Intercropping of Maize and the topical grass Brachiaria humidicola alters soil respiration and nitrous oxide emissions through changes in exudate quantity and composition

Abstract:

Intercropping has been used as an agricultural management to improve soil cover, fertility and land use efficiency. Brazilian farmers have been using different forage grasses intercropped with Maize to provide cover crop after Maize harvest to be used as mulch for no-till practices or feed cattle during the fall-winter season. Among the commonly used forage grasses are many species of the Brachiaria genus. One species of this genus (B. humidicola) is known to exudate a diterpene that inhibits the activity of Nitrosonomas europaea and reduces the abundance of ammonia oxidizers, while other species of forage grasses have also showed similar potential. However, although the inhibitory effect of some compounds has been observed, the role of plant exudates on N cycling can vary, with an exudate priming effect linked to N mineralization/immobilization also contributing to observed fluxes. Here, using a hydroponics approach and separate soil incubations to isolate the contributions of root exudates to this observed phenomenon, we investigated the impact of intercropping Maize with B. humidicola on plant carbon (C) allocation (shoot and root biomass and relative C exudation rate), exudate chemical composition, as well as the impact of intercropping on soil respiration, N2O emission and inorganic N pools. Intercropping altered B. humidicola C allocation, which resulted in lower biomass (shoot and root) and higher relative C exudation rate. The primary metabolites present in plant exudates also differed according to crop and an intercropping interaction. A Structural Equation Model (SEM) was used to explore how plant traits (exudation rate, shoot:root, exudation chemistry) relate to the change in ammonium availability (mineralization, immobilization, nitrification), net nitrification rate, respiration and nitrous oxide emission. Our results indicate that plant traits influenced soil fluxes. Higher shoot:root ratios of B. humidicola were negatively associated with N immobilization (ammonium pool and soil respiration) and nitrification (ammonium pool) parameters. Higher relative exudation rates were observed for intercropped B. humidicola and were positively correlated with the ammonium pool, while the root exudate chemistry of intercropped B. humidicola was associated with lower N2O emissions, suggesting that intercropping may enhance the concentration of biological nitrification inhibitors in B. humidicola exudates. Maize exudate chemistry was associated with higher soil respiration and N2O emissions, suggesting higher N mineralization when monocropped and immobilization when intercropped. These results indicate that, as expected, the amount of C input through exudates and associated plant traits are key drivers of soil respiration and inorganic N availability. Evidence for higher N immobilization associated with exudates of intercropped Maize and lower nitrification for intercropped B. humidicola may be one explanation for higher Maize grain yield observed in field intercropping experiments. Furthermore, as intercropping induced changes in exudate chemistry and amount, these additional plant functional traits should be considered when designing management approaches to enhance soil N retention and plant N use efficiency.