Responses of plant biomass and nutrients to fire vary with functional group and slope aspect in a semiarid restored grassland on the Loess Plateau

Fire and topography both have important impacts on grassland ecosystems. Still, the interactive effects between them on plant nutrients remain unclear, hindering the precise prediction of fire effects at larger spatial scales. Here, we examined the effects of fire on aboveground biomass, as well as on carbon and nutrient concentrations, in a semiarid restored grassland (under grazing exclusion) on the Loess Plateau. We compared these effects between graminoid and forb, as well as at the total community level. The influence of slope aspect and slope position on these effects were also examined. Fire significantly increased the aboveground biomass of graminoid but did not affect that of forb. Fire significantly decreased the concentrations of most nutrients in aboveground biomass. Graminoid on the north-facing slope had higher dominance and aboveground biomass but lower nutrient concentrations than that on the south-facing slope. Forb on the north slope had similar aboveground biomass and nutrient concentrations but lower dominance than that on the south slope. Furthermore, the effects of fire on nutrient concentrations in graminoid varied with slope aspect, with more fire-induced decrease in nitrogen and potassium but less decrease in carbon on the north slope than that on the south slope. However, the effects of fire on nutrients in forb and in total community were not varied with slope aspect. Slope position had minimal effect on aboveground biomass and nutrient concentrations and their responses to fire. These results indicate that slope aspect and fire have interaCtive effects on plant nutrient concentrations in semiarid restored grasslands on the Loess Plateau, and thus should be considered when predicting nutrients cycling in grasslands.