FACTORS AFFECTING THE APPLICABILITY OF THE AET MODEL FOR DECOMPOSITION IN ARID ENVIRONMENTS

There is considerable interest in the applicability of the model developed by Meentemeyer (1978), which predicts decomposition as a function of actual evapotranspiration (AET) and lignin, to environments other than forests. Whitford et al. (1981b) discussed exceptions to the Meentemeyer AET model in deserts and clearcut forests and concluded that in these environments, biological activity that is relatively independent of AET, such as activity of soil microarthropods and termites, accounts for the deviations (higher rates) from the model. We recently completed a study of decomposition in a semiarid desert-grassland transitional area that provides another independent test of the AET model. Our study was conducted at the site of the proposed nuclear waste isolation pilot plant 40 km east of Carlsbad, New Mexico, USA. Rainfall averages =260 mm/ yr with most precipitation occurring in late summer and autumn, and in late spring. We used litter characteristic of three plant communities: creosotebush, Larrea tridentata, shinnery oak, Quercus harvardii, and a mixture of two grasses: black gramma, Bouteloua eripoda, and burrograss, Scleropogon brevifolia. The creosotebush community is on shallow, sandyloam soils. The shinnery oak community is on deep, sandy, unstable soils which form dunes, and the grass community is on shallow, silty-loam soils. Forty bags of each litter type, containing 10 + 0.2 g of litter per bag, were placed on the soil surface in the plant community from which the litter was collected. Litter bags were 20 x 20 cm fiberglass screen, 1.5-mm mesh size. Ten bags were collected at 1 mo, 3 mo, 6 mo, and 1 yr; mineral soil was separated from the remaining litter, and the litter was dried, weighed, and burned in a muffle furnace to obtain organic matter loss. Lignin content of samples of the original collections was estimated by the Van Soest method (Van Soest 1963). We used rainfall data from the National Oceanic and Atmospheric Administration station at Potash Company of America, 30 km northwest of the site, to estimate actual evapotranspiration. The average lignin contents of the litter species were, Q. harvardii: 10.8 + 0.5%; L. tridentata: 9.5 ? 0.7%; and mixed 100