Nitrogen fertilization and genotype jointly drive bermudagrass (Cynodon spp.) productivity but are not associated with differences in SOC

Pastureland contributes a large share of the global soil C stock, much of which derives from root systems. Management practices like fertilization and the introduction of improved forages have clear benefits to aboveground forage production, but their impacts on belowground biomass (BGB) and hence soil C are less clear, especially in relatively understudied subtropical pastures. If fertilization and improved cultivars increase BGB, C sequestration may benefit. However, long-term soil C stocks, and their associated ecosystem services, may be compromised if these practices sacrifice roots in favor of shoot production. We studied the aboveground and belowground biomass of nine bermudagrass (Cynodon spp.) genotypes in response to four escalating NPK fertilization rates and compared the soil C and N stocks among them. As expected, increasing fertilization improved forage accumulation (FA) although gains from additional N diminished at higher fertilization rates. A positive relationship between fertilization and BGB emerged but varied among genotypes. The latter identified potential tradeoffs between aboveground and belowground allocation in newly released and commercial forage varieties, which may affect pasture persistence and contributions to soil organic matter over time. Overall, we found subtle differences in soil organic C/soil organic N stocks among NPK fertilization rates and genotypes, with the strongest signal emerging from C isotopic analysis. Our results suggest that fertilization at the recommended rate and improved genotype selection minimized negative tradeoffs between aboveground and belowground biomass and did not elicit differences in SOC in the top 15 cm but likely contributed to ecosystem disservices as it relates to N losses.