Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2005
Publication Date: 2/5/2006
Citation: Gill, R.A., Anderson, L.J., Polley, H.W., Johnson, H.B., Jackson, R.B. 2006. Potential nitrogen constraints on soil carbon sequestration under low and elevated atmospheric CO2. Ecology. 87(1):41-52. Interpretive Summary: Air temperatures are predicted to climb during coming decades as the concentration of carbon dioxide (CO2) gas in the atmosphere increases. Higher CO2 also stimulates plant growth, so the rise in CO2 may increase the amount of carbon that is removed from air and stored in soil as partially-decomposed plant material (organic matter). If storage of organic matter increases, the rise in atmospheric CO2 concentration will be slowed and the amount of warming will be lessened. To predict the amount of additional carbon that can be stored in soils requires an understanding of how nitrogen and carbon interact in plants and soils. Because carbon and nitrogen accumulate together in soil organic matter, it is predicted that the accumulation of soil organic matter will lessen the amount of nitrogen that is available to sustain plant production, thus limiting the amount of CO2 that can be removed from the atmosphere. We studied interactions between carbon and nitrogen dynamics in a grassland in central Texas that was exposed for four years to a continuous gradient in CO2 that spanned low concentrations of the past to the elevated levels expected within the century. We found that nitrogen was reallocated from soil organic matter to plant biomass as CO2 increased. This transfer of N to plants permitted plant production to increase as CO2 levels rose, but limited the amount of organic carbon that accumulated in soil at elevated CO2. Our results are consistent with the prediction that the availability of nitrogen ultimately limits the potential of grasslands to slow the rise in atmospheric CO2 concentration by accumulating soil organic matter.
Technical Abstract: The interaction between nitrogen cycling and carbon sequestration is critical in predicting the consequences of anthropogenic increases in atmospheric CO2 (Ca). The progressive N limitation (PNL) theory predicts that accumulation of organic material in ecosystems will immobilize mineral nutrients necessary to sustain increased plant production and ultimately constrain long-term C sequestration in terrestrial ecosystems. Here we report on the interaction between C and N dynamics during a four year field experiment where an intact C3/C4 grassland was exposed to a gradient in Ca from 200-560 umol/mol. We found a general movement of N out of soil organic matter and into aboveground plant biomass with increased Ca. There was a significant negative exponential relationship between net N-mineralization and Ca, consistent with the predictions of PNL. There were strong species effects on decomposition dynamics and N mineralization during initial decomposition in laboratory incubations, with C loss and N mineralization from litter of the C3 forb Solanum dimidiatum consistently responding to Ca while the C4 grass Bothriochloa ischaemum was relatively unresponsive to Ca. In the field experiment we found evidence that there has been a redistribution of N from recalcitrant soil C fractions with narrow C:N ratios to more labile soil fractions with broader C:N ratios. The observed reallocation of N from soil to plants over the last three years of the experiment supports the PNL theory that reductions in N availability with rising Ca could initially be overcome by a transfer of N from low C:N ratio fractions to those with higher C:N ratios. While this transfer of N allowed plant production to increase with increasing Ca, there was no net soil C sequestration at elevated Ca, presumably because relatively stable C is being decomposed to meet microbial biomass and plant N requirements. Ultimately, if the C gained by increased plant production is rapidly lost through decomposition, the shift in N from older soil organic matter to rapidly decomposing plant tissue may result in little or no net C sequestration with increased plant production.