EFFECT OF VASCULAR PLANT-GROWTH ON CHEMICAL WEATHERING PROCESSES - OBSERVATIONS FROM EXPERIMENTAL ECOSYSTEM STUDY

Silicate mineral weathering was investigated in mesoscale experimental ecosystems, i.e. sandboxes, at Hubbard Brook Experimental Forest (HBEF), New Hampshire. The sandbox study was established to investigate elemental cycles in simple systems under a variety of vegetation cover. The focus of our study was to detect the effect of vascular plant growth on dissolution processes and base cation cycles.

Weathering fluxes were estimated using mass balance approach for three time intervals in large (7.5x7.5x1.5m) fully lined sandboxes. One sandbox was planted with red pine (Pinus resinosa Ait.), and another was kept free of vascular vegetation. Mass-balance equations accounted for base cations (Ca, Mg, and K) in precipitation inputs, drainage outputs, biomass and soil-extractable pool. Scanning Electron Microscopy (SEM) studies of the coarse glacial outwash sandbox material were also used for confirming the effect of red pine and its ectomycorrhizal fungal associates on biochemical weathering. SEM images were analyzed with ImageJ software to compare weathered areas on mineral surfaces between the boxes.

In the non-vascular system chemical weathering fluxes did not change significantly during the monitored period (20 years). In the red pine system weathering was very high in the rapid growth stage of the ecosystem (1984-1988), consistent with Bormann et al. (1998) findings, but it decreased 5-fold when growth slowed down (1988-1998). After 1998, when the trees were harvested and regrowth was prevented, weathering rates showed similarities with the non-vascular system. The SEM study supports the chemical mass balance results with more abundant weathering features (etch pits, cracks, holes, channels, and secondary minerals) on red pine mineral surfaces. 18% of the biotite surfaces were weathered from the red pine sandbox whereas only 7.5% of the biotite surfaces exhibited altered areas. The sandbox experiment provides insights into the short-term effect of ecosystem development on chemical weathering in pine forest ecosystems. It may be possible to extend these findings to other forested ecosystems and make some predictions about long-term processes.