Quantifying geostatistical properties of 137Cs and 210Pbex at small scales for improving sampling design and soil erosion estimation

Authors: Zhang, Xunchang; Polyakov, Viktor; LIU, BAOYUAN - Northwest Agricultural & Forestry University; Nearing, Mark

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2018
Publication Date: 8/8/2018
Citation: Zhang, X.J., Polyakov, V.O., Liu, B., Nearing, M.A. 2018. Quantifying geostatistical properties of 137Cs and 210Pbex at small scales for improving sampling design and soil erosion estimation. Geoderma. 334:155-164. https://doi.org/10.1016/j.geoderma.2018.08.002.
DOI: https://doi.org/10.1016/j.geoderma.2018.08.002

Interpretive Summary: Knowledge of spatial structures of the radionuclides 137Cs and unsupported 210Pb (210Pbex) is vital for developing sound sampling designs that are crucial for deriving quantitative soil erosion estimates. The objectives are to characterize spatial structures of 137Cs and 210Pbex inventories at small spatial scales under different land uses, and to quantify the effects of corer sizes on (1) estimated means and variances of the 137Cs and 210Pbex inventories and (2) sample numbers required for reliable estimation of the inventories at a given confidence level. Three different corer sizes were used to take soil samples along three 10-m transects at 0.25-m or 0.5-m intervals for each land use. Land uses included crop, grass, forest, and range. 330 samples were analyzed for 137Cs and 210Pbex inventories. Semivariograms were obtained by plotting empirical semivariances with separation distances. The semivariances at the separation distances of >0.25 m were very close to the variances of the fields for all four land uses, indicating that the spatial distributions of 137Cs and 210Pbex were nearly stationary and had little spatial dependence at scales between 0.25 and 5 m. The results suggested that samples taken at a separation distance of > 0.5 m would be largely independent and could be composited to form a representative sample for the sampling location. Given the large spatial variability at such a small scale, quantitative soil erosion rates cannot be estimated from a single soil core, because remarkably different soil erosion rates can be estimated from soil cores taken 0.5 m apart. Core size variation between 38 mm and 86 mm has no effect on estimating means and standard deviations of 137Cs and 210Pbex inventories, except for gravely soils. In general, 15-30 samples are needed to estimate 137Cs reference inventory on reference sites, but may be more for gravely soils. More samples are required for forest and cultivated sites than for uncultivated grassland sites. On measuring sites, it is strongly recommended that 5-15 samples be combined to form a composite sample for a reliable estimate of 137Cs inventory for a location or land form unit for most soils and land uses. This work will be useful to soil conservationists and erosion scientists for improving sampling designs and the accuracy of soil erosion estimation.

Technical Abstract: Knowledge of spatial structures of the radionuclides 137Cs and unsupported 210Pb (210Pbex) is vital for developing sound sampling designs that are crucial for deriving quantitative soil erosion estimates. The objectives are to characterize spatial structures of 137Cs and 210Pbex inventories at small spatial scales under different land uses, and to quantify the effects of core sizes on (1) estimated means and variances of the 137Cs and 210Pbex inventories and (2) sample numbers required for reliable estimation of the inventories at a given confidence level. Three different core sizes were used to take soil samples along three 10-m transects at 0.25-m or 0.5-m intervals for each land use. Land uses included crop, grass, forest, and range. 330 samples were analyzed for 137Cs and 210Pbex inventories. Semivariograms were obtained by plotting empirical semivariances with separation distances. The semivariances at the separation distances of >0.25 m were very close to the variances of the fields for all four land uses, indicating that the spatial distributions of 137Cs and 210Pbex were nearly stationary and had little spatial dependence at scales between 0.25 and 5 m. The results suggested that samples taken at a separation distance of > 0.5 m would be largely independent and could be composited to form a representative sample for the sampling location. Given the large spatial variability at such a small scale, quantitative soil erosion rates cannot be estimated from a single soil core, because remarkably different soil erosion rates can be estimated from soil cores taken 0.5 m apart. Core size variation between 38 mm and 86 mm has no effect on estimating means and standard deviations of 137Cs and 210Pbex inventories, except for gravely soils. In general, 15-30 samples are needed to estimate 137Cs reference inventory on reference sites, but may be more for gravely soils. More samples are required for forest and cultivated sites than for uncultivated grassland sites. On measuring sites, it is strongly recommended that 5-15 samples be combined to form a composite sample for a reliable estimate of 137Cs inventory for a location or land form unit for most soils and land uses. This work will be useful for improving sampling designs and consequently the accuracy of soil erosion estimation.