Responses Of Soil Carbon And Nitrogen Dynamics To Global Environmental Changing Factors

Under the context of global climatic and environmental changes, it is critical tounderstand how soil carbon processes respond to these prevalent changes such as increased airtemperature, increased nitrogen deposition and land use changes. Overall, the wholedissertation reported global changing factors such as land uses, climatic warming and nitrogendeposition significantly affected soil carbon and nitrogen cycles.Considering that the quality and quantity of dissolved organic carbon (DOC) in the soilcan be used as indicators of the effects of perturbations on soil C. We first studied the effectsof land use systems (native forest, grassland and arable land) in both Alberta, in westernCanada, and Heilongjiang, in northeast China, on the quality and quantity of soil DOC. Westudied water-extractable organic C, operationally defined as DOC, the UV absorption andhumification index (HIX) and biodegradability of DOC. The relationship betweenbiodegradability and the structural chemistry of soil organic matter studied with13C-nuclearmagnetic resonance (NMR) spectroscopy was also studied. The UV absorption and HIX,biodegradability of DOC, and the proportion of organic matter functional groups were notdifferent among the land use types in both Canada and China. When the samples from bothcountries were considered together, the extractability of DOC was negatively correlated withsoil organic carbon content and carbonyl C, and positively correlated with O-alkyl C; thebiodegradability of DOC was positively correlated with soil C/N ratio, and negativelycorrelated with the specific UV absorbance and HIX. The main effect of land use type on soilorganic matter was on its content but not the labile C characteristics or organic matterfunctional group composition, indicating the dominant control of the climate on the quality ofDOC and of the organic matter under different land use types we studied in the two coldtemperate regions. Secondly, we conducted studies in climate warming, nitrogen (N)deposition and land use change affecting ecosystem processes such as soil carbon (C) and Ndynamics, especially the interactive effects of those drivers on ecosystem processes which arepoorly understood so far. This part of the study aimed to understand mechanisms ofinteractive effects of temperature, form of N deposition and land use type on soil C and N mineralization. We studied, in a laboratory incubation experiment, the effects of temperature(15vs.25°C) and species of N deposition (NH4-N vs. NO3-N) on soil CO2efflux, dissolvedorganic C (DOC) and N (DON, DIN) concentrations using intact soil columns collected fromadjacent forest and grassland ecosystems in north-central Alberta. Temperature and land usetype interacted to affect soil CO2efflux, concentrations of DON, NH4-N and NO3-N in mostmeasurement times, with high temperature resulted in the highest CO2efflux and NH4-Nconcentrations in forest soils and the highest DON and NO3-N concentrations in grasslandsoils. Temperature and land use type affected the cumulative soil CO2efflux, and DOC, DON,NH4-N and NO3-N concentrations. The form of the N addition or its interaction with the othertwo factors did not affect any of the C and N cycling parameters. Temperature and land usetype were dominant factors affecting soil C loss, with the soil C in grassland soils more stableand resistant to temperature changes. The lack of short-term effects of the deposition of N ofdifferent species on soil C and N mineralization suggest that maybe there was a threshold forthe N effect to kick in and long-term experiments should be conducted to further elucidate theN deposition effects on soil C and N cycling in the studied systems. Thirdly, anthropogenicactivities have doubled the rate of nitrogen (N) inputs to most terrestrial ecosystems in thepast century and the trend is likely to increase over the next several decades. The increased Ncould affect soil carbon (C) cycling through influencing enzymatic activities of soil microbes.In the soils of two different land use types (forest and grassland), we studied the effects ofnitrate (NO3-) and ammonia (NH4+) addition on microbial enzymatic activities includingligninolytic and cellulolytic enzymatic activities, and microbial functional structure. Overall,N-acetyl-glucosaminidase and peroxidase were affected by the land use, with higher activitiesin forest soils. Nitrogen additions increased cellobiohydrolase activity in forest soil. TheAWCD and Shanon diversity index H’ were affected by land use, and forest soils had highermicrobial functional diversity. In a PCA analysis, the first two principal componentsexplained54%of total variation in substrate utilization profiles. Land use had strong loadingson the first component while N fertilizer treatment had strong loadings on the secondcomponent. Our findings indicated that global changes could affect soil carbon and nitrogendynamics through influencing microbial enzymatic activities.