I have reported ratios (for plants, soils, porewater nutrients) using either molar or mass ratios, with the choice depending on where I was publishing and what I was doing with the data. If you are reporting the ratios as simple descriptors of the peat and soil scientists generally use mass ratios, go ahead and do that too. If you are comparing plant and soil ratios to assess relative nutrient limitation, I’ll suggest that it doesn’t really matter as long as you use the same mass or molar units for both plants and soil (although, as John L. Stoddard pointed out, there is a compelling argument for using molar ratios here). If you are specifically interested in stoichiometry (e.g., is there enough N and P to make a specific biogeochemical reaction happen), then molar ratios probably make the most sense.

Whatever you choose, please be sure to clearly report your units. There’s nothing worse than seeing ratios and not knowing if they are mass or molar ratios; numbers are pretty useless without units.

For elemental ratios always use moles. Also, always clearly define it in the methods. Do not present the ratio as both, it will confuse people. Most of the stoichiometry literature is in moles. Frequently you will that people switch between using molar elemental ratios and element-based mass for concentrations. This is not ideal, but it is the way it is.

Soil science likes mass because it is convenient for expressing g of a fixed thing per grams of a variable/unknowable thing, e.g. grams of carbon (a fixed thing) per gram of soil (a highly variable thing). You can't convert that to moles of soil without knowing the quantity of every atom in a sample.