Long-term grazing affects relationships between nitrogen form uptake and biomass of alpine meadow plants

Trade-off between nutrient uptake rate and product accumulation has been found among species characterized as acquisitive and conservative strategies in resource utilization. However, long-term grazing causes changes in soil nutrient availability and plant species abundance by selective foraging and resource allocation between above- and belowground organs, which may cover up such trade-off. However, little is known whether the trade-off can be observed among species in community without grazing disturbance, and how grazing influences the trade-off. We conducted a 15 N labelling experiment in winter grazing and grazing release alpine meadow communities on the Tibet Plateau. We examined changes in N form uptake of 11 common species and relationship of N chemical uptake rate with aboveground biomass. Grazing release increased soil NH_4^ + –N and NO_3^ - –N, and increased NO_3^ - –N uptake rate in two species and NH_4^ + –N uptake rate in three species. Meanwhile, grazing release decreased aboveground biomass of three species and two of them belong to those species’ increased N uptake rate. Contrarily, grazing release increased aboveground biomass of four species and none belongs to the changed N uptake rate. Thus, grazing release caused changes of plant nutrient uptake rate and aboveground production in different directions, which explains the negative relationship of N uptake rate with aboveground biomass in ungrazed community. Our results indicate that the increase in nutrient uptake is probably one of the mechanisms for acquisitive species to cope with the raising nutrient availability and/or competition from the conservative dominant grasses after grazing release.