Humans are both intentionally (fertilization) and unintentionally (atmospheric nutrient deposition) adding nutrients worldwide. Increasing availability of biologically reactive nitrogen (N) is one of the major drivers of plant species loss. It remains unclear, however, whether plant diversity will be equally reduced by inputs of reactive N coming from either small and frequent N deposition events or large and infrequent N fertilization events. By independently manipulating the rate and frequency of reactive N inputs, our study teases apart these potentially contrasting effects. Plant species richness decreased more quickly at high rates and at low frequency of N addition, which suggests that previous fertilization studies have likely over-estimated the effects of N deposition on plant species loss. N-induced species loss resulted from both acidification and ammonium toxicity. Further study of small and frequent N additions will be necessary to project future rates of plant species loss under increasing aerial N deposition.
Abstract. Recent studies have underscored the importance of grasslands as potential carbon (C) sinks.We performed a grazing experiment with seven stocking rates (SR0, SR1.5, SR3.0, SR4.5, SR6.0, SR7.5, and SR9.0 for 0, 1.5, 3.0, 4.5, 6.0, 7.5, and 9.0 sheep ha
À1, respectively) to investigate the effect of increasing grazing pressure on soil C and nitrogen (N) storage in the temperate grasslands of northern China. The results revealed that C and N storage in both 0-10 cm and 10-30 cm soil layers decreased linearly with increasing stocking rates. Carbon storage in the 0-10 cm soil layer was significantly higher in lightly grazed grasslands than in heavily grazed grasslands after a 5-yr grazing treatment. Our findings suggest an underlying transformation from soil C sequestration under light grazing to C loss under heavy grazing, and that the threshold for this transformation is 4.5 sheep ha À1 (grazing period from June to September).Results confirmed that grasslands used for grazing in northern China have the capacity to sequester C in the soil under appropriate grazing pressure, but that they lose C under heavy grazing. Therefore, appropriate grazer densities will promote soil C sequestration in the grasslands of northern China.
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