ALBERT

Notes: The effects of elevated carbon dioxide (CO2) on plant litter are critical determinants of ecosystem feedback to changing atmospheric CO2 concentrations. We measured concentrations of nitrogen (N) and carbon (C) and calculated C : N ratios of green leaves of two desert perennial shrubs, and the same quality parameters plus lignin and cellulose content of leaf litter from four shrub species exposed to elevated CO2 (FACE technology; Hendrey & Kimball, 1994) for 3 years in an intact Mojave Desert ecosystem. Shrubs tested were Larrea tridentata, Lycium pallidum, Lycium andersonii and Ambrosia dumosa. We calculated resorption efficiency from green tissue and leaf litter N data and measured lignin and cellulose content in litter in the last year study. Green leaves of L. tridentata grown under elevated CO2 had significantly lower N concentrations and higher C : N ratios than shrubs grown in ambient conditions in 1999 (P 〈 0.05). Lycium pallidum green leaves grown under elevated CO2 had significantly lower N concentrations and higher C : N ratios than shrubs grown under ambient conditions in 2000 (P 〈 0.05). There was no CO2 effect on C content of either species. We found no effect of CO2 on N or C content, C : N ratios, or lignin or cellulose concentrations in leaf litter of L. tridentata, L. pallidum, L. andersonii, or A. dumosa. There was no significant effect of CO2 on estimates of shrub resorption efficiency. There was a seasonal effect on green tissue and litter tissue quality for L. tridentata, with lower tissue N content in summer than in spring or winter months. These data suggest that any productivity increases with elevated CO2 in desert ecosystems may not be limited by lower leaf litter quality and that resorption efficiency calculations are best performed on an individual leaf basis.

Notes: Rising atmospheric CO2 has been predicted to reduce litter decomposition as a result of CO2-induced reductions in litter quality. However, available data have not supported this hypothesis in mesic ecosystems, and no data are available for desert or semi-arid ecosystems, which account for more than 35% of the Earth's land area. The objective of our study was to explore controls on litter decomposition in the Mojave Desert using elevated CO2 and interannual climate variability as driving environmental factors. In particular, we sought to evaluate the extent to which decomposition is modulated by litter chemistry (C:N) and litter species and tissue composition. Naturally senesced litter was collected from each of nine 25 m diameter experimental plots, with six plots exposed to ambient [CO2] or 367 μL CO2 L−1 and three plots continuously fumigated with elevated [CO2] (550 μL CO2 L−1) using FACE technology beginning in April 1997. All litter collected in 1998 (a wet, or El Niño year; 306 mm precipitation) was pooled as was litter collected in 1999 (a dry year; 94 mm). Samples were allowed to decompose for 4 and 12 months starting in May 2001 in mesh litterbags in the locations from which litter was collected. Decomposition of litter produced under elevated CO2 and ambient CO2 did not differ. Litter produced in the wetter year showed more rapid initial decomposition (over the first 4 months) than that produced in the drier year (27±2% yr−1 or 7.8±0.7 g m−2 yr−1 for 1998 litter; 18±3% yr−1 or 2.2±0.4 g m−2 yr−1 for 1999 litter). C:N ratios of litter produced under elevated CO2 (wet year: 37±0.5; dry year: 42±2.5) were higher than those of litter produced under ambient CO2 (wet year: 34±1.1; dry year: 35±1.4). Litter production in the wet year (amb. CO2: 25.1±1.1 g m−2 yr−1; elev. CO2: 35.0±1.1 g m−2 yr−1) was more than twice as high as that in the dry year (amb. CO2: 11.6±1.7 g m−2, elev. CO2: 13.3±3.4 g m−2), and contained a greater proportion of Lycium pallidum and a lower proportion of Larrea tridentata than litter produced in the dry year. Decomposition, viewed across all treatments, decreased with increasing C:N ratios, decreased with increasing proportions of Larrea tridentata and increased with increasing proportions of Lycium pallidum and Lycium andersonii. Because litter C:N did not vary by litter production year, and CO2 did not alter decomposition or litter species/tissue composition, it is likely that the impact of year-to-year variation in precipitation on the proportion of key plant species in the litter may be the most important way in which litter decomposition will be modulated in the Mojave Desert under future rising atmospheric CO2.