Soil Science Society of America

Decadal changes in Potassium, Calcium, and Magnesium in a deciduous forest soil

Decadal changes in soil exchangeable K+, Ca2+, and Mg2+ concentrations and contents from 1972 to 2004 in eight intensively monitored plots on Walker Branch Watershed were compared with estimates of increments or decrements in vegetation and detritus. The results from these eight plots compared favorably with those from a more extensive set from 24 soil sampling plots sampled in 1972 and 2004. Increases in exchangeable K+ were noted between 1972 and 1982, but few changes were noted between 1982 and 2004 despite significant increments in vegetation and detritus and significant potential losses by leaching. Total K contents of soils in the 0- to 60-cm sampling depth were very large and a slight amount of weathering could have replenished the K+ lost from exchanges sites. With one notable exception, exchangeable Ca2+ and Mg2+ concentrations and contents decreased continuously during the sampling period. Decreases in exchangeable Ca2+ could be attributed mostly to increments in biomass and detritus, whereas decreases in exchangeable Mg2+ could not and were attributed to leaching. The major exception to these patterns was in the case of exchangeable Ca2+, where significant increases were noted in one plot and attributed to Ca release from the decomposition of Ca-rich coarse woody debris from oak (Quercus spp.) mortality. With minor exceptions, soils and changes in soils among the eight intensively sampled core plots were similar to those in a more extensive set of plots distributed across the watershed. This study shows that averaging among plots can mask significant and important spatial patterns in soil change that must be taken into account in assessing long-term trends.

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