Nutrient status in Alpine soils of the Colorado front range using the Nitrogen/Phosphorus ratio index

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The suitability of the foliar N/P ratio was evaluated as a predictor of nutrient limitation in an alpine ecosystem of the Colorado Front Range. We hypothesized that foliar N/P ratios are directly correlated with the alpine soil nutrient status. We used a long-term fertilization experiment conducted in three alpine plant communities, where 48 plots were established consisting of four replicates of control, N, P, and N + P additions. We characterized four extractable P fractions, maximum P sorption capacity, and extractable NO3–N and NH4–N, measured N and P adsorption fluxes, determined the soil moisture content, and measured the total N and P concentrations in soils and below- and aboveground plant material. The fertilized plots exhibited significantly higher nutrient concentrations in above- and belowground plant materials and adsorption fluxes of N and P (P < 0.001 for all additions) than the control plots. The N-added plots were significantly acidified (<3.8) vs. the control plots (>4.14), which may have partially contributed to increased P flux. A correlation analysis showed that N and P accumulations in aboveground plant material could not be predicted by the traditional extractable N and P tests. Nutrient adsorption flux measurements and especially P showed better correlation with nutrient accumulation in below- and aboveground plant material (r = 0.53 and 0.43, respectively, P < 0.001). This moderate correlation suggested, however, that the N/P ratio index is somewhat limited in providing a definitive answer for the nutrient limitation status of this alpine ecosystem.

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