It is widely accepted that nitrogen availability constrains net primary production (NPP) in terrestrial ecosystems. Soil nitrogen availability is influenced by many processes, such as mineralization, leaching, gaseous losses, fixation, release from rock weathering in addition to deposition. Turnover of each of those processes will influence soil nitrogen availability and then NPP in terrestrial ecosystems. Although those processes have been extensively examined in the past, it has not been carefully evaluated how rapidly those processes adjust in response to global change, especially in the drylands.
We have made a comprehensive assessment of N limitation and ecosystem adjustment based on the available data. Specifically, we summarize results from a few meta-analyses on those nitrogen processes from experimental and field studies of N additions, elevated CO2 fumigation, and forest succession. Then we evaluate those results from a systems perspective to understand dynamic coupling of carbon and nitrogen cycles.
Results/Conclusions
Our results indicate that most of the added N is rapidly lost through leaching and gaseous emissions within a few days of treatments in response to N addition. A global meta-analysis indicates that approximately 9% of the added N goes to aboveground plant N pool, 2% goes to litter N pool and belowground plant N pool, and 25% to soil total N pool. The rest is lost from the ecosystems via leaching and gas emission. The great loss of added N leads to the return of soil inorganic N concentrations to ambient levels within two years after cessation of 12 years continue N addition according to one study in UK. In contrast, CO2 enrichment significantly enhanced N influx to ecosystems through biological N fixation, due to the stimulating activities of symbiotic and free-living heterotrophic N fixing bacteria. In addition, N losses through leaching is reduced under elevated CO2, due to the decrease in NO3- and the increasing root growth that immobilize more inorganic N into soils. Thus, soil available N can rapidly adjust to environment change to reach dynamic coupling between carbon and nitrogen cycles. It is highly desirable to carefully evaluate time scales of adjustments of those processes in response to global change in the future.
In this talk, we will particularly highlight how those nitrogen processes adjust in drylands in response to global change.