THE BIOGEOCHEMISTRY OF PHOSPHORUS CYCLING AND PHOSPHORUS AVAILABILITY IN A DESERT ECOSYSTEM (NUTRIENT CYCLING, CHRONOSEQUENCE, NEW MEXICO)

The geochemistry of the weathering, landscape movement, chemical transformations, and the plant availability of phosphorus was examined in a chronosequence of soils developed from quartz monzonite alluvium in southern New Mexico. All soils contained pedogenic carbonates in upper horizons with caliche layers at depth. Total P in the soil profile decreased with increasing soil age, and was removed from the ecosystem as readily as the most easily leachable base cations. Although Ca-bound forms of P decreased with increasing soil age, Ca-P remained the single largest fraction of total P in all soils.; There was little evidence for the biological conservation of P within this ecosystem; both soil organic P and microbial P represented very small pools of total soil P. Phosphorus availability, measured by in situ resin bags, was not well correlated with soil age or total soil P, and P concentrations in shrub tissues did not reflect changes in forms or total amounts of soil P.; In greenhouse experiments, seedling response to experimental additions of CaCO(,3) was species-specific. Parthenium incanum seedlings responded negatively to the presence of CaCO(,3), but this response was not specific to P uptake and did not appear to be caused by a reduction in P availability. Seedlings of Larrea tridentata responded to soil CaCO(,3) with increased root:shoot ratios, increased N:P ratios in tissues, and decreases in the specific absorption rate of P, suggesting that CaCo(,3) directly inhibited the uptake or the availability of P to roots. In field fertilization experiments, however, P was not a limiting element for the growth or leaf retention of mature larrea shrubs.; While pedogenic CaCO(,3) appeared to be the main geochemical factor retaining P within the soil profile in these Aridisols, P was not as tightly conserved as in more mesic ecosystems with high levels of free iron and aluminum oxides. Similarly, pedogenic CaCO(,3) did not appear to be as effective as iron or aluminum oxides in limiting the availability of P to plants.