Biogeochemical responses of two forest streams to a 2-month calcium addition

1. Calcium (Ca) has been lost from forest soils ait the Hubbard Brook Exper imental Forest (HBEF) because of decreased atmospheric input of Ca and high input of acid anions. Through time, this Ca loss has led to low streamwate r Ca concentration and this change may affect stream ecosystem processes. 2. To test both the biogeochemical response of streams to increased calcium concentration and the role of streams in retaining calcium lost from soils , we added c. 120 mu eq Ca L-1 as CaCl2 to two second-order streams at HBEF for 2 months. One stream (buffered) also received an equivalent amount of NaHCO3 to simulate the increase in pH and alkalinity if Ca were added with associated HCO3- ion. The other stream (unbuffered) received only CaCl2. We collected water samples along a transect above and below the addition site at 11 dates: two before, seven during, and two after the addition. 3. Increase in pH in the buffered stream ranged from 5.6 to about 7.0 in th e treated section. There was a net uptake of Ca on all sampling dates durin g the addition and these uptake rates were positively related to pH. betwee n 10 and 50% of the added Ca was taken up during the release in the 80-m st udy reach. In the unbuffered stream, there was net uptake of Ca on only two dates, suggesting lower Ca uptake. 4. Water samples collected after the addition was stopped showed that a sma ll fraction of the added Ca desorbed from sediments; the remainder was appa rently in longer-term storage in the sediments. No Ca desorbed from the str eam sediments in the unbuffered stream, showing that sorption/desorption ma y be controlled by a pH-induced increase in the number of exchange sites. 5. These streams appeared to be a significant sink for Ca over a 2-month ti me scale, and thus, change in streamwater Ca during a year may be due to pr ocessing of Ca within the stream channel, its well as to changes in inputs from the catchment.