Piñon mortality and summer monsoon rains affect extra cellular enzyme activity of soil microbial communities living beneath tree canopies in a Piñon-Juniper woodland

Background/Question/Methods

Climate change processes, including drought and elevated temperatures, are significantly affecting numerous arid ecosystems in New Mexico.  One of the more adversely affected ecosystems in New Mexico is piñon (Pinus edulis) juniper (Juniperus monosperma) (PJ) woodlands.  Because changes in temperature and water availability directly affect ecosystem processes easily assessed aboveground such as mortality, net ecosystem exchange of carbon and gross primary productivity, it is likely that changes in these variables will also manifest themselves as differences in key soil process rates such as decomposition, respiration, and nutrient cycling.  To address these questions, we collected soil samples in both the dry (June and July) and wet (August and September) seasons, from multiple tree to tree gradients at an experimental PJ woodland site near Mountainair, NM.  In 2008, all piñon trees greater than 7cm at DBH were girdled, simulating death due to prolonged drought stress.  We established transects (n=6) that included the following gradients, live piñon to live piñon (LP/LP), live piñon to dead piñon (LP/DP), live piñon to live juniper (LP/LJ), dead piñon to live juniper (DP/LJ), and live juniper to live juniper (LJ/LJ).  Samples collected from this site were analyzed, to assess multiple soil physicochemical properties, none of which showed any significant trends across our different tree-to-tree gradients, as well as the activity rates of the hydrolytic extracellular enzymes alanine aminopeptidase, alkaline phosphatase, β-D-glucosidase, and β-N-acetyl glucosaminidase.

Results/Conclusions

Our results showed that the activity rates from samples collected under the trees in LJ/LJ vs. the trees in LP/LP were significantly greater for at least one enzyme in all months, except August.  For example, for β-D-glucosidase assays, activity rates beneath trees from LJ/LJ vs. trees from LP/LP that were 30% higher in June, 13% in July, and 70% in September.  In contrast there was only one instance where hydrolytic activity rates under piñons were higher than those under juniper.  In September, alanine amino peptidase activity was 82% higher under piñon trees.  These results illustrate how drought-stress within one tree species can significantly affect the activity of root associated microorganisms and alter nutrient availability and cycling within soil associated with the tree’s root systems.  Further, the magnitude of these effects varies significantly across the growing season, in association with the dynamics of summer monsoon rains.