Coarse woody debris influences microbial processes and grasses in cold deserts

Background/Question/Methods

To stem catastrophic wildfires and create firebreaks, whole Juniperus osteosperma trees are being mechanically shredded into coarse woody debris (CWD) and deposited on soils. CWD often alters belowground processes, ultimately increasing or decreasing the availability of limiting nutrients for establishing plant species. However, semi-arid woodlands are exposed to tree-induced changes creating “tree-islands of fertility.”  These drastic differences in belowground characteristics between barren plant interspace and tree-island soils may alter the impact of CWD on nutrient cycling. We evaluated C cycling, N transformation rates, and P availability in three Juniperus tree-island microsites (i.e., interspace soils, soils beneath the canopy, and at the edge of the canopy) exposed to field CWD manipulation in over forty cold desert sites.  For each microsite, we linked our measured belowground processes to the frequency of one exotic grass (Bromus tectorum) and three native perennial grasses (i.e., Pseudoroegneria spicata, Elymus elymoides, and Poa secunda) using structural equation modeling.  Microsites were evaluated at two depths (0-2 cm and 15-17 cm).  C, P, and N cycling were evaluated in soil microcosms incubated for ten days at constant gravimetric water content (0.3 g H₂O g dry soil^-1). Microsites were evaluated at two depths (0-2 cm and 15-17 cm). 

Results/Conclusions

Microsites regulated the effects of CWD on N transformations and influenced B. tectorum and P. secunda. N mineralization and nitrification rates increased in interspace and decreased in edge soils. The frequency of B. tectorum increased in interspace soils and demonstrated a positive relationship with higher levels of nitrification in deep soils (15-17 cm, R²=0.43, P=0.01). Thus, decomposition of roots from shredded trees may provide inorganic N for the exotic grass. Further, B. tectorum frequency increased with time since CWD addition (P=0.005), suggesting that B. tectorum benefits from the accumulation of N in interspaces. Alternatively, P. secunda frequency decreased in canopy edge and canopy soils, but this grass did demonstrate a positive relationship with increasing N mineralization (R²=0.16, P=0.04). P availability increased following CWD additions in all microsites and so did the frequency of E. elymoides and P. spicata. However, these species were not influenced by P in our models and instead correlated with changes in soil respiration. CWD additions increased DOC, microbial biomass, and respiration in interspace and canopy edge soils. Tree-islands of fertility microsites did alter the impact of CWD on belowground processes, but microsite-specific changes to N transformations were the only processes linked to grass species distribution.