Understanding stress effects on ecosystem function: The influence of anthropogenic acidification on nitrogen cycling in headwater streams

Background/Question/Methods

Recently, ecologists have been urged to focus on the physiological stress response of microorganisms to better understand and predict the consequences of changing environmental conditions on ecosystem function. Anthropogenic stream acidification remains a widespread phenomenon with detrimental effects on community structure; however, the influence of chronic acidity on nutrient cycling is unknown. Observations of slower leaf decomposition rates and reduced microbial biomass in acidified streams suggest the impairment of nutrient uptake processes. We collected decomposing leaves (Quercus prinus) from five streams in Shenandoah National Park, VA, spanning a gradient of chronic pH (5.1 – 6.7) and investigated respiration and nitrogen (N) uptake by associated fungal biofilms under two levels of N availability (25 and 100 μg NH₄-N L^-1). We used the metabolic quotient qCO₂ (mg CO₂ respired-C·g^-1 Fungal C·h^-1) as an indicator of stress and likewise standardized N uptake (μg NH₄-N·mg^-1 Fungal Biomass·h^-1).

Results/Conclusions

Strong patterns of increasing qCO₂ (i.e., increasing stress) and increasing N uptake were observed with decreasing stream pH. Experimentally-increased N availability stimulated N uptake by biofilms collected from all streams but had no effect on qCO₂. Significant positive relationships between qCO₂ and biomass-specific N uptake were observed at both low (r² = 0.83; P < 0.001) and high (r² = 0.91: P < 0.001) levels of N availability. Fungal biomass was lower on leaves from more acidic streams, resulting in reduced respiration and N uptake in low pH streams when rates were standardized by leaf biomass. Results suggest that the stress of chronic acidification decreases microbial metabolic efficiency by allocating greater resource flow to maintenance and survival with associated increases in N removal. At the same time, greater N availability increased N uptake without increased CO₂ production, implying greater growth efficiencies. At the ecosystem level, reductions in growth due to chronic acidification lower microbial biomass and impair processes of decomposition and N uptake; however, in systems where N is initially scarce increased N availability may counterbalance these effects. Ecosystem response to large-scale stressors may be better understood with greater focus on microbial physiology, coupled elemental cycling, and responses across multiple scales of investigation.