Anthropogenic water drawdown and carbon accumulation mediates by soil aggregates in ecotones (Morichales) of the Orinoco lowland

Background/Question/Methods

Morichal in the Orinoco basin are one of the most carbon-accumulating ecosystems. These groundwater dependent ecosystems are markedly affected by human water extraction from the phreatic level. The water drawdown probably decreases the capability of Morichal to accumulate carbon. Soil aggregate is a known path of stabilization for soil carbon. Therefore, soil aggregation could be used as an indicator of the potential for carbon accumulation. The key property of carbon sequestration by soil aggregates was evaluated. Three study-sites were selected along a lowland Morichal ecotone Sunsunes, on basis of the depletion of a groundwater-fed stream. Site one features an interrupted stream (IRS). Site two has an intermittent stream (IMS), and site three a permanent stream (PMS). At each study-site soil samples were taken at the channel, the flood-prone area and adjacent well drained savanna. Carbon storage derived from soil aggregation was measured by fractioning soil in 4 pools. Air-dry soil was sieved to generate aggregate fractions (<0.053 mm, 0.053-0.25 mm, 0.25–2 mm and >2 mm).

Results/Conclusions

All sites were similar texture (loamy sand), with clay+silt between 16 -29%. At the Morichal, soil carbon (4-11 %) was higher than that in well drained savanna (3%). Difference between channel bed and flood-prone area was not significant. The >2.0 mm size fraction of well drained savanna soil was absent and only rock dominated up to 10%. At the flood-prone area and channel bed, 25-35 % of soil was formed by aggregates of >2.0 mm in size and rich in carbon content (15%C). In the three sites the most abundant size fraction was 0.25-2.0 mm, forming between 45-55% of total soil. Although, the well drained savanna soil was dominated by sand particulates and not by soil aggregates. In the flood-prone area and the channel bed, significant carbon (ca. 50%C) was storage in aggregates of size fraction >0.25 mm, as compared with that in the well drained savanna soil. Size distribution of aggregates was different in permanent stream as result of the effect of high water level, leading to carbon accumulation in soil aggregates in the fractions >2.0 mm and 0.25-2.0 mm.