Increasing global atmospheric carbon dioxide (CO2) concentration has led to concerns regarding its potential effects on terrestrial ecosystems and the long-term storage of carbon (C) and nitrogen (N) in soil. This study examined responses to elevated CO2 in a grass ecosystem invaded with a leguminous shrub Acacia farnesiana (L.) Willd (Huisache). Seedlings of Acacia along with grass species were grown for 13 months at CO2 concentrations of 385 (ambient), 690, and 980？μmol？mol？1. Elevated CO2 increased both C and N inputs from plant growth which would result in higher soil C from litter fall, root turnover, and excretions. Results from the incubation indicated an initial (20 days) decrease in N mineralization which resulted in no change in C mineralization. However, after 40 and 60 days, an increase in both C and N mineralization was observed. These increases would indicate that increases in soil C storage may not occur in grass ecosystems that are invaded with Acacia over the long term. 1. Introduction The rise of CO2 in the atmosphere is well documented ; what has not been documented are the sinks for this C, with an estimated unknown sink of ？g？C？yr？1 arising from the global C balance . Carbon dioxide is a prime chemical input to the metabolism of higher plants and has a major role in governing plant-water relations and water use efficiency. The increased growth of most plants under higher levels of CO2 [3–6] has prompted recent speculation on the ability of terrestrial ecosystems to sequester C . However, the fate of C within ecosystems is affected by a biological chain of events which includes competition between plants. The ability of terrestrial ecosystems to sequester C will depend on the cycling of C among the various biomass and soil C pools and on the residence time of C in these pools. The rate of C mineralization during decomposition of residue derived from plants grown under elevated CO2 has not been resolved. It has been theorized that the commonly observed increase in plant C？:？N ratio under elevated CO2 could lead to slower residue decomposition resulting in increased soil C storage and reduction in available N for plant production . However, slower decomposition of leaf litter due to elevated CO2 is not supported by the literature on litter quality . Others have suggested that increased biomass might enhance microbial activity, resulting in a “priming effect” thereby leading to no increase in C storage . Alternatively, microbial preference for easily decomposable plant material produced under CO2-enriched conditions
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