Neoformation of soil aggregates after a volcano eruption in meadows of Northern Patagonia, Argentina

ENRIQUEZ A.S.; CREMONA M.V.; PERI P.L.; SIX J.

Río de Janeiro

Congreso; 21st World Congress of Soil Science; 2018

Brazilian Soil Science Society

In June 2011, a natural event occurred in Patagonia, Argentina: The ?Puyehue-Cordón Caulle? volcanic complex erupted, and particles from tephras (1-5cm) to very fine volcanic ash (44-500?m) where deposited in the direction of prevailing winds (West to East). The ash was biologically inert, with no organic compounds, neutral pH and low electrical conductivity. The objective was to study neoformation processes of soil aggregates in wet meadows after volcano eruption. For this, five years after the volcano event, we collected 6 soil samples to a depth of 0-5cm (the depth under influence of ash deposition in Northern Patagonia) in 3 wet meadows (acuic mollisols) under different grassland conditions (good and poor) due to different long-term grazing pressures. In the laboratory, soil samples were separated into different aggregate-size classes by wet sieving: large macroaggregates (LM: > 2000µm); small macroagregates (SM: 250?2000µm); microagregates (micro: 53?250µm); silt+clay (s+c: < 53µm). The macroaggregates-M were separated into coarse POM (c-POM: > 250µm), microaggregates in M (microM: 53-250µm) and silt + clay in M (s+cM: < 53µm). Total carbon-C and nitrogen-N were determined in each fraction with a LECO. Five years after the eruption, the C content in the first 5cm soil layer was 52% lower than before. Ash texture was 59% silt, 3% clay, and 34% sand. The proportion of classes derived from the whole dry soil basis from good meadows accounted for 25%-LM+SM, 45%-micro, and 29%-s+c; classes derived frommacroaggregates accounted for 31%-c-POM, 32%-microM, 37-s+cM. No differences between grassland conditions were found in the aggregate proportion; overall, C and N content of the aggregates was significantly higher (ANOVA, p< 0.05) in good conditions than in poor ones. We believe that a sequence of neoformation ofaggregates is occurring. Aggregates different from micro and s+c (dominant ash granulometry) will develop after different processes, where time, organic matter and biological activity interact. Macroaggregation is low, but with ahigh C and N content; small macroaggregates predominate that fraction. In contrast, micros and s+c particles are dominant but with small C and N content. On the other hand, grassland conditions modified the quality of theaggregates, so management influence soil formation. To confirm these patterns, similar meadow soils without volcanic ash deposition should be studied, or continue monitoring this processes over time.