Short- and medium-term effects of fuel reduction mulch treatments on soil nitrogen availability in Colorado conifer forests

Mechanical fuel reduction treatments have been implemented on millions of hectares of western North American forests. The redistribution of standing forest biomass to the soil surface by mulching treatments has no ecological analog, and this practice may alter soil processes and forest productivity. We evaluated the effects of mulch addition on soil nitrogen availability at 15 fuel reduction projects in the southern Rocky Mountains and Colorado Plateau regions of Colorado. Mulching treatments removed 38 Mg ha−1 of standing forest biomass on average and added 2– 4 cm of irregular woody fragments to the O horizon. Mulching lowered maximum summer soil temperatures and increased soil moisture. 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 material of similar size in untreated areas. Plant-available soil N, measured in situ with ion exchange resins was reduced under heavily-mulched experimental plots the year mulching occurred, but the effect did not persist for a second year. The nitrogen content of freshly-applied mulch increased by 9, 24 and 55 kg N ha−1 year−1 in plots receiving 22, 49 and 105 Mg ha−1 of mulch material on average. In contrast, 5-year-old mulch released N regardless of amount of mulch added. Three to five years after treatment, available N was 32% higher in mulched fuel reduction treatments compared to untreated forests. Heavy mulch application has the potential to temporarily reduce soil N availability in limited areas, but as implemented in Colorado conifer forests, fuel reduction mulch treatments increase soil N availability.