Soil use effect on organic and inorganic carbon sequestration in the Argentine Pampas

BERHONGARAY, GONZALO; ALVAREZ, ROBERTO

Colorado Springs

Simposio; International Symposium on Soil Organic Matter Dynamics: Land Use, Management and Global Change; 2009

Colorado State University

The evaluation of the carbon content of the soils and the changes produced by different land uses is important because of the possibility for sequestering carbon in soils and mitigate climate change. Our objective was to establish the impact of soil use on organic and inorganic carbon stocks in soils of the Argentinean Pampas. At eighty-two locations widespread along the Pampas, presenting a very wide range of soil and climatic conditions, five soil type uses were selected in each location: planted forest, undisturbed or very low disturbed grassland sites never cultivated on well drained soil, cropped field, seeded pasture on sites subjected to rotation with agriculture, and flooding site under natural grassland. Soils were sample to 1 m depth in layers of 25 cm with a special device that allowed sample extraction and soil bulk density determination. Soil samples were air-dried, ground through a 2 mm mesh and total carbon concentration determined by wet digestion. Carbonate was determined by acid hydrolysis and organic carbon estimated by difference. Soil carbon content data were analyzed by ANVA and LSD. An average content of 97 t ha-1 for organic carbon and 48 t ha-1 for inorganic carbon were estimated. Significant differences were detected between different soil uses only in organic carbon (Table 1).   Table 1. Carbon contents of pampean soils to 1 m depth as means of 82 locations in relation to soil use. Different letters in a column represent significant differences between soil uses.  Soil use          Organic C     Inorganic C                            (t ha-1) -------------------------------------------   Planted forest       134 a          42 a Undisturbed          104 b          47 a Seeded pasture        90 c          48 a Cropped field         87 c          52 a Flooding site         70 d          50 a -------------------------------------------   Organic carbon in soils under planted forest was ca. 30 % greater than under undisturbed control sites indicating intense carbon sequestration in soils under forest. Soils subjected to agriculture lost ca. 15 % of their organic carbon content comparing with control sites, which may be attributed to erosion and negative carbon balances. Flooding sites had lower organic carbon content than controls possible due to lower net primary productivity under flooding conditions. The ratio cultivated surface/planted forest surface is ca. 100 for the whole region. Consequently, a net flux of 400 Mt carbon from soils to the atmosphere was estimated due to agriculture. This preliminary estimation doubles previous figures based on surface soil samplings.