Initial responses of grass litter tissue chemistry and N:P stoichiometry to varied N and P input rates and ratios in Inner Mongolia

Anthropogenic nitrogen (N) and phosphorus (P) inputs can alter the stoichiometry of senesced plant tissues, a key trait controlling nutrient cycling. However, it is unclear how fertilization rate affects plant litter tissue chemistry under varied N:P supply ratios. In a 2-year study, we investigated the effects of N and P supply rates at three N:P input ratios (4:1, 16:1, and 60:1) on the chemical constitution and N:P stoichiometry of the litter of two grasses: Leymus chinensis and Stipa krylovii. We further evaluated the differential responses of chemical constitution and N:P stoichiometry in leaf and culm litter of L. chinensis. Combined N and P fertilization increased soil acidity and plant-available N, but decreased plant-available P, especially when fertilization occurred at N:P ratio = 60:1. Litter N and P concentrations showed positive response to N and P inputs, and N concentration increased with fertilization rate under N:P ratio = 4:1, but P concentration decreased under N:P ratio = 60:1. Furthermore, we found stronger responses of N and P in L. chinensis and culms than in S. krylovii and leaves. Stoichiometric responses became more positive with increasing N and P fertilization level at each ratio. Nitrogen and P inputs also significantly improved potassium, copper, and sodium concentrations in senesced shoots independent of fertilization rates except for sodium at N:P ratio = 16:1, which had weaker responses in L. chinensis and leaves than in S. krylovii and culms. The effects of N and P inputs on other elements were primarily influenced by species and organs, but were also idiosyncratically affected by input levels at each ratio. These results indicate that decreasing evenness of N and P inputs may have increasingly severe non-linear impacts on nutrient cycling and that these impacts will be greater in L. chinensis-dominated ecosystems compared to those dominated by S. krylovii.