Monitoring nitrogen deposition in throughfall using ion exchange resin columns: a field test in the San Bernardino Mountains.

Conventional throughfall collection methods are labor intensive and analytically expensive to implement at broad scales. This study was conducted to test an alternative approach requiring infrequent sample collection and a greatly reduced number of chemical analyses. The major objective of the study was to determine the feasibility of using ion exchange resin (IER) to measure N deposition in throughfall with field deployment periods of 3 to 12 mo. Nitrogen deposition measurements in bulk throughfall collected under pine (Pinus sp.) canopies and in forest clearings were compared between co-located conventional throughfall solution collectors and IER throughfall collectors using mixed bed IER columns. Deposition data were collected for 1 yr at a high deposition site (Camp Paivika, CP) and a relatively low one (Barton Flats, BF) in the San Bernardino Mountains in southern California: Annual throughfall deposition values (kg ha(-1) of NH(4)-N + NO(3)-N) under large ponderosa pine trees (Pinus ponderosa Laws.) were 145.8 and 143.9 at CP and 17.0 and 15.0 at BF according to the IER and conventional methods, respectively. Analogous values for bulk deposition in forest clearings were 15.6 and 12.3 at CP and 4.0 and 3.3 at BF. It was concluded that the IER collectors can be used for routine monitoring of deposition in throughfall and bulk deposition, provided that field blanks are used to account for background levels of N in the IER columns, which at times are slightly elevated, possibly from slow release of amine groups from the anion exchange resin during field exposures.