Developing more sustainable forage systems requires efficient decision support tools for fertilization management. Soil phosphorus (P) tests have long been used as decision support tools for fertilization management but, more recently, plant nutrition indices using the P concentration of shoot biomass were developed to assess the P nutrition status of grasslands. The objectives of this study were to (i) evaluate the relationship between the phosphorus nutrition index (PNI) and the yield response to P fertilization (ii) analyze relationships between PNI and soil plant-available P (SPAP) indicators, and (iii) evaluate PNI assets for P diagnosis in forage system. Five long-term (≥ 9 years) grassland P fertilization experiments under different soil and climate environments in Canada, Switzerland, France and Romania were used. Three SPAP indicators were tested: C_P, the soil solution orthophosphate ions (oPion) concentration (mg P L⁻¹), Olsen P (mg P kg⁻¹), and, a process-based assessment (Qw + Pr) from the sum of oPion in the soil solution (Qw, mg P kg⁻¹) and diffusive oPion with time and C_P (Pr, mg P kg⁻¹). PNI was calculated as sward P concentration divided by the critical P concentration.

The cumulative effect of P fertilization resulted in a wide range of SPAP values. Overall, C_P varied from 0.03 to 3.6 mg P L⁻¹, (Qw + Pr60 min) from 6−52 mg kg⁻¹, and Olsen P from 4−40 mg kg⁻¹. The PNI varied from 48–94% in plots with no applied P, and from 83–121% in P-fertilized plots. A generally positive relationship between relative forage dry matter yield and PNI was established, with a critical PNI value of 92% that distinguishes P-limited and non-P-limited grassland nutrition. Positive relationships between PNI and the three SPAP indicators confirmed that the soil P status influenced the grassland P nutrition status. Critical values on a stock basis for a target PNI value of 92% were similar for Olsen P (12.9 kg P ha⁻¹) and (Qw + Pr60 min) (13.5 kg P ha⁻¹). This study opens perspectives for P diagnosis improvement in forage systems.

开发更可持续的饲草系统需要有效的决策支持工具来进行施肥管理。长期以来，土壤磷（P）试验一直用作施肥管理的决策支持工具，但最近，开发了利用茎生物量中P浓度的植物营养指数，以评估草原的P营养状况。这项研究的目的是（i）评估磷营养指数（PNI）与磷肥产量响应之间的关系（ii）分析PNI与土壤有效磷（SPAP）指标之间的关系，以及（iii）评估用于饲料系统中P诊断的PNI资产。在加拿大，瑞士，法国和罗马尼亚的不同土壤和气候环境下，进行了五个长期（≥9年）的草地磷施肥实验。_P，土壤溶液中的正磷酸盐离子（oPion）浓度（mg PL ^-1），Olsen P（mg P kg ^-1），以及基于土壤溶液中oPion的总和的基于过程的评估（Qw + Pr）（ Qw，mg P kg ^-1）和随时间和C _P的扩散op （Pr，mg P kg ^-1）。将PNI计算为草地P浓度除以临界P浓度。

磷肥的累积效应导致了很宽的SPAP值。总体而言，C _P从0.03到3.6 mg PL ^-1变化，（Qw + Pr60 min）从6-52 mg kg ^-1变化，Olsen P从4-40 mg kg ^-1变化。在没有施用磷的地块中，PNI从48％到94％不等，在受磷的地块中，PNI从83％到121％不等。建立了相对饲草干物质产量与PNI之间的总体正相关关系，临界PNI值为92％，可区分P限制和非P限制的草地营养。PNI与三个SPAP指标之间的正相关关系证实了土壤P状况会影响草地P的营养状况。对于目标PNI值为92％的库存，临界值与Olsen P（12.9 kg P ha^-1）和（Qw + Pr60 min）（13.5 kg P ha ^-1）。这项研究为改善饲草系统中磷的诊断开辟了前景。