Summary Most soil tests for available phosphorus (P) perform rather poorly in predicting crop response. This study was set up to compare different established soil tests in their capacity to predict crop response across contrasting types of soil. Soil samples from long‐term field experiments, the oldest >100 years old, were collected in five European countries. The total number of soil samples (n = 218), which differed in cropping and P treatment, and originated from 11 different soil types, were analysed with five tests: ammonium oxalate (Ox), ammonium lactate (AL), Olsen P, 0.01 m CaCl2 and the diffusive gradient in thin film (DGT). The first three tests denote available P quantity (Q), whereas the last two indicate P intensity (I) of the soil solution. All five tests were positively related to the crop yield data (n = 317). The Q‐tests generally outperformed I‐tests when evaluated with goodness of fit in Mitscherlich models, but critical P values of the I‐tests varied the least among different types of soil. No test was clearly superior to the others, except for the oxalate extraction, which was generally poor. The combination of Q‐ and I‐tests performed slightly better for predicting crop yield than any single soil P test. This Q + I analysis explains why recent successes with I‐tests (e.g. DGT) were found for soils with larger P sorption than for those in the present study. This systematic evaluation of soil tests using a unique compilation of established field trials provides critical soil P values that are valid across Europe. Highlights We compared soil P tests for predicting crop response across contrasting soil types. No test was clearly superior to the others except for the oxalate extraction, which was generally poor. This study suggests that intensity tests do not perform markedly better than quantity tests. The evaluation of soil P tests on this unique dataset provided critical soil P values across Europe.

Summary The reduced use of phosphorus (P) fertilizer in fertile soil has reverted the P balance to negative in some regions. It is unclear how long current soil P stocks will ensure adequate P supply to crops. In addition, it is unknown if current soil tests for available P describe bioavailable P adequately in soil where P is becoming depleted. We set up an accelerated soil P mining test to address these questions. Perennial ryegrass (Lolium perenne, Melpetra tetra) was grown for 2 years in a greenhouse on 5‐cm‐deep soil layers of eight contrasting soils with periodic grass clipping. Each soil was split into four fertilizer treatments (i.e. no P (–P) and adequate P (+P)) and two nitrogen levels, the latter to alter the rate of P uptake. The long‐term P mining induced P‐related yield losses in seven of the 16 soil treatments. The cumulative uptake of shoot P at which yield loss started to exceed 10% (–P versus +P) varied over a small range of 37–74 mg P kg−1 soil among the soils. This critical cumulative P uptake (CCP) was related to the soil P content prior to mining measured by five soil P tests (ammonium oxalate, ammonium lactate (AL), Olsen P, 0.01 m CaCl2 and the diffusive gradient in thin film technique (DGT)); the largest R2 values were observed for P‐AL (R2 = 0.72) and P‐DGT (R2 = 0.73). However, none of the tests was diagnostic for yield loss during the depletion period. Increased N supply accelerated growth and rates of P uptake and decreased the CCP by a factor of 1.7 on average, illustrating the effect of the rate of biomass production. The CCP values obtained in the treatment with reduced N fertilizer application are likely to be the most relevant for the field and suggest that current stocks allow adequate P supply for arable crops for 3–8 years under zero P application (0–23 cm) in soils similar to those tested. The lack of a successful diagnosis for P deficiency during this depletion experiment calls for further calibration of soil tests for available P in the field. Highlights The availability of legacy P in well‐fertilized soil was evaluated with a P mining pot trial 10% loss of crop growth occurred when soil P was depleted by 37–74 mg P kg−1 soil Accelerated plant growth with increased N supply decreased total P uptake beyond which P deficiency occurs In a depletion scenario, current soil P tests are not diagnostic but they can be used for prediction

Many lowland regions are afflicted with episodic high phosphorus (P) concentrations in streams during low flow periods. It has been hypothesised that these P peaks are due to internal loading after reductive dissolution of ferric iron (Fe(III)) minerals in sediments with a high P/Fe ratio. Here, we experimentally tested that hypothesis by measuring the sediment/water fluxes of P in streams and by testing responses to in-stream oxygenation. A phosphate stock solution with a bromide (Br -) tracer was administered during 3 h in four streams with varying sediment P/Fe ratios during summer (low DO concentration). The experiment was repeated in winter (high DO concentration) in one of the streams. The sediments were either net P sinks (three cases), net unreactive (one case) or a net source (one case), with the net source occurring in the stream with the highest sediment P/Fe and at lowest DO concentration. A direct oxygenation experiment in one stream increased downstream DO concentration with about 1 mg O2 L -1 and decreased the P concentration with, on average, about 0.3 mg P L -1 over a two-week period. The efficiency of oxygenation to reduce P concentrations enhanced when flow rates were lower. In conclusion, this experimental study confirmed the hypothesis that internal P loading in streams is largest in high P/Fe sediments at low DO conditions. Lowering P concentrations in lowland streams should combine reduced P emission and strategies avoiding low DO concentration, i.e. reduced biological oxygen demand (BOD) and nutrient emissions.

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Many lowland regions are afflicted with episodic high phosphorus (P) concentrations in streams during low flow periods. It has been hypothesised that these P peaks are due to internal loading after reductive dissolution of ferric iron (Fe(III)) minerals in sediments with a high P/Fe ratio. Here, we experimentally tested that hypothesis by measuring the sediment/water fluxes of P in streams and by testing responses to in-stream oxygenation. A phosphate stock solution with a bromide (Br -) tracer was administered during 3 h in four streams with varying sediment P/Fe ratios during summer (low DO concentration). The experiment was repeated in winter (high DO concentration) in one of the streams. The sediments were either net P sinks (three cases), net unreactive (one case) or a net source (one case), with the net source occurring in the stream with the highest sediment P/Fe and at lowest DO concentration. A direct oxygenation experiment in one stream increased downstream DO concentration with about 1 mg O2 L -1 and decreased the P concentration with, on average, about 0.3 mg P L -1 over a two-week period. The efficiency of oxygenation to reduce P concentrations enhanced when flow rates were lower. In conclusion, this experimental study confirmed the hypothesis that internal P loading in streams is largest in high P/Fe sediments at low DO conditions. Lowering P concentrations in lowland streams should combine reduced P emission and strategies avoiding low DO concentration, i.e. reduced biological oxygen demand (BOD) and nutrient emissions.