Conversion of forest into irrigated pasture I. Changes in the chemical and biological properties of the soil

- The conversion of forest to pasture increases soil organic matter at the surface.
- Losses of carbon and nutrients happened at deeper layers on sandy and irrigated soils.
- Labile organic matter and microbial activity were sensitive in the land conversion

The conversion of forest into pasture affects several properties and functions of the soil including the cycling carbon and other nutrients. Changes in selected chemical and biological properties of the soil were studied in an area of intensive irrigated pasture (PAST) and compared to soil under native vegetation (NV). The selected soil properties evaluated were: total organic carbon (TOC), total nitrogen (TN), levels of N-NH4+ and N-NO3−, humic substances (fulvic and humic acids and humin), physical fractionation of the soil organic matter, water-soluble carbon (WSC), C and N microbial biomass (MB-C and MB-N), C-CO2 evolution, and the presence of spores of arbuscular mycorrhizal fungi. The conversion of forest into pasture, and the adopted management (rotational grazing, irrigation and fertilisation) led to significant increases in soil organic matter at the surface, when compared to natural conditions. Some losses however were also seen at deeper layers, as water-soluble carbon, indicating that irrigation of soils with a sandy texture should be better planned to avoid loss of organic matter and nutrients, or contamination of the groundwater. The greater metabolic quotient in PAST than in NV is an indication of the large loss of carbon (C), which may explain the C-CO2 emissions and may demonstrate disturbances in an area of intense productivity with elevated levels of C. Labile organic matter, such as water-soluble or related to microbial activity, were the most sensitive to the effects of land use, reflecting behaviour that can compromise the use of land as pasture under the conditions of this study.