ORGANIC CARBON AND NITROGEN CONCENTRATIONS AS INDICATORS OF SOIL QUALITY IN CATTLE FARMS IN THE BELLBIRD BIOLOGICAL CORRIDOR, COSTA RICA

The deforestation of the tropics is an important issue facing the world. Deforestation leads to increased soil erosion and loss of soil organic matter (SOM), which is an important influence on nutrient availability, soil structure, and water retention. In Costa Rica, deforestation began on a large scale in the 1940s, and in 1987, only 21% of the original forest remained. The Bellbird Biological Corridor, Costa Rica, was deforested by the 1950s and converted to growing coffee, corn, and beans with limited to no tilling. Cropland was then transformed into cattle pasture in the 1980-90s. The farms we sampled are currently low intensity, small hold cattle farms that use rotational grazing. We collected 31 soil profiles from four small cattle farms in Monteverde (Andisols) and San Luis (Ultisols). We measured soil organic carbon (SOC) and total soil nitrogen (TSN) concentrations as an indicator of soil quality. The SOC and TSN concentrations in each of the profiles decreased with depth. Median SOC concentrations were 65 mgC/g at the surface and decreased to 25 mgC/g at 18 cm, and median TSN concentrations were 7 mgN/g at the surface and decreased to 3 mgN/g at 18 cm. Both SOC and TSN concentrations decreased rapidly in top 8 cm of the profiles before stabilizing. The differences in concentrations of SOC and TSN between Ultisols and Andisols were not statistically significant, although the Ultisols showed less variability than the Andisols. TSN showed a strong correlation with SOC (r²=0.96) in these soils. Therefore, we interpreted total soil nitrogen to be equivalent to soil organic nitrogen. The rapid decrease in SOC and TSN in the upper 8 cm was consistent with the rooting depth of African Stargrass, which is the dominant fodder crop. The C:N ratio of the soils was 11.2:1, indicating microbial processing of organic matter derived from roots. Assuming that deforestation and conversion to cropland on steep slopes would be degrade soils, the results suggest that 30-40 years of rotational grazing was long enough for soils to recover from the degradation.