Are grassland soils of the pampas sequestering carbon?

A belief that grassland soils act as carbon sinks and mitigate greenhouse gas emissions generated by livestock production is common among some farming organizations. A recent theoretical analysis proposed that grazing lands in South America sequester carbon as soil organic matter but this result is controversial. Here, we search for empirical evidence of changes in the carbon stock in the grassland soils of the Pampean region of Argentina. For this purpose, 22 natural grassland sites were selected, distributed across the region. The sites were first sampled in 2007 as part of a regional survey of soil carbon stocks; they were re-sampled in 2019. The sites represent a wide range of soil and climate conditions. Samples were taken to a depth of 50 cm. Bulk density, and inorganic and organic carbon, were determined, and the carbon stocks were estimated on an equivalent mass basis. The results were analyzed using ANOVA. No statistically significant differences were found in either inorganic or organic carbon stocks between the sampling times. Across all the sites, the average organic carbon stock was 59.0 t ha-1 in 2007 and 56.2 t ha-1 in 2019. These results suggest that the Pampas grasslands are not sequestering carbon.

Soil carbon and belowground carbon balance of a short-rotation coppice: assessments from three different approaches.

Uncertainty in soil carbon (C) fluxes across different land-use transitions is an issue that needs to be addressed for the further deployment of perennial bioenergy crops. A large-scale short-rotation coppice (SRC) site with poplar (Populus) and willow (Salix) was established to examine the land-use transitions of arable and pasture to bioenergy. Soil C pools, output fluxes of soil CO 2, CH 4, dissolved organic carbon (DOC) and volatile organic compounds, as well as input fluxes from litter fall and from roots, were measured over a 4-year period, along with environmental parameters. Three approaches were used to estimate changes in the soil C. The largest C pool in the soil was the soil organic carbon (SOC) pool and increased after four years of SRC from 10.9 to 13.9 kg C m-2. The belowground woody biomass (coarse roots) represented the second largest C pool, followed by the fine roots (Fr). The annual leaf fall represented the largest C input to the soil, followed by weeds and Fr. After the first harvest, we observed a very large C input into the soil from high Fr mortality. The weed inputs decreased as trees grew older and bigger. Soil respiration averaged 568.9 g C m-2 yr-1. Leaching of DOC increased over the three years from 7.9 to 14.5 g C m-2. The pool-based approach indicated an increase of 3360 g C m-2 in the SOC pool over the 4-year period, which was high when compared with the -27 g C m-2 estimated by the flux-based approach and the -956 g C m-2 of the combined eddy-covariance + biometric approach. High uncertainties were associated to the pool-based approach. Our results suggest using the C flux approach for the assessment of the short-/medium-term SOC balance at our site, while SOC pool changes can only be used for long-term C balance assessments.

Fine root biomass and turnover of two fast-growing poplar genotypes in a short-rotation coppice culture.

BACKGROUND AND AIMS: The quantification of root dynamics remains a major challenge in ecological research because root sampling is laborious and prone to error due to unavoidable disturbance of the delicate soil-root interface. The objective of the present study was to quantify the distribution of the biomass and turnover of roots of poplars (Populus) and associated understory vegetation during the second growing season of a high-density short rotation coppice culture. METHODS: Roots were manually picked from soil samples collected with a soil core from narrow (75 cm apart) and wide rows (150 cm apart) of the double-row planting system from two genetically contrasting poplar genotypes. Several methods of estimating root production and turnover were compared. RESULTS: Poplar fine root biomass was higher in the narrow rows than in the wide rows. In spite of genetic differences in above-ground biomass, annual fine root productivity was similar for both genotypes (ca. 44 g DM m-2 year-1). Weed root biomass was equally distributed over the ground surface, and root productivity was more than two times higher compared to poplar fine roots (ca. 109 g DM m-2 year-1). CONCLUSIONS: Early in SRC plantation development, weeds result in significant root competition to the crop tree poplars, but may confer certain ecosystem services such as carbon input to soil and retention of available soil N until the trees fully occupy the site.