Extensive implementation of mechanical fuel reduction treatments represents a new, but poorly-understood manipulation of western forest ecosystems. The shredded woody residue created by fuel reduction operations forms a mulch layer upon the forest floor that can change the biogeochemical processes that regulate nutrient availability and ecosystem productivity. Mulch layers affect both soil microclimate and inputs of water, nutrient and carbon. Plant demand for soil resources and the consequences of mulch application likely differ between ecosystems and fuel reduction treatments. The objective of work reported here was to determine the relation between mulch depth and soil nitrogen fertility. We created 2 x 2 m residue beds of varying thickness (0, 7.5, 15 cm) in three conifer forest types at fourteen sites throughout Colorado. We used ion exchange resin (IER) bags installed beneath mulch beds to assay the movement of inorganic forms of nitrogen (ammonium and nitrate) in the upper 5 cm of mineral soil.
Results/Conclusions
Nitrate, a highly-mobile anion, comprises 50 to 90% of IER-N. In general, both ammonium and nitrate declined with mulching. IER-N was 50 to 63% lower in mulched compared to untreated plots regardless of mulch depth, though deeper layers reduced IER-N 36 to 95%, whereas the reduction beneath thinner layers was somewhat less (20 to 85%). When mulch treatments are assessed across forest landscapes, spatial variability of mulch deposition and variability in soil characteristics and plant populations complicate the biogeochemical signals created by mulch applied at the plot scale.
