Natural primary production mediates the effects of nitrogen and carbon addition on plant functional groups biomass and temporal stability in the Tibetan alpine steppe-meadow

Background and aims: Extensive studies have been made on the effect of the addition of nitrogen (N) and organic carbon (C) on grasses, sedges and forbs of the alpine ecosystems; however, few such studies have been made on alpine plant communities and plant functional groups at extremely high altitudes. We questioned whether the addition of N and C, two important soil elements, would have different effects on plant functional groups and on the main plant species in an alpine steppe meadow ecosystem at an altitude of 4700 m. Methods: A two-factor completely randomized design study was used, with the addition of three levels of N (0, 50 and 100 kg N ha(-1); urea as N source) and three levels of C (0, 60 and 120 kg C ha(-1); sucrose as C source) and four replicates, each year from 2011 - 2014. Effects at the community, plant functional groups and species levels were examined in an alpine steppe meadow. Results and discussion: With N and C as a single additive, biomass of grasses increased while forbs decreased. Sedges remained unchanged with N addition and increased with C addition. These results may be due to competition among Stipa prpurea, Poa alpigena, Kobresia humilis and forbs for light resources. This effect could be explained by the increase in cover of the major gramineous species, Stipa purpurea, while the cover of the sedge, Kobresia humilis, tended to decrease or showed no change with N addition. High level of N addition alone produced highest biomass in a year of low natural primary production (without C and/or N addition). After the addition of N and C together, the biomass of the three plant functional groups did not change or even decreased compared with either C or N addition. Furthermore, C addition increased community stability but N addition did not. Conclusions: The effects of N and C addition on the biomass of plant functional groups were complicated by the interaction between N and C, as well as by a major effect of initial standing biomass. With the addition of both N and C, the biomass of grasses increased and forbs decreased, especially in low natural primary production years. With C as a single additive, the biomass of sedges increased as did community stability. Consequently, management of plant biomass using N and/or C addition depends on both the plant functional group target and the species within the group, as well as the natural primary production. Predicting environmental factors that potentially affect primary production are crucial in a decision-making nutrient management system.