2.3. Sampling Strategy
Soil sampling took place from April to June 2014. Sampling locations were randomly selected in sites implementing conventional cultivation, while sites having SWC structures were sampled in a stratified random manner that covered three contrasting ground features, namely trench, mound and undisturbed ground (Fig. 1; Saiz and Albrecht, 2015). Undis- turbed ground is defined as that not directly affected by either the addi- tion or removal of soil resultant from the building and maintenance of trenches and mounds. The area occupied by each stratum was mea- sured at each site. Soil sampling was carried out across four replicate sampling points at each of the strata described above, which resulted in a total of 56 locations being sampled per stratum (4 replicates x 14 sites having SWC structures). In the case of stands hosting semi- natural vegetation sampling locations were chosen following a stratified sampling strategy that has proved well suited to best account for the in- herent heterogeneity of SOC that is characteristic of these ecosystems (Bird et al., 2004; Wynn et al., 2006; Saiz et al., 2012). This approach consists of taking samples near trees (‘Tree samples’ at half canopy radi- us from trunks) and away from trees (‘Grass samples’ at half the maxi- mum distance between trees).
At each sampling location surface litter was removed when present. Two samples were taken at 0–0.05 m with the aid of a stainless steel corer (40 mm inner diameter), before being bulked together to smooth out the typically large local heterogeneity existing at very shallow depths. Subsequently, deep soil sampling was carried out using the same corer at 0.05–0.30 m, 0.30–0.50 m and up to 0.85 m depth (impen- etrable layers permitting), with each sample being individually collect- ed. The total number of soil samples taken was 989.