**4.1. Atmospheric CO2**

There is strong evidence on the relationship between atmospheric CO2 concentration and plant stoichiometry. It is expected that the increases of atmospheric CO2 will stimulate the plant photosynthesis and, perhaps, growth and overall production.

As a result, there is potential for C sequestration in plant biomass as atmospheric CO2 increases [61]. However, the length of plant growth in any location is probably influenced by the resources available in the soil, particularly N [62].

Atmospheric CO2 fixation tends to increase the root/plant ratio [63] and leaf area [64], which will influence the C:N:P ratios of the entire plant and, ultimately, photosynthetic capacity [65].

At the molecular level, rubisco, a key photosynthetic enzyme, operates more efficiently at higher levels of CO2 emissions (intracellular levels), especially in C3 plants [66], by minimiz‐ ing the need for gene expression of the enzyme to compensate for the losses to photorespira‐ tion [67]. The resources (for example, N) which are not used to produce rubisco can then be diverted to increase production [68].

In general, a higher concentration of CO2 should result in a greater C:N ratio in plant biomass and increases in plant size [69].

### **4.2. Global warming**

Global warming will likely influence plant stoichiometry, plant species, community primary production through impacts on phenology and plant growth conditions [61].

However, these effects will be moderated by drought. For example, in the long term, warming with increasing drought conditions in the Amazon can induce massive changes in biomass carbon [62].

However, restrictions on the use of nutrients [45] and changes in development and the way plants share resources across the types of tissue [70] suggest changes in C concentrations on a large scale; they will also be accompanied by absolute changes in levels of soil nutrients.

### **4.3. Varying increases in supplementation with N and P**

The majority of terrestrial ecosystems has historically been adapted to a natural limitation of key nutrients [71]. Combustion of fossil fuels, use of fertilizers, agricultural production of legumes [72], deforestation and changes in land [73] allowed for a large‐scale duplication of input of biologically available N in ecosystems around the world. The anthropogenic effects of P in the biosphere appear to be even greater, because the cycle of this nutrient was ampli‐ fied four times by human action [74].

In the short term, more availability of N and P can increase the productivity of plant species through a greater leaf area index [65] among other routes, and biomass [18]. In the long term, increases mediated by nutrient deposition in the soil can shape community composition dif‐ ferentially, changing the growth rate and the success of resident plant species [75].
