Soil samples were taken from a chronosequence of OP with stand ages of 12, 18, 30, 32, 45, and 56 years, the three first blocks correspond to first cycle plantations and the last three to second cycle plantations. OP blocks were of sizes between 20 and 30 ha. In addition, three adjacent cattle ranching farms were sampled as reference sites. All selected sites were located within an area that covered roughly 5000 ha.

Soil samples from OP plots were collected using a modified transect methodology (60). This sampling strategy allows us to have mixed samples that represent well the spatial variability in OP plantations due to management practices. Furthermore, it is also well adapted for measures in replanted plantations, since the spatial distribution of management zones differs between OP cycles. Twenty sampling points evenly spaced were marked along a 50-m diagonal transect at ca. >60° (considering a selected palm at row 1 of the transect as reference point). The diagonal transects crossed six OP rows, and the four management zones were typically found in commercial OP plantations. To capture the spatial variability at each of the OP blocks, three parallel transects were made, except in the 12- and 32-year OP blocks, and sampled in 2016, with some complementary sampling in 2017. One transect was established in a centered position relative to the number of palm rows and average number of palms per row in each block. The other two transects were established at least 120 m away from each side of the first transect. Transects were established at least 50 m away from the plot’s edges. At the three Brachiaria pasture sites, one diagonal transect of 50 m long oriented east-west with 20 evenly spaced sampling points was made. At all diagonal transects (OP blocks and pasture sites), a total of 20 soil cores were taken. Half of those cores were taken to a depth of 30 cm and the other half went down to 50 cm in an alternative manner, using a 6-cm-diameter soil auger. Cores were divided into four depth intervals: 0 to 10 cm, 10 to 20 cm, 20 to 30 cm, and 30 to 50 cm. Therefore, samples of the 0 to 10 cm, 10 to 20 cm, and 20 to 30 cm were composed of 20 cores, and the 30- to 50-cm samples were made of 10 cores. Final samples in each transect were bulked into a composite sample, resulting in one sample for each depth in each transect. Samples were homogenized, air-dried, sieved through 2 mm, sealed in bags, and stored at room temperature until transportation to the laboratory in Lausanne (Switzerland). Soil physical, chemical, and isotopic analyses were carried out after drying at 35°C in forced-air ovens.

Pits were dug in a middle representative position along the first 50-m linear transects to determine soil BD in each OP block and one reference site to 70-cm depth. Two stainless steel volume cores were inserted horizontally into a pit wall at each depth increment. Soils from the two cores were mixed and oven-dried at 105°C for 48 hours to calculate BD.

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