| dc.description.abstract | Global observations and model simulations show that atmospheric carbon dioxide (CO2) concentrations and surface temperatures have been and will keep increasing. These environmental changes have significant influences on terrestrial biogeochemical cycles. On the other way, how changes in terrestrial biogeochemistry in response to the environmental changes can either amplify or alleviate climate change. Soils, the primary research subject of this dissertation, store more than twice as much carbon (C) as the atmosphere. As such, small changes in soil C may have large impacts on the magnitude of atmospheric CO2 concentrations and therefore climate change. However, due to the huge storage and relatively long residence time, how soil C responds to increasing atmospheric CO2 concentrations and surface temperatures is still unclear. The unclear response of soil C is one of the most important reasons for the uncertainties of the magnitude of global change in this century. In this dissertation, I attempted to study the responses of soil C and related biogeochemical processes to increased temperature and CO2 concentrations, through syntheses and data-model integration.
In the first study, I estimated the responses of two critical soil C dynamic processes, replenishment and priming effect, to increased C input. With the responses of the two processes, I estimated the net change of soil organic C by increased C input. Results show that approximately 58% of newly added C is transferred into soil organic C (SOC) via replenishment, whereas the additional loss of old SOC due to priming effect only accounts for 8.4% of the added new C in the first year after a one-time new C input. As a result, the new C input leads to a net increase in SOC, ranging from 40% to 49% of the added new C. The magnitude of the net increase in SOC is positively correlated with the nitrogen-to-C ratio of the added substrates. Furthermore, a 100-year modeling experiment confirms that an increase in new C input leads to significant SOC accumulation over time. The findings suggest that increasing plant productivity and the consequent increase in C input to soils likely promote SOC storage despite the enhanced decomposition of old C, potentially mitigating further climate change.
The first study evaluated impacts of C input on soil C dynamics. My second study evaluated how nitrogen (N) regulates C input under elevated CO2. A popular hypothesis of the N constraint to the CO2 fertilization effect is progressive N limitation (PNL), which postulates that the stimulation of plant growth by CO2 enrichment results in more N sequestered in plant, litter and soil organic matter (SOM) so that, the N availability for plant growth progressively declines in soils over time. The reduced N availability then in turn constrains the further CO2 fertilization effect on plant growth over longer time scales. Although extensive research has explored whether or not PNL occurs under CO2 enrichment, a comprehensive assessment of the N processes that regulate PNL is still lacking. In the second study, I quantitatively synthesized the responses of all major processes and pools in the terrestrial N cycle with meta-analysis of CO2 experimental data available in the literature. The results showed that CO2 enrichment significantly increased N sequestration in the plant and litter pools but not in the soil pool, partially supporting one of the basic assumptions in the PNL hypothesis that elevated CO2 results in more N sequestered in organic pools. However, CO2 enrichment significantly increased the N influx via biological N fixation and the loss via nitrous oxide (N2O) emission, but decreased the N efflux via leaching. In addition, no general diminution was observed in effects of CO2 fertilization on plant growth. Overall, the analyses suggest that the extra N supply by the increased biological N fixation and decreased leaching may potentially alleviate PNL under elevated CO2 conditions in spite of the increases in plant N sequestration and N2O emission. Moreover, the syntheses indicate that CO2 enrichment increases soil ammonium (NH4+) to nitrate (NO3-) ratio. The changed NH4+/NO3- ratio and subsequent biological processes may result in changes in soil microenvironments, above-belowground community structures and associated interactions, which could potentially affect the terrestrial biogeochemical cycles. In addition, the data synthesis suggests that more long-term studies, especially in regions other than temperate ones, are needed for comprehensive assessments of the PNL hypothesis.
In the third study, I evaluated methods for estimating the temperature sensitivity (Q10) of SOC decomposition since the Q10 estimate substantially depends on their specific assumptions. I compared several commonly used methods (i.e., one-pool (1P) model, two-discrete-pool (2P) model, three-discrete-pool (3P) model, and time-for-substrate (T4S) Q10 method) plus a new and more process-oriented approach for estimating Q10 of SOC decomposition from laboratory incubation data. The process-oriented approach is a three-transfer-pool (3PX) model that resembles the decomposition sub-model commonly used in Earth system models. The estimated Q10s generally increased with the soil recalcitrance, but decreased with the incubation temperature increase. The results indicated that the 1P model did not adequately simulate the dynamics of SOC decomposition and thus was not adequate for the Q10 estimation. All the multi-pool models fitted the soil incubation data well. The Akaike information criterion (AIC) analysis suggested that the 2P model is the most parsimonious. As the incubation progressed, Q10 estimated by the 3PX model was smaller than those by the 2P and 3P models because the continuous C transfers from the slow and passive pools to the active pool were included in the 3PX model. Although the T4S method could estimate the Q10 of labile carbon appropriately, the analyses showed that it overestimated that of recalcitrant SOM. The similar structure of 3PX model with the decomposition sub-model of Earth system models provides a possible approach, via the data assimilation techniques, to incorporate results from numerous incubation experiments into Earth system models.
In the fourth study, I studied how warming affect SOC storage in Alaskan tundra. By combining a process-based model and a unique field experiment, this study shows that warming reduced the base turnover rate of SOC, which is the representation of unresolved microbial community and activity on the resolved scale. The reduced base turnover rate of SOC suggests that microbial decomposers acclimate to warming in Alaskan tundra. Although warming still accelerates SOC loss, the acclimation counterbalances the SOC loss acceleration by 62%. Our study suggests that it is critical to incorporate changes in biological properties (as parameters) to improve the model performance in predicting C dynamics and its feedback to climate change.
This dissertation demonstrates that integrating data and model can advance our understanding of biogeochemical cycles in the context of global change. Future research is needed to study the integrated effect of global change factors on the responses and feedbacks of biogeochemical cycles to global change. | en_US |
