Submitted to: Ecosystems
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/15/2004
Publication Date: 5/31/2006
Citation: Keller, C.K., O'Brien, R., Havig, Smith, J.L., Bormann, B.T., Wang, D. 2006. Tree harvest in an experimental sand ecosystem: Plant effects on nutrient dynamics and solute generation. Ecosystems 9: 1-14. Interpretive Summary: Disturbed natural systems, such as forests, can alter chemical movement and potentially contaminate streams and rivers. We monitored the chemistry of soil water and discharge (drainage) water in an outdoor forest 'sandbox' system over a five-year period. The sandboxes were constructed and then planted with native trees, thus the system was simple and all components were known. The trees were harvested after 15 years of growth and the water chemistry before and after harvesting was compared (2 years before and 3 years after harvest). During the first year after harvest potassium (K) began to increase in the water leaching through the soil, then calcium and magnesium in the water began to increase. These elements are in high concentrations in plant litter and root material and increased in the water due to the decomposition of these materials. These elements were also in elevated concentrations due to the lack of plant uptake after harvest. Nitrogen as nitrate (NO3) would also be expected to be present in increasing concentrations; however, soil microorganisms were probably limited by nitrogen for growth in this system and thus consumed any available N until their needs were satisfied. This nitrogen consumption lasted until over a year after harvest, then NO3 increased in leaching water. The amount and timing of nutrient release is important with regards to nutrient fluxes to streams and rivers in which water quality may be impaired due to high nutrient levels. This type of research may help to optimize forest harvest activities.
Technical Abstract: We monitored the hydrochemistry of soil water and discharge (drainage) water in an outdoor forest 'sandbox' lysimeter over a five-year period. Homogeneous materials, a comparison lysimeter without vascular plants, and replicated, frequent sampling allowed us to discern the responses of spatial and temporal hydrochemical patterns to careful and complete removal of aboveground tree biomass. Over the monitoring period, we recognized three stages of ecosystem processes driving nutrient dynamics. Before the harvest i.e. after 15 years of tree growth, dissolved and soil-extractable K and Ca exhibited shallow concentration and depletion respectively, relative to nonvascular conditions. This contrast and the similarity of the uptake fluxes for these two cations over the growth period imply different modes of uptake. Potassium concentrations in discharge, typically 30-40 mM, were controlled by strong biocycling of this labile, growth-limiting element while Ca discharge concentrations, typically 2-3 times greater than both shallow-soil-water Ca and discharge K, were due to progressive CO2 weathering along downward flow paths through the sandbox. Nitrate levels during this first stage were below detection. During the first growing season after the harvest K concentrations in shallow soil and discharge waters increased and decreased with soil temperature, peaking at approximately three times typical pre-harvest levels. Calcium and NO3 concentrations did not definitely increase until the third stage, in the second growing season after harvest, when C-limited decomposition and nitrification in the shallow horizons of the sandbox triggered NO3 and Ca discharge concentration peaks approaching 400 and 250 mM respectively. Stable K:Ca concentration ratios of 0.4 to 0.5 in discharge during this hydrochemically dynamic stage were consistent with decomposition sources for both cations and a steady decomposer community, but these ratios could also be explained by the CO2 weathering which strongly imprinted bulk discharge hydrochemistry through the entire 5-year period. This stable weathering regime which confers well-buffered high-ionic-strength terrestrial hydrochemistry, and the persistence of N limitation for a long period following disturbance, may be characteristic of early phase primary-successional systems.