Association of tropical forage grasses in pastures: an opportunity for sustainable intensification?

• Autor(a) principal: Gomes, Caio Macret
• Data de Publicação: 2023
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://www.teses.usp.br/teses/disponiveis/11/11139/tde-11092023-090137/
• topic: Ciclo do nitrogênio; Estoque de carbono; Gases do efeito estufa; Greenhouse gases; Microbiologia do solo; Multispecific grasslands; Pastagens multiespecíficas; Soil carbon stock; Soil microbiology; Soil nitrogen cycling

Biodiverse grasslands are reported to increase productivity and soil organic carbon stocks (SOC) as well as to reduce nitrous oxide (N2O) emissions compared to monocultures. The positive effects of biodiversity are generally attributed to complementarity (niche partitioning and facilitation) among different species, through which their morphophysiological dissimilarities allow for greater overall exploitation of light, water and nutrients across space, time and soil chemical gradients. However, there is no information on how traits of tropical C4 perennial forage grasses commonly used under intensive grazing management and their association affect soil N and C dynamics. The objective of the study was to identify wether a mixture of C4 perennial grasses with contrasting growth strategies may be an alternative to mitigate GHG emissions relative to their monocultures. The field experiment was carried out in Piracicaba/SP, Brazil, from Jan/2020 until Mar/2022, using a randomized complete block design, with four replications. Treatments corresponded to three tropical perennial forage grass species: Andropogon gayanus cv. Planaltina (gamba grass), Megathyrsus maximum cv. Massai (guinea grass) and Urochloa brizantha cv. BRS Piata (palisade grass) cultivated in monoculture and in association (the three grass species sown in equal proportions). Species traits varied among the monocultures, which influenced both N2O emissions and SOC stocks. Gamba grass showed poorer root development, but greater plant nitrogen content, which seems to have stimulated N cycling and N2O efflux during the whole experimental period. The greater nitrogen content of gamba grass also influenced the stabilization of plant derived carbon input into the mineral-associated form (MAOC), presumably by stimulating microbial activity. Guinea and palisade grass presented contrasting rooting strategies of precision (length development efficiency) and scale (greater biomass), respectively. The species strategies reduced N2O emissions during different seasons of the year, according to the respective soil NO3- and NH4+ content. Palisade grass greater herbage and root biomass positively influenced the soil particulate organic carbon (POC) accumulation, as it represented greater plant C input. On the other hand, greater length development and lower N content of guinea grass roots were related to lower and negative values of MAOC and POC, which seems to be the result of priming effect. The dissimilar traits of species used generated complementarity in their association, as shown by increased root biomass and N-stock compared to the expected from the monocultures. Also, guinea and gamba grass shifted their functional traits towards a more conservative mode when grown in association. This shift may have slowed N-cycling in the association, as indicated by the smaller ammonia oxidizing archaea gene abundance, soil NO3- content and greater soil NH4+ content, compared to the expected values from the monocultures. Both complementarity and intraspecific shift of species grown in association contributed to reduce N2O efflux in the multispecies swards. Root over yielding by the association was also related to increased POC, but no effect was observed in the MAOC. We concluded that the species traits and their association influence soil C and N cycles, allowing for sustainable intensification of animal production systems from pastures.