Passage VII

Three studies examined how the depth of plant roots, and the soils carbon (C) and nitrogen (N) contents, change once shrubs have replaced grasses as the dominant plant species in an area.

In one area, Sites (Sites X, Y, and Z) were selected. Each site consisted of 2 adjacent plots of land: one covered only with grasses (grass plot) and one that had once been covered only with grasses but was now covered only with shrubs (shrub plot). The average annual precipitation for each site is shown in Table 1.

Study 1

Five vertical soil cores were taken from each of the 2 plots at Sites X, Y, and Z. Each core was 0.06 m in diameter and 3.5 m deep, which was a few centimeters deeper than the deepest plant roots. Starting at the top, each core was cut horizontally into 10 cm sections. Each section was dried, crushed, and passed through screens so that all root material could be collected and weighed. The average 95% root mass depth (see Figure 1) for each plot was determined (see Table 1).

Study 2

For each of 5 additional soil cores taken from each plot at Site X, soil samples were taken from the top of the core, and then every 0.25 m down the core to a depth of 2.0 m. The 5 samples from a given depth and plot were thoroughly mixed. Each resulting mixture was then analyzed for C content, in percent by mass. These procedures were repeated for 5 additional cores from each plot at Site Y and for 5 additional cores from each plot at Site Z (see Figure 2).

Study 3

Study 2 was repeated except that each mixture was analyzed N content (see Figure 3).