Using 137Cs measurements to estimate soil erosion rates in forest stands affected by wildfires. Results from plot experiments.

The destruction of forest cover by wildfires has important consequences on the stability of forest ecosystems. It is well recognized that forests play a key role in regulating the hydrological cycle by modifying rainfall interception and evapotranspiration but also affecting hydrological and erosion responses of the soil surface. For these reasons, in areas devoted to forestry, wildfires can cause loss of biomass production and serious off-site problems related to overland flow and the amount of sediment delivered downstream. Quantifying the consequences caused by wildfires on the soil surface in terms of runoff and soil loss is a great challenge for scientists and foresters. In this paper, a plot experiment aimed at evaluating the effects on soil erosion by wildfires of different severity is firstly described. The experiment was carried out in 1999 in three small plots, located in Southern Italy, covered by pine trees planted in the early '60s. The results obtained from this experiment provided evidence of a short-time effect of wildfire on soil erosion. However, the normal conditions of the pre-fire period seemed to be re-established after a few months following fire. In order to explore possible long-term effects on soil loss, estimates of soil erosion provided by recent 137Cs measurements carried out in the same plots were made. The overall results indicated, for the study area, the absence of a long-term effect on soil erosion caused by the passage of fire and suggested that the 137Cs technique can be a useful tool to explore the effect of wildfires in forested ecosystems.

Exploring the potential for using 210Pbex measurements within a re-sampling approach to document recent changes in soil redistribution rates within a small catchment in southern Italy.

In recent years, the fallout radionuclides caesium-137 (137Cs) and unsupported lead-210 (210Pbex) have been successfully used to document rates of soil erosion in many areas of the world, as an alternative to conventional measurements. By virtue of their different half-lives, these two radionuclides are capable of providing information related to different time windows. 137Cs measurements are commonly used to generate information on mean annual erosion rates over the past ca. 50-60 years, whereas 210Pbex measurements are able to provide information relating to a longer period of up to ca. 100 years. However, the time-integrated nature of the estimates of soil redistribution provided by 137Cs and 210Pbex measurements can be seen as a limitation, particularly when viewed in the context of global change and interest in the response of soil redistribution rates to contemporary climate change and land use change. Re-sampling techniques used with these two fallout radionuclides potentially provide a basis for providing information on recent changes in soil redistribution rates. By virtue of the effectively continuous fallout input, of 210Pb, the response of the 210Pbex inventory of a soil profile to changing soil redistribution rates and thus its potential for use with the re-sampling approach differs from that of 137Cs. Its greater sensitivity to recent changes in soil redistribution rates suggests that 210Pbex may have advantages over 137Cs for use in the re-sampling approach. The potential for using 210Pbex measurements in re-sampling studies is explored further in this contribution. Attention focuses on a small (1.38 ha) forested catchment in southern Italy. The catchment was originally sampled for 210Pbex measurements in 2001 and equivalent samples were collected from points very close to the original sampling points again in 2013. This made it possible to compare the estimates of mean annual erosion related to two different time windows. This comparison suggests that mean annual rates of net soil loss had increased during the period between the two sampling campaigns and that this increase was associated with a shift to an increased sediment delivery ratio. This change was consistent with independent information on likely changes in the sediment response of the study catchment provided by the available records of annual sediment yield and changes in the annual rainfall documented for the local area.

Use of a (137)Cs re-sampling technique to investigate temporal changes in soil erosion and sediment mobilisation for a small forested catchment in southern Italy.

Soil erosion and both its on-site and off-site impacts are increasingly seen as a serious environmental problem across the world. The need for an improved evidence base on soil loss and soil redistribution rates has directed attention to the use of fallout radionuclides, and particularly (137)Cs, for documenting soil redistribution rates. This approach possesses important advantages over more traditional means of documenting soil erosion and soil redistribution. However, one key limitation of the approach is the time-averaged or lumped nature of the estimated erosion rates. In nearly all cases, these will relate to the period extending from the main period of bomb fallout to the time of sampling. Increasing concern for the impact of global change, particularly that related to changing land use and climate change, has frequently directed attention to the need to document changes in soil redistribution rates within this period. Re-sampling techniques, which should be distinguished from repeat-sampling techniques, have the potential to meet this requirement. As an example, the use of a re-sampling technique to derive estimates of the mean annual net soil loss from a small (1.38 ha) forested catchment in southern Italy is reported. The catchment was originally sampled in 1998 and samples were collected from points very close to the original sampling points again in 2013. This made it possible to compare the estimate of mean annual erosion for the period 1954-1998 with that for the period 1999-2013. The availability of measurements of sediment yield from the catchment for parts of the overall period made it possible to compare the results provided by the (137)Cs re-sampling study with the estimates of sediment yield for the same periods. In order to compare the estimates of soil loss and sediment yield for the two different periods, it was necessary to establish the uncertainty associated with the individual estimates. In the absence of a generally accepted procedure for such calculations, key factors influencing the uncertainty of the estimates were identified and a procedure developed. The results of the study demonstrated that there had been no significant change in mean annual soil loss in recent years and this was consistent with the information provided by the estimates of sediment yield from the catchment for the same periods. The study demonstrates the potential for using a re-sampling technique to document recent changes in soil redistribution rates.