Effects of high Ca and Mg stress on plants water use efficiency in a Karst ecosystem

BACKGROUND: Karst ecosystems are widely distributed in the world, with one of the largest continuous Karst landforms in Southwest China. Karst regions are characterized by water shortage, high soil calcium (Ca) and magnesium (Mg) content, and soil nutrient leaching, resulting in drought stress and growth limitation of plants. METHODS: This study compared nitrogen (N), phosphorus (P), potassium (K), Ca, and Mg of herbaceous and woody plants in a small Karst ecosystem in Southwest China. The indexes of water use efficiency (WUE) were calculated to identify the drought stress of plants in this Karst ecosystem. Meanwhile, the relationship between Ca and Mg accumulation and WUE was evaluated in herbaceous and woody plants. RESULTS: Herbaceous plants showed a higher content of leaf N (13.4 to 40.1 g·kg(−1)), leaf P (2.2 to 4.8 g·kg(−1)) and leaf K (14.6 to 35.5 g·kg(−1)) than woody plants (N: 10.4 g to 22.4 g·kg(−1); P: 0.4 to 2.3 g·kg(−1); K: 5.7 to 15.5 g·kg(−1)). Herbaceous plants showed a significantly positive correlation between WUE and K:Ca ratio (R = 0.79), while WUE has a strongly positive correlation with K:Mg ratio in woody plants (R = 0.63). CONCLUSION: Herbaceous plants suffered from nitrogen (N) limitation, and woody plants were constrained by P or N+P content. Herbaceous plants had higher leaf N, P, and K than woody plants, while Ca and Mg showed no significant differences, probably resulting from the Karst environment of high Ca and Mg contents. Under high Karst Ca and Mg stress, herbaceous and woody plants responded differently to Ca and Mg stress, respectively. WUE of herbaceous plants is more sensitive to Ca stress, while that of woody plants is more sensitive to Mg stress. These findings establish a link between plant nutrients and hydraulic processes in a unique Karst ecosystem, further facilitating studies of the nutrient-water cycling system in the ecosystem.