Phosphorus addition, rather than mycorrhizal fungi or nitrogen addition, alleviate plant phosphorus deficits in phosphorus-impoverished grassland

• The common soil nutrient enrichment (nitrogen, N; phosphorus, P) typical of the agro-pasture ecotone may impact plants-arbuscular mycorrhizal fungi (AMF) symbiotic relationships. However, the stoichiometric responses of plants to increased soil nutrient availability and AMF are not yet clear.
• Thus, we carried a 3-year in situ study to understand how AMF as well as N and P additon alter the dominant plant ecological stoichiometry in a P-impoverished grassland in Northwest China. The grassland type is desert stepp. The soil type is light sierozem, with 1.8 mg kg⁻¹ soil available P and 6.7 mg kg⁻¹ soil inorganic N. A randomized block design (three-way factors) was conducted with four fertilization treatments: control, N input (N), P input (P), and combined N and P input (NP). Each fertilization treatment comprised two AMF treatments: a non-benomyl and a benomyl treatment. N input was provided as an NH₄NO₃ fertilizer (10 g N m⁻² year⁻¹) evenly input by hand to N and NP plots in late June (early growing season) in each year. P input was provided as a Ca(H₂PO₄)₂ fertilizer (10 g P m⁻² year⁻¹), similarly distributed as the N input.
• Overall, our study indicated that P input considerably exacerbated plant P limitations by diminishing their C:P and N:P; the impact of P input on plant C:P and N:P was higher than that of AMF and N input. This observed reduction in plant N:P and C:P owing to the P supply might be due to the higher soil available P level and quick increase in plant P concentration by P input. Conversely, N input and AMF suppression decreased two dominant grasses C:N ratios by increasing their N concentration. Accordingly, plant and soil available N:P could predict plant biomass changes under N and P addition in P-deficient grasslands.
• Our findings highlight that the importance of P input, but not AMF, in changing plant C:N:P stoichiometry in P-impoverished grasslands.