Soil carbon sequestration potential of legume-based improvements of native grasslands: results from a national scale field survey in Uruguay

Volume 1, 2025 - 324282

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Abstract

Soil organic carbon (SOC) content is the primary indicator of soil health and productive potential, determining multiple soil physical and chemical properties. As the largest terrestrial carbon reservoir, SOC also plays a key role regulating processes that influence the planetary climate. Therefore, its conservation represents a fundamental pillar for the development of sustainable agricultural systems. The inclusion of legumes and fertilization in grasslands has shown positive results in the SOC balance in different parts of the world. However, as SOC stocks are affected by many factors and management practices, contradictory results have also been observed. One of the main challenges for estimating global SOC sequestration potential is the lack of local information on the achievable SOC stock change rates for different management practices and land uses. Following a paired sampling strategy, a nationwide field survey was conducted in Uruguay to assess the response of SOC stock to the inclusion of exotic legumes and phosphorous fertilization on native grassland, an extended management practice aimed at increasing productivity. Stocks of SOC and nitrogen (N) were determined in 30 native grasslands oversown with legumes and fertilized with phosphorus, and in paired control native grasslands. Sampling sites were located on commercial farms distributed throughout the country. Case studies were selected based on a previous study assessing biological nitrogen fixation and informing on consistent long-term land use and management (>20 years). Stratified soil sampling was conducted at each site, with composite samples taken for each treatment at 0-10 cm, 10-20 cm, 20-30 cm, and 30-60 cm depths. Bulk density, total carbon and nitrogen content (dry combustion), pH, and inorganic carbon were determined. The stocks of SOC and N were calculated considering a fixed depth, since no differences in soil bulk density were observed. There was an ample variation in SOC and N stocks, ranging from 27.6 to 146.9 Mg C.ha^-1, and from 3.6 to 11.7 Mg N.ha^-1 at 0-30 cm; and from 46.3 to 255.5 Mg C.ha^-1, and from 5.3 to 19.6 Mg N.ha^-1 at 0-60 cm. However, SOC and N stocks did not respond to the introduction of legumes and phosphorous fertilization in these native grasslands. Hence, opportunities for SOC sequestration in native grasslands of Uruguay by introducing legumes and phosphorus fertilization seem scant. These results highlight nominally managed native grasslands as a best-case scenario, in addition to their value as SOC reference stocks in this Pampa’s Biome region.

Institutions

¹ Instituto Nacional de Investigación Agropecuaria - INIA

Topic

Sustainable agricultural and forestry production for carbon sequestration, soil health, and food security

Keywords

soil carbon stock

grassland

management