Optimal Soil Calcium for the Growth of Mulberry Seedlings Is Altered by Nitrogen Addition

Increasing soil N not only aggravates calcium (Ca) stress by stimulating Ca leaching from the soil but also impacts the sensitivity of plants to Ca stress. However, how increasing N influences the soil Ca demand of seedlings is largely unknown. We studied the influence of different concentrations of exogenous Ca (i.e., 0, 200, 400, 600, and 800 mg center dot kg(-1) Ca2+) on the growth, photosynthesis, Ca absorption, and intrinsic water use efficiency (iWUE) of mulberry (Morus alba) seedlings under two N levels (i.e., 200 and 600 mg center dot kg(-1) NH4NO3). We found that there was an optimal concentration of soil Ca for the growth and net photosynthetic rate (Pn) of mulberry seedlings; the optimal Ca concentration was 200 mg center dot kg(-1) under low N conditions and 400 mg center dot kg(-1) under high N conditions. Therefore, the application of N fertilizer increased the optimal Ca concentration. Different from the unimodal relationship between biomass and Ca levels, the iWUE of mulberry was significantly and positively correlated with soil Ca levels. At the same time, except under the 800 mg center dot kg(-1) Ca treatment, the soil Ca levels were reflected by foliar Ca concentrations. The N deposition, large-scale N fertilizer application, and drought increase Ca demand in plants, thus causing the application of Ca fertilizer to be necessary in low-Ca soil while alleviating Ca stress in high-Ca soil. The balance between the optimal Ca level needed for growth and drought resistance should be considered when determining the amount of Ca fertilizer required.