Persistent Effects of Fuel Reduction Mulch Treatments on Soil Nitrogen Dynamics in Colorado Conifer Forests.

Mechanical fuel reduction treatments have been implemented on millions of hectares of western North American forests in recent years.  Mulching transfers woody biomass to the soil surface, creating a treatment with no ecological analogue.  This relatively-new management practice may have lasting effects on soil nitrogen (N) dynamics and forest productivity.  Over the past decade, we have studied the effects of forest mulching on plant available soil N and net N transformations at fifteen fuel reduction projects in the Southern Rocky Mountains and Colorado Plateau regions of Colorado.  Mulching removed 38 Mg ha^-1 of standing forest biomass and added 2 to 4 cm of irregular woody fragments to the O horizon.  The N added in mulch was equivalent to half the amount contained in untreated O horizons, and mulch had a lower N concentration and wider C:N ratio than similar material in untreated areas.  Mulching increased soil moisture in general and this effect was most pronounced under dry conditions.  Plant available soil N was 32% higher in operational mulch treatment areas compared to untreated forests three to five years after treatment, and this pattern and net N transformations remained higher in mulched areas throughout the study.  Conversely, we found a lasting reduction in plant-available soil nitrate and net nitrification in heavily-mulched experimental plots that received mulch in excess of rates typical of Colorado fuel reduction treatments.  High rates of mulch application may reduce soil N dynamics, but in general forest mulching as conducted operationally in Colorado has a positive effect on soil N.