Page 45 - Soil Degradation, Conservation and Remediation

30 1 Soil Resources and Soil Degradation

Fig. 1.5 The soil has badly suffered from overgrazing (Photo courtesy of Sky Jacobs of Wild
Sonora)

Abril and Bucher ( 1999 ) measured the changes in soil characteristics, nutrient
availability, and microbial activity on sites utilized by different grazing intensities
in Argentina. Three sites were selected for comparison: a highly restored (no graz-
ing for 20 years), a moderately restored (8 years of restoration), and a highly
degraded (extremely overgrazed). The following parameters decreased as the graz-
ing intensity increased: the soil moisture (4.5–2.25 %), the organic matter (4.68–
1.45 %), and the nitrogen content (0.28–0.14 %). Microbial activity ranged from
0.89 at the restored sites to 0.22 mg CO 2 /g/week at the highly degraded site. The
seasonal variations in the density and the activity of microorganisms increased
from the highly restored to the highly degraded site, probably as a response to an
increased lack of humidity. The cellulolytic and nitrifier groups were the most

affected, whereas the ammonifier and free-living N-fixing organisms decreased in

the highly degraded site only. N fixation was more intense at the moderately
restored site followed by the highly degraded site. The observed values are inter-
preted as resulting from the interaction between organic matter availability (as
energy source) and N deficiency. The results suggest a strong influence of over-

grazing on the soil fertility, as well as on the soil ability to buffer water stress dur-
ing the dry season. According to Ling Hao et al. ( 1997 ), an average of 12.4 % of
the total carbon initially stored in soils (0–20 cm soil layer) has been lost due to
overgrazing over the 40-year period.