Net N Mineralization Following Mastication of Pinyon-Juniper Woodland in Southwest Colorado.

Pinyon-juniper and juniper woodlands dominate 19 million ha in the western United States. Natural fires were the major stand disturbance in the pinyon-juniper woodlands prior to the introduction of livestock. The loss of the herbaceous cover and the increase in tree density has been linked to past livestock grazing practices and aggressive fire suppression policies. Wildfire risks in these woodlands has been exacerbated by the fuel buildups related current extended drought and infestation of pinyon ips (Ips confuses) that have killed millions of trees over large areas of the region. Large wildfires were historically uncommon in the woodland ecosystems of southwest Colorado, yet between 1996 and 2003 six large wildfires occurred. Mastication technique for treating fuels in wildland-urban-interface areas is preferred because of its high production and reduced cost relative to thinning, piling and burning. Mastication of woody material has raised concerns that a shift in the carbon: nitrogen (C:N) ratio due to the residual plant material left on the soil surface may slow decomposition rates and limit plant available N needed for long-term site productivity. We compared net N mineralization in the upper 15 cm of mineral soil from thinned and burned, masticated, and untreated plots at three sites in southwest Colorado, along with microbial biomass and community structure measured using phospholipid fatty acids,. This technique allows us to determine if there has been a structural shift of microbial communities with emphasis on changes to the fungal:bacterial (F:B) ratios in organic and soil horizons. The plant residue additions to the soil surface should increase C:N ratios and total surface biomass following mastication. Does this alteration also influence the mineral soil C:N ratios by reallocating N from the upper mineral soil and immobilizing it in masticated plant material by fungal decomposers, thus decreasing N availability for a plant growth?