Great Basin Naturalist


In a piñon-juniper woodland in northwestern Arizona, connected basal cladodes of a prickly pear cactus (Opuntia littoralis var. martiniana) form check dams that cause deposition of N-rich detritus interspaces otherwise lacking litter. Seventy-eight percent of connected basal cladodes measured in transects grew at an angle (with respect to the slope contour) ≤ 45°—an orientation facilitating deposition of flood-borne debris.

Soil total N was significantly greater (P < 0.01) and organic C was greater, but not significantly, above cactus dams compared to below cactus dams. Soil total N and organic C both above and below cactus dams were significantly greater (P = 0.0001) compared to adjacent interspaces. Soil total N and organic C above cactus dams were equal to areas beneath canopies (tree and shrub combined). Net NO3 (0–5 cm depth) above cactus dams was significantly greater (P = 0.0001) than below cactus dams, at interspaces, and beneath canopies. Net NH4+ (0–5 cm soil depth) above cactus dams was significantly greater (P < 0.01) than below cactus dams and interspaces, and was greater (but not significantly) than beneath canopies. At 5–10 cm soil depth, differences in net NH4+ and net NO3 between sampling locations were not significant except for the difference in net NO3 above and below cactus dams (P < 0.05). The litter layer above cactus dams had twice as much total N (P < 0.01) as the litter layer beneath canopies (tree and shrub combined); differences in net mineralized N were not significant between litter layers. Over the course of a single rainy season, detritus depth behind cactus dams increased up to 23 cm, with a mean increase of 4.3 cm (s = 0.625, P = 0.0001).