This paper specifically focuses on the importance of organic phosphorus as a source of phosphorus for plant nutrition. In particular, the importance of rhizosphere phosphatase activity and its role in hydrolysing organic phosphorus in soil is considered. The bioavailability and utilization of organic phosphorus by plants as well as in the soil are also discussed.

Aim  Although many studies support the prevailing paradigm of nitrogen (N)‐driven biodiversity loss, some have argued that phosphorus (P) may be the main culprit. This questions the generality of the global threat through N enrichment. The major objective here was to quantify the relative importance of soil N and P in explaining patterns of plant species richness, under different levels of N and P limitation. Location  North‐western Europe. Methods  We collected soil, productivity and plant species data from 132 semi‐natural grasslands located along a gradient of nutrient availability and atmospheric N deposition. We used linear mixed models to investigate the relation between soil nutrients, acidity, limitation and productivity on one side, and indices for plant species richness on the other. Results  Mixed models explained between 38 and 50% of the total variation in species numbers, forbs and endangered species. Soil P was significantly negatively related to total species number, forbs and endangered species. Soil N was only significantly negatively related to number of forbs and endangered species. Compared with soil P, the explained variation attributed to soil N was between five‐ and twenty‐fold lower. P‐limited grasslands exhibited higher species richness, numbers of forbs and endangered species. Species richness and number of forbs decreased with lower soil acidity. N deposition was negatively related to the number of forbs and endangered species, as well as to soil acidity. Productivity was weakly positively related to soil P and negatively to species and forb numbers. We found no interaction factors between the explanatory variables. Main conclusions  P enrichment can present a greater threat to biodiversity than N enrichment in at least some terrestrial ecosystems. However, as N‐ and P‐driven species loss appeared independent, our results suggest that simultaneously reducing N and P inputs is a prerequisite for maintaining maximum plant diversity.

The impact of mobile colloids on the transport of phosphorus in the subsurface environment is not well understood. We hypothesized that interactions between metals, organic matter, and P control the dynamics of mobile colloidal P species in excessively fertilized sandy soils. The effect of UV irradiation and additions of 32P, orthophosphate, Fe, Al, and NaF on the concentration of colloidal P was examined using gel filtration chromatography. In addition, molybdate unreactive P (MUP) was characterized using phosphomonoesterase assays. The high molecular mass reactive P (HMMRP) fraction did not react to orthophosphate additions, increased upon Al and Fe additions and decreased upon NaF addition and UV irradiation. These results support the hypothesis that HMMRP is present as organic matter-metal-orthophosphate complexes. The concentration of high molecular mass unreactive P (HMMUP) decreased upon UV irradiation. The MUP concentration slightly decreased upon incubation with phytase and acid phosphatase. These observations fitted well to the "protection" hypothesis, where hydrolyzable P bonds are protected from monoesterase attack through occlusion in colloidal material. Taken together, this study indicates the high potential for subsurface P loss by colloidal particles in soils excessively fertilized with animal manure.

Summary1. Both nitrogen (N) and phosphorus (P) enrichment have been identified to drive plant species losses from nutrient-poor semi-natural grasslands. The relative contribution of N vs. P to species loss remains unclear, however. 2. We investigated how soil N and P availability affect the occurrence of 61 grassland species across North-western Europe. We selected 132 study sites, located in Great Britain, Belgium and France, along a soil fertility gradient based on variability in atmospheric N deposition and on nutrient input from adjacent agricultural land. To gain insight into the underlying ecological mechanisms of species loss, we examined the role of a suite of plant traits that may mediate a species' response to increased N or P availability. 3. Mixed logistic regression showed that the occurrence of 24 plant species (39AE3%) was affected by soil nutrient availability. Of these species, 18 were negatively affected by increased P (29AE5%) and five by increased N (8AE2%). Regionally declining plant species were absent from both P-rich and N-rich grasslands. Higher susceptibility to elevated P was associated with stress tolerance, low maximum canopy height and symbiosis with arbuscular mycorrhizae. 4. Synthesis and applications. Although we also identified negative effects on plant diversity through N enrichment, our results strongly suggest that P enrichment is a more important driver of species loss from semi-natural grasslands. Species in symbiosis with mycorrhizae and with low canopy height are especially at risk. Because detrimental effects of P enrichment are very difficult to mitigate due to the persistence of P in the soil, nature management should give absolute priority to preventing P input in grasslands through fertilization, agricultural run-off or inundation with P-polluted surface water. To restore species-rich grasslands on P-enriched soils, top soil removal appears crucial and more research regarding alternative removal strategies is essential.

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