Soil organic carbon stocks under different páramo vegetation covers in Ecuador’s northern Andes

Abstract

<p>The Ecuadorian páramo, a neotropical ecosystem located in the upper Andes, acts as a constant source of high-quality water. It also stores significant amounts of C at the regional scale. In this region, volcanic ash soils sustain most of the paramo, and C storage results partly from their propensity to accumulate organic matter. Vegetation type is known to influence the balance between plant C inputs and soil C losses, ultimately affecting the soil organic C (SOC) content and stock. Tussock-forming grass (spp. Calamagrostis Intermedia; TU), cushion-like plants (spp. Azorella pedunculata; CU) and shrubs and trees (Polylepis stands) are commonly found in the páramo. Our understanding of SOC stocks and dynamics in the páramo remains limited, despite mounting concerns that human activities are increasingly affecting vegetation and potentially, the capacity of these ecosystems to store C.</p><p>Here, we compare the organic C content and stock in soils under tussock-forming grass (spp. Calamagrostis Intermedia; TU) and soils under cushion-like plants (spp. Azorella pedunculata; CU). The study took place at Jatunhuayco, a watershed on the western slopes of Antisana volcano in the northern Ecuadorian Andes. Two areas of similar size (~0.35 km<sup>2</sup>) were surveyed. Fourty soil samples were collected randomly in each area to depths varying from 10 to 30 cm (A horizon) and from 30 to 75 cm (2Ab horizon). The soils are Vitric Andosols and the 2Ab horizon corresponds to a soil buried by the tephra fall from the Quilotoa eruption about 800 yr. BP. Sixteen intact soil samples were collected in Kopecky's cylinders for bulk density (BD) determination of each horizon.</p><p>The average SOC content in the A horizon of the CU sites (9.4±0.5%) is significantly higher (Mann-Whitney U test, p<0.05) than that of the TU sites (8.0±0.4%), probably reflecting a larger input of root biomass from the cushion-forming plants. The 2Ab horizon contains less organic C (i.e. TU: 4.3±0.3% and CU: 4.0±0.4%) than the A horizon, but the SOC contents are undistinguishable between the two vegetation types. This suggests that the influence of vegetation type on SOC is limited to the A horizon. The average SOC stocks (in the first 30 cm from the soil) for TU and CU are 20.04±1.1 and 18.23±1.0 kg/m<sup>2</sup>,<sup></sup>respectively. These values are almost two times greater than the global average reported for Vitric Andosols (~8.2 kg/m<sup>2</sup> ), but are lower than the estimates obtained for some wetter Andean páramos (22.5±5 kg/m<sup>2</sup>, 270% higher rainfall) from Ecuador. Our stock values further indicate that vegetation type has a limited effect on C storage in the young volcanic ash soils found at Jatunhuyaco. Despite a higher SOC content, the CU soils store a stock of organic C similar to that estimated for the TU soils. This likely reflects the comparatively lower BD of the former soils (650±100 vs. 840±30 kg/m<sup>3</sup>). Additional studies are needed in order to establish the vegetation-related factors driving the SOC content and stability in the TU and CU soils.</p>

Vegetation effects on soil pore structure and hydraulic properties in volcanic ash soils of the high Andes

“…The mean hourly discharge for the period 2019-2020 was 1.76 l s À1 , and the maximum discharge was 7.57 l s À1 (Figure 2). Soils were classified as vitric Andosols following IUSS Working Group WRB (2014) (Calispa et al, 2021), with a sequence of buried soil horizons and tephra layers up to depths of 5-7 m. Soils are overlaying scoria-rich layers and glaciofluvial sediments that can reach up to 27 m in depth (Hall et al, 2017). The last significant ash falls in the region are from Quilotoa (800 years BP) and Guagua Pichincha (1100 years BP) volcanoes (Hall et al, 2017).…”

Delineation of water flow paths in a tropical Andean headwater catchment with deep soils and permeable bedrock

“…(c) Monitoring setup at JTU_01 including the tussock grass (TU, red circles) and cushion plant (CU, green circles) experimental hillslopes showing the location of the soil water samplers at the upper (UP), upper replica (UR), middle (MI), and lower (LO) parts of the hillslope, the wetlands (WE) at the bottom of each hillslope (blue circles), and streamflow station (white circle). Subplot (c) also shows an example of a representative profile of the soils monitored at the TU and CU experimental hillslopes (horizons A, 2A, 2BC) within the catchment (Calispa et al, 2021). Suction cups (SC) were used to collect soil water at the middle of the A, 2A, and 2BC horizons and wick samplers (WS) were used to collect soil water at the interface of the A‐2A and 2A‐2BC horizons as indicated by the open circles in the profile.…”

“…In nearby areas, buried soils have been observed at depths of up to 27 meters beneath scoria and glacio‐fluvial sediments (Hall et al, 2017). The soils across JTU_01 are young and slightly altered vitric Andosols (Calispa et al, 2021), which developed from ashfall deposits and pyroclastic material from Quilotoa and Cotopaxi volcanos that fell in the area around 800 to 1100 years ago (Hall et al, 2017; Onderet, 2018). Histosols are commonly found in flat zones where organic matter is accumulated (Onderet, 2018).…”

“…At each sampling position, the soils were described up to a depth of 135 cm following the FAO (2015) guidelines. Calispa et al (2021) determined five soil horizons, namely O, A, 2A, 2BC, 3BC whereof the isotopic composition of soil water in the A, 2A, 2BC, and 3BC horizons was monitored.…”

“…The SCs were 0.50 m in length and had a ceramic cup with a maximum pore size of 1 μm. During each sampling campaign, a negative pressure between 45 and 50 KPa was applied to the SCs to T A B L E 1 Characterization of the soil pits located within the tussock grass (TU) and cushion plant (CU) experimental hillslopes (Calispa et al, 2021). UP, UR, MI, and LO correspond to upper, upper replica, middle, and lower sampling positions along each of the experimental hillslopes, respectively 2 and Figure 1c).…”