities of microbes were more strongly associated with environmental variables.

Overall, my dissertation revealed strong impacts of grassland degradation on several aspects of nutrient dynamics and limitation as well as on microbial communities in terrestrial and aquatic ecosystems in the Qinghai Lake watershed.



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