The use of fallout radionuclides to estimate soil redistribution in a logged coupe in the south east of NSW

Abstract

An investigation into the effects of logging on a native forest in the south east of NSW was carried out. The principle aim was to measure the five years of soil redistribution and soil loss that has occurred due to the harvesting process.

Fallout radionuclide technology was employed to quantify soil redistribution, in particular the anthropogenic radioisotop 137Cs and the lithogenic radioisotope 210Pbex. These tracers are distributed onto the earth's surface principally by wet precipitation, and move about the surface not in solution but in association with soil particles.

The landscape units that result from logging are identified as the snig tracks, log landings, buffer strip, cross banks and general harvest area. Reference values were determined for the two tracers from unlogged, undisturbed forests adjacent to the study area. The reference activity values were then conrpared to the activity of the logged landscape units to determine a sediment budget. The 137Cs sediment budget result was 111% (± 14), and the 210Pbex sediment budget was 78% (± 12).

Various options were investigated to account for the 20% difference in the two budgets. From two point estimates it was found that more of the 210Pbex total activity inventory (17% and 5%) resided in the surface organic layer than did 137Cs (1.3% and 0.1%). It was possible that the differences between the two sediment budget could be balanced by a loss of the surface organic layer (due to fire) and some associated soil loss.

The two point estimates total areal concentration for each tracer was compared to the total areal concentrations derived from 30 spatially representative reference cores. While 137Cs point estimate values were within uncertainty of the 30 cores, the 2l0Pbex point estimates were not. It was concluded that the two sediment budgets could not be balanced as the two point estimates for 210Pbex activity inventory were not a confident estimate. It was further concluded that the 210Pbex tracer is more variable in this environment and not enough samples have been taken to confidently characterise it.

Consequently, only the 137Cs sediment budget was used to quantify soil edistribution. Areas of soil loss were the log landing (- 2%) and snig tracks (-10%). Areas of soil gain were the general harvest area (18%), and the buffer strip (4%). The cross banks (2%) were considered part of the snig track activity.

Soil loss was quantified from the log landings and snig tracks using the 137Cs areal concentration reference curve. The mean areal concentration for the two landscape units was plotted against the depth dependent curve to yield a soil loss result of 23-49 mm for the snig tracks and 49-59 mm for the log landing. Show more