CARBON-ISOTOPE DYNAMICS OF FREE-AIR CO2-ENRICHED COTTON AND SOILS

A role for soils as global carbon sink or source under increasing atmo spheric CO2 concentrations has been speculative. Free-air carbon dioxi de enrichment (FACE) experiments with cotton, conducted from 1989 to 1 991 at the Maricopa Agricultural Center in Arizona, maintained circula r plots at 550 mumol mol-1 CO2 with tank CO2 while adjacent ambient co ntrol plots averaged about 370 mumol mol-1 CO2. This provided an excep tional test for entry of carbon into soils because the petrochemically derived tank CO2 used to enrich the air above the FACE plots was depl eted in both radiocarbon (C-14 content was 0% modern carbon (pmC)) and C-13 (delta13 C almost-equal-to -36 parts per thousand) relative to b ackground air, thus serving as a potent isotopic tracer. Flask air sam ples, and plant and soil samples were collected in conjunction with th e 1991 experiment. Most of the isotopic analyses on the plants were pe rformed on the holecellulose component. Soil organic carbon was obtain ed by first removing carbonate with HCl, floating off plant fragments with a NaCl solution, and picking out remaining plant fragments under magnification. The delta C-13 of the air above the FACE plots was appr oximately - 15 to - 19 parts per thousand, i.e. much more C-13 deplete d than the background air of approximately -7.5parts per thousand. The delta C-13 values of plants and soils in the FACE plots were 10-12 pa rts per thousand and 2 parts per thousand C-13-depleted, respectively, compared with their control counterparts. The C-14 content of the FAC E cotton plants was approximately 40 pmC lower than that of the contro l cotton, but the C-14 results from soils were conflicting and therefo re not as revealing as the delta C-13 of soils. Soil stable-carbon iso tope patterns were consistent, and mass balance calculations indicate that about 10% of the present organic carbon content in the FACE soil derived from the 3 year FACE experiment. At a minimum, this is an impo rtant quantitative measure of carbon turnover, but the presence of C-1 3-depleted carbon, even in the recalcitrant 6 N HCl resistant soil org anic fraction (average age 2200 years before present (BP)), suggests t hat at least some portion of this 10% is an actual increase in carbon accumulation. Similar isotopic studies on FACE experiments in differen t ecosystems could permit more definitive assessment of carbon turnove r rates and perhaps provide insight into the extent to which soil orga nic matter can accommodate the 'missing' carbon in the global carbon c ycle.