Organic carbon within soil aggregates under forestry systems and pasture in a southeast region of Brazil

Abstract

The location of organic matter within the aggregate is a key factor for stabilizing and storing soil organic carbon (SOC). In this context, the purpose of this study was to answer the following question: How much is the contribution of aggregate associated-C in total SOC accumulation up to 1 m under different land uses in the Atlantic rainforest zone of Brazil? The experimental areas were: a secondary-forest; 3 and 5-year-old eucalyptus plantation; 35-year-old rubber tree plantation; and 50-year-old unfertilized pasture. Soil samples were collected at six different depths (0–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm), air dried and separated by wet-sieving into three fraction-size classes (2000–250 μm; 250–53 μm and <53 μm) – corresponding to macroaggregate, microaggregate and silt-and-clay size fraction, respectively. The ultrasonic method was used to quantify the C physically-protected in macro and microaggregates. Macroaggregates accumulated higher amounts of C (CMa) than microaggregates (CMi). The amount of CMa stock in all land-use systems ranged from 33 to 8 Mg ha−1, and the CMi ranged from 11 to 7 Mg ha−1. This represents around 22 to 4% and 7 to 4%, respectively, of physically protected C in the SOC storage up to 100 cm. The amount of CMa + CMi up to 1 m was 43 Mg ha−1 and 36 Mg ha−1 under 3- and 5-year-old eucalyptus plantations; 30 Mg ha−1 in the in secondary forest; and 19 Mg ha−1 in the rubber plantation and pasture. The C stabilization within aggregates represents a significant mechanism of C protection in these soils.