Carole C. Klopatek

THE EFFECTS OF LITTER QUALITY AND CLIMATE ON DECOMPOSITION ALONG AN ELEVATIONAL GRADIENT

The process of decomposition is controlled by both biotic and abiotic factors. While it has been widely hypothesized that litter quality and climatic conditions regulate decomposition, the relative importance of these factors appears to vary across biomes. This study examines the decomposition of native plant litter along an elevational gradient in northern Arizona to determine the influence of litter quality and climate on the rate of decomposition in semiarid communities. A litter-bag experiment was performed using needle/leaf litter from Pinus ponderosa, Pinus edulis, Juniperus monosperma, Gutierrezia sarothrae, and Bouteloua gracilis. The five litter types are representative of the dominant local vegetation and offer a range of litter qualities. The bags were placed along a gradient, running from Great Basin Desert scrub (1960 m) through a pinyon-juniper woodland (2100 m) and up into a ponderosa pine forest (2280 m). Samples were collected and analyzed over a period of 2 yr. Decomposition was closely correlated with the relative proportion of easily decomposed carbon fractions to recalcitrant fractions for the first year. Litter from G. sarothrae and B. gracilis contained relatively low levels of lignin and high levels of cellulose and carbohydrates, and these litter types exhibited significantly faster rates of decay than the highly lignified pine and juniper litter. The order of the relative rates of decomposition was G. sarothrae k B. gracilis . J. monosperma . P. ponderosa 5 P. edulis. There was no correlation between initial litter nitrogen content and the rate of decomposition, suggesting that decomposition is limited by carbon substrates rather than by nutrient content. Decomposition rates were significantly greater at the upper elevation sites, which were colder and wetter. Evidence strongly suggests that decomposition is limited by moisture in these ecosystems. Warmer temperatures resulting from climate change may not increase the rate of decomposition in the Southwest unless accompanied by increases in available moisture.

Effects of simulated fire on vesicular-arbuscular mycorrhizae in pinyon-juniper woodland soil

Effects of fire on vesicular-arbuscular mycorrhizal fungi were tested using microcosms constructed from soil, litter, and duff collected beneath canopies of pinyon pine, Utah juniper, and in the open space (interspace). Burning was conducted over wet and dry soils. Soil temperatures were monitored continuously throughout the microcosms during burning. Plants grown in soils burned when dry had a lower VAM colonization than soils burned when wet. Juniper soils demonstrated the greatest reduction, over 95%, compared to their respective controls. Plants grown in interspace soils burned when wet were least affected. There was a positive correlation (r2=0.90) between the decrease in VAM colonization and the soil temperature as a result of the fire. Temperature effects, and associated reductions in VAM, were related to amount of litter burned in each microcosm and the moisture content of the soils.

Nitrifiers and mycorrhizae in pristine and grazed pinyon‐juniper ecosystems

We compared soils from grazed and pristine pinyon‐juniper ecosystems for the numbers of chemoautotrophic nitrifying bacteria and determined the distribution of vesicular‐arbuscular (VA) mycorrhizae and ectomycorrhizae (EM). Additionally, we report on the presence of allelopathic substances in organic horizons. Ammonium (NH4+)‐oxidizing bacteria were in greater numbers in the interspaces between the trees, ranging from 9.60 × 104 to 2.13 × 105 bacteria‐1 soil, than under canopies, 3.5 × 104 to 4.8 × 104 bacteria‐1 soil. Grazed interspace soils had over twice the number of NH4+oxidizing bacteria than any other location. There were no differences in the numbers of nitrite‐oxidizing bacteria between either vegetative covers or sites. The rate of nitrate production did not correlate with the number of nitrifiers. However, there was a significant correlation (r2 = 0.85) between mineralization coefficients of total nitrogen and the total number of nitrifiers. Fourteen known and ten unknown monoterpenes were foun...