Changes in Soil Quality Following Land Use Conversion of Native Mediterranean Forest to Avocado (Persea americana Mill. var. Hass) Orchards in Central Chile.

Land use changes from forest to agriculture affect soil physical and chemical properties through soil management and surface soil rotation and removal. The changes in vegetation coverage alter natural nutrients cycling, specially soil carbon stability, which could affect permanently soil quality and sustainability. During the last decade, in the Chilean Mediterranean region thousands of hectares of native shrubland have been converted to agriculture to establish avocado orchards planted in steep hills. The main objective of this study was to evaluate changes in soil quality, with emphasis in soil carbon, five years following conversion from native shrubland to avocado orchards in the Coastal range piedmont of central Chile (33°45’ south, 71°24’ west). A twin plot approach was used to compare the effect of land use change. Six pairs of twin plots were established during May 2011. The pairing criteria were defined based on location, aspect, slope and surface soil type. These factors are known to affect soil nutrients, and soil organic matter stability. In order to incorporate vegetation variability and uniformity, two strata were identified and sampled at each land use; beneath and outside the trees canopy in the native shrubland, and terrace (planting line) and furrow in the avocado plots. At the beginning of the study, surface soil samples were collected from each stratum to measure bulk density, soil texture, pH, CEC, soil carbon and nitrogen, aggregate stability and the size of the carbon pools associated to each particle size. Field soil respiration, temperature, and water content have been measured every two weeks across the twin plots using a non-dispersive infrared gas analyzer (EGM-4, PP-system). Preliminary results indicated no difference between the two areas sampled within the native shrubland. In the avocado plots, the terraces showed significantly greater organic matter content compared to the furrows. Differences in soil quality parameters were significantly higher in the native shrubland in relation to the avocado soils, indicating high levels of soil degradation five years after land conversion.