Effects of throughfall manipulation on soil nutrient status: results of 12 years of sustained wet and dry treatments
Johnson, D. 2008. Global Change Biology 14: 1661-1675
Abstract
To investigate the potential effects of changing precipitation on forest ecosystems, the
Throughfall Displacement Experiment (TDE) was established on Walker Branch
Watershed, Tennessee, in 1993. Three different throughfall amounts were tested: -33%
(DRY); ambient (no change, AMB); and +33% (WET). Throughfall manipulations had no
statistically significant effects on total C, N, exchangeable Ca2+, Mg2+, bicarbonateextractable
P, or extractable SO42- treatments. Increased
K inputs in the WET treatment resulted in relative increases in exchangeable K1
compared with the AMB and DRY treatments. Soil C, N, and extractable P declined in all
treatments over the 12-year study, and the declines in N were inexplicably large. Field
observations contrasted with earlier simulations from the Nutrient Cycling Model
(NuCM), which predicted greater decreases in exchangeable K+, Ca2+, Mg2+, and
extractable P in the order WET>AMB>DRY, and no change in C, N, and extractable
SO42-. The failure of the NuCM model to accurately predict observed changes is
attributed to the lack of mechanisms for deep rooting and the transfer of throughfall
K1 from one plot to another in the model. Measurements of element availability using
resin membranes during the final years showed higher values in wet and lower values in
dry treatments compared with ambient conditions for mineral N, K, Mn, Zn, and Al, but
the opposite for B, Ca, and Mg. In the cases of Ca and Mg, the patterns in resin values
were similar to those at the soil exchange sites (greatest in the dry treatment) and
appeared to reflect pretreatment differences. This study showed that while longer term
changes in soil nutrients are likely to occur with changes in precipitation, potential
changes over this 12-year interval were buffered by ecosystem processes such as deep
rooting.
Key Words
calcium, carbon, climate, magnesium, nitrogen, phosphorus, potassium, resin membranes,soil, throughfall
