Nitrogen Stimulation of Plant Biomass Growth Mediated by Foliar Magnesium and Calcium Worldwide

Liebig’s Law of the Minimum underscores the need for balanced nutrition for optimal plant growth; any deficiency in essential nutrients can influence plant growth and subsequent biomass accumulation. Anthropogenic nitrogen (N) deposition can alleviate the prevailing N limitation and stimulate plant growth in many terrestrial ecosystems, thereby helping to mitigate climate change. However, the N stimulation effects may diminish under conditions where plant growth is limited by soil cation availability that is susceptible to N-induced soil acidification. How variation in soil cations influences N stimulation of plant growth is unresolved. Here, we synthesized data from 282 field experiments and found that in agreement with the optimal allocation theory, N addition asymmetrically increased plant biomass aboveground (42.5 ± 7.4%) and belowground (20.8 ± 10.1%). The increment in aboveground biomass was soil pH dependent, shifting from neutral in low pH (pH ≤ 4.5) to positive in medium (4.5 < pH ≤ 7.5) and high pH (pH > 7.5) soils. In contrast, changes in belowground biomass were independent of soil pH. The variations in biomass increments across different soil pH ranges were mediated by the levels of foliar magnesium (Mg) and calcium (Ca), with the responses exhibiting a shift from negative in low pH soils to neutral in medium and high pH soils. These findings suggest that reduction in foliar Mg and Ca levels diminishes the N stimulation of plant biomass despite the enhancement of root growth in low pH soils. Given the widespread stimulation of plant biomass through N addition, the extent of this effect is soil pH dependent and mediated by the foliar cation status of plants.