Long-term and seasonal variations in CO2: linkages to catchment alkalinity generation

Abstract

As atmospheric emissions of S have declined in the Northern Hemisphere, there has been an expectation of increased pH and alkalinity in streams believed to have been acidified by excess S and N. Many streams and lakes have not recovered. Evidence from East Bear Brook in Maine, USA and modelling with the groundwater acid-base model MAGIC (Cosby et al. 1985a,b) indicate that seasonal and yearly variations in soil PCO 2 are adequate to enhance or even reverse acid-base (alkalinity) changes anticipated from modest decreases of SO 4 in surface waters. Alkalinity is generated in the soil by exchange of H + from dissociation of H 2 CO 3 , which in turn is derived from the dissolving of soil CO 2 . The variation in soil PCO 2 produces an alkalinity variation of up to 15 meq L -1 in stream water. Detecting and relating increases in alkalinity to decreases in stream SO 4 are significantly more difficult in the short term because of this effect. For example, modelled alkalinity recovery at Bear Brook due to a decline of 20 meq SO 4 L -1 in soil solution is compensated by a decline from 0.4 to 0.2% for soil air PCO 2 . This compensation ability decays over time as base saturation declines. Variable PCO 2 has less effect in more acidic soils. Short-term decreases of PCO 2 below the long-term average value produce short-term decreases in alkalinity, whereas short-term increases in PCO 2 produce shortterm alkalization. Trend analysis for detecting recovery of streams and lakes from acidification after reduced atmospheric emissions will require a longer monitoring period for statistical significance than previously appreciated.

Twenty-year inter-annual trends and seasonal variations in precipitation and stream water chemistry at the Bear Brook Watershed in Maine, USA

“…The higher PCO 2 caused transient acidification that was countered by increased desorption of base cations. The counter anion was HCO − 3 and the result was higher pH in the stream (Norton et al 2001). Comparable soil air data are lacking for any other period at the BBWM, but they illustrate the ability of climate fluctuations to cause measurable variation in stream chemistry.…”

Variability in soil respiration across riparian-hillslope transitions

“…This corresponded to the time of the deepest snowpack and greatest snow water equivalent of the year (120 and 46 cm, respectively, Figures 5 & 6). A deep snowpack can lead to increased soil CO 2 concentrations due to its insulating effects (Sommerfeld et al 1996;Schadt et al 2003) (allowing for relatively high production compared to colder soils) and low gas diffusivity (Hamada and Tanaka 2001;Norton et al 2001). Differences in the timing of peak winter CO 2 concentrations (4-8 weeks later in the hillslopes) were likely the result of differences in the snow energy balance.…”

Mobilization of metals and phosphorus from intact forest soil cores by dissolved inorganic carbon

“…Concerns about global climate change have focused primarily on carbon dioxide (CO 2 ; Robinson et al 1998). Although changing the CO 2 concentration in the atmosphere from 10 −3.5 to 10 −3.3 does little to the soil atmosphere, high concentrations of CO 2 in subsoils due to microbial and root respiration can increase metal mobilization, including base cations (Norton et al 2001), Fe, and Al (David and Vance 1989;Su and Suarez 1997). Dissolved inorganic carbon (DIC) is the sum of carbonic acid (H 2 CO 3 * ≈ CO 2(aq) ), bicarbonate anion (HCO 3 − ), and carbonate anion (CO 3 2− ) in water, with the concentration of each species depending on solution pH.…”