Time-dependent changes in distribution patterns of soil bulk density and penetration resistance in a rangeland under overgrazing

High soil bulk density and penetration resistance as an indication of soil compaction affect soil infiltration and runoff. Overgrazed rangelands are under severe erosion risk because of soil compaction. The objective of this study was to determine changes in soil bulk density and penetration resistance in a high altitude rangeland within a 3-month grazing period. A 5-ha rangeland (200 x 250 m) was transected with intervals of 25 x 5 m, and soil penetration resistance was measured at 459 points in the 20-cm surface soil layer on 3 different dates (15 July, 15 August, and 15 September). At each intersection of the 50 x 50 m interval grid system, 30 undisturbed soil samples were taken to determine soil bulk density. Semivariogram and punctual kriging analyses were used to define spatial variability in bulk density and penetration resistance measurements. The results indicated that the mean bulk density did not show a clear trend with grazing intervals and produced values of 0.88, 0.93, and 0.82 g cm(-3) for July, August, and September, respectively. Soil penetration resistance was higher than the critical penetration resistance (3 MPa) for root growth at all measurement points. The mean soil penetration resistance increased about 8.5% in August as compared with July, and it was more or less constant in the following month.