Responses of nitrogen cycling and ammonia-oxidizing communities to warming and altered precipitation in a New England old field
Auyeung, D. S. N. 2012.
Abstract
Shifts in nitrogen (N) cycling rates, especially N mineralization and nitrification, due to
global environmental changes such as warming and altered precipitation can alter the
availability of soil inorganic N, an important plant nutrient. Changes in soil inorganic N
due to warming and altered precipitation have the potential to alter climate change
feedbacks by influencing carbon uptake by plant communities or rates of denitrification,
a process that leads to the release of the greenhouse gas nitrous oxide. In order to better
understand the responses of N cycling to global environmental changes and the
mechanisms underlying these responses, I examined the responses of N mineralization,
nitrification, soil inorganic N, and the ammonia-oxidizing community to warming and
altered precipitation. All of this research took place at the Boston Area Climate
Experiment, located in an old-field ecosystem in Waltham, Massachusetts.
Using a combination of field soil incubations and laboratory assays, I measured
net N mineralization and net and potential nitrification rates throughout the year to
determine the seasonal shifts in the N cycling responses to warming and altered
precipitation. I used soil extractions and ion exchange membranes to capture short-term
and longer-term measurements, respectively, of soil inorganic N availability. These measurements were compared with N cycling rates, animal activity, and leaf area index, a
proxy for plant N uptake, to determine how well they were correlated. Finally, I
examined the response of potential nitrification kinetics and the abundance and
composition of ammonia oxidizers to warming and altered precipitation in order to
determine links between process rates and microbial communities. The potential
nitrification kinetics of the nitrifying microbial community were assessed using potential
nitrification assays at multiple ammonium concentrations. The community composition
and abundance of ammonia oxidizers were assessed using terminal restriction fragment
length polymorphism (T-RFLP) and real-time quantitative PCR (qPCR), respectively.
Based on results collected from October 2008 to January 2009 and from August
2009 to October 2010, net N mineralization and net and potential nitrification rates rarely
responded to warming and altered precipitation so there was little evidence of consistent
seasonal differences in these responses. Although N mineralization and nitrification rates
were not very sensitive to warming or altered precipitation, warming and drought
strongly increased soil inorganic N pools, especially during the growing season. This
effect may be partly due to the influence of the N uptake by plant communities: leaf area index was strongly correlated with soil inorganic N pools and tended to decrease in
response to drought. If decreased leaf area index corresponded with lower rates of N
removal by plants, this may explain why soil inorganic N pools were larger in drought
plots. Drought also influenced potential nitrification kinetics. At low ammonium
concentrations, potential nitrification rates were higher in drought than ambient or wet
plots; at high ammonium concentrations, potential nitrification rates were lower in
drought than ambient or wet plots. In terms of the microbial community, however, thesampling date explained more of the variation in ammonia oxidizer composition than the
treatments, and there were only slight differences between the responses of the ammonia-
oxidizing archaea (AOA) and the ammonia-oxidizing bacteria (AOB) to warming and
altered precipitation. This suggests that changes in nitrification rates may be attributed
more to physiological changes in the microbial community than to changes in the
community structure or abundance. Overall, this research suggests that N mineralization
and nitrification may not be as tightly coupled to soil inorganic N availability in a future,
warmer climate with different precipitation regimes, and changes microbial physiology
can play an important role in influencing the rates of biological processes.
