Plant phosphorus nutrition indicators evaluated in agricultural grasslands managed at different intensities

A precise assessment of the phosphorus (P) nutrition status of plants is necessary for an efficient P management in agricultural grasslands. Plant mineral analysis is a tool to identify the nutrition status of grasslands and several P indicators derived thereof are available. However, the interpretation of plant P indicators in grassland samples is complex due to variation in botanical composition, changing nutrient concentrations during growth and interactions between nutrients. The aim of this study was to compare indicators on the P nutrition status of plants and eventually to improve their application in agricultural grasslands. We studied three agricultural grassland types in Switzerland that were managed at different intensity either for high biodiversity or high forage production. Each grassland type was for 5–25 years subjected to treatments of low, recommended or high rates of P, nitrogen (N) and potassium (K) fertilizer. The recommended P rates ranged from 15 to 48 kg P ha−1 year−1. We measured plant P, N and K concentrations in the aboveground biomass of grasses, legumes and forbs. The evaluated P indicators were: P concentration, N:P and K:P ratios and the P nutrition index (PNI). The PNI is calculated as P concentration divided by the linear relationship describing optimal sward P concentration as a function of N concentration (PNI = 100P/(0.06 × 5N + 1.5)). We observed significant yield reduction compared to the expected yield only when one or more nutrients were omitted from fertilization. Fertilizer P input higher than recommended did not significantly increase yields above the yield expected for the respective management and altitude. Under P limiting conditions, forbs and legumes had significantly higher P concentrations than grasses. Additionally, the proportion of legumes affected the P indicators integrating N. Therefore, we used the P indicators in the grass fraction, which was always the main botanical fraction. In grasses all P indicators differentiated between P fertilized and non-P-fertilized treatments. Concentrations of P from 2.1 to 3.0 mg g−1 indicated sufficient P supply, while yield was reduced at lower and not increased at higher concentrations, suggesting luxurious P consumption. Our results for N:P and K:P suggested optimal ranges of 5.5–9.0 and 6.0–10.5, respectively. The PNI showed a clear differentiation between deficient, sufficient or surplus fertilizer inputs. For the precise and correct interpretation of the plant P nutrition status in agricultural grasslands under different management, we propose to use the PNI in the grass fraction. Finally, the interpretation of the indicators was valid for the agricultural grasslands managed at different intensities in spite that the grass fraction was composed by different species.

► We applied plant P indicators to grasslands managed at different intensity. ► The grass fraction was most suited to analyse P status of mixed species grasslands. ► Sufficient P nutrition was reflected by P concentrations of 2.1–3.0 mg g−1. ► N:P and K:P ratios did not separate between sufficient and luxurious P nutrition. ► The P nutrition index identified P deficient, sufficient and luxurious nutrition.