Aggregate stability and related properties in NW Patagonian Andisols

Resilience is an ecological concept explaining responses to disturbance or stress. For the soil system, it is defined as the ability of the soil to recover its condition of origin. Not much is known about the factors and processes that determine soil resilience and this research project was conducted to study soil resilience through analysis of changes in aggregate stability of Andisols supporting Pinus ponderosa Dougl. that replaced former Nothofagus sp. forest. Five sites with three treatments each were selected. Treatments included native forest, thinned exotic plantation forest and non-thinned exotic plantation forest. Bulk density and aggregate stability of the first 5 cm of the A-horizon were determined in undisturbed samples at field moisture, and the aggregate size distribution was determined involving the fractions of 8–2; 2–0.25; 0.25–0.053 and < 0.053 mm (diameter). The results indicated that the percentage of aggregates between 2 and 0.25 mm was significantly higher under Nothofagus (39.7%) than under pine without thinning (28.8%), with soil under thinned pine (32.0%) in between. The decrease in macroaggregates under pine without thinning was accompanied by a decrease in organic carbon content (from 70.2 g kg- 1 to 42.4 g kg- 1). Other results of importance were i) the high water stable aggregates under the different types of vegetation (> 80% of > 0.25 mm aggregates), indicating the relatively high stability of such Andisols; ii) highest organic matter contents found in the smallest sized aggregates, and iii) formation of stable soil aggregates to be significantly correlated with organic matter, Al activity, base content and soil reaction.