The effects of acid rock drainage on Carex aquatilis leaf litter decomposition in Rocky Mountain fens

Fens occur in many Colorado Rocky Mountain watersheds that have historicall y been affected by metal mining activities. A persistent effect of mining i s acid rack drainage (ARD), which flows from mine adits and tailings piles. ARD water has low pH and high concentrations of heavy metals and can pollu te surface and ground waters supporting fens. There are few floristic diffe rences between polluted and pristine fens, and it is unclear what long-term affects ARD has on fens. We investigated decomposition of native leaf litt er and a standard litter for two years in four Carer aquatilis-dominated su balpine fens. Two of the fens are affected by ARD, and their source waters have pH < 4.0 and high concentrations of dissolved Zn, Cu, Mn, and Pb. The other two fens are relatively pristine, with source water pH > 6.0 and low metal concentrations. ARD significantly reduced native litter decomposition rates over two years (ANOVA, p < 0.01), with 63% of the initial mass of le af litter remaining at the most polluted site (Peru Creek fen), 47% at a mo derately polluted site (Argentine fen), and 23% at the most pristine site ( Deer Creek fen). ARD also reduced the quality of litter produced. Standard litter decomposed more rapidly than native litter at all sites, particularl y Peru Creek fen. Site, representing fen environment, and litter origin, re presenting native vs, standard litter, significantly affected decomposition rates (ANOVA, p < 0.0001). Pristine fens had decomposition rates comparabl e with boreal rich fens, while polluted fens had decomposition rates compar able with boreal poor fens and bogs. Acid rock drainage causes ecosystem-le vel responses in organic matter processing, peat accumulation rates, and mo st likely in nutrient cycling, which could have long-term affects on fen pr imary and secondary production and other functions.