Tree Harvest in an Experimental Sand Ecosystem: Plant Effects on Nutrient Dynamics and Solute Generation

The hydrochemical signatures of forested ecosystems are known to be determined by a time-variant combination of physical-hydrologic, geochemical, and biologic processes. We studied subsurface potassium (K), calcium (Ca), and nitrate (NO3) in an experimental red -pine mesocosm to determine how trees affect the behavior of these nutrients in soil water, both during growth and after a harvest disturbance. Solution chemistry was monitored for 2 years at the end of a 15-year period of tree growth, and then for 3 more years after harvest and removal of aboveground biomass. Concentrations were characterized by three distinct temporal patterns that we ascribe to changes in solute generation mechanisms. Prior to harvest, K soil-water concentrations were relatively uniform with depth, whereas Ca soil-water concentrations doubled with depth. Nitrate concentrations were below detection in soil water and discharge (drainage) water. Plant uptake and water/nutrient cycling exerted strong control during this interval. During the 1st year after harvest, K concentrations tripled in shallow soil water, relative to preharvest levels, and showed a strong seasonal peak in discharge that mimicked soil temperature. Summer soil temperatures and annual water flux also increased. Decomposition of labile litter, with complete nitrogen (N) immobilization, characterized this interval. In the third interval (years 2 and 3 after harvest), decomposition shifted from N to carbon (C) limitation, and Ca and NO3 concentrations in discharge spiked to nearly 200 and 400 μM, respectively. Relatively stable ionic strength and carbonate chemistry in discharge, throughout the study period, indicate that carbonic-acid weathering was sustained by belowground decomposition long after the harvest. This stable chemical weathering regime, along with the persistence of N limitation for a long period after disturbance, may be characteristic of early-phase primary-successional systems.