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Plants are likely to be phosphorus (P)-limited because of low availability of P to roots or mycorrhizae due to slow diffusion and high sorption in soils.  Phosphorus sub-cycles (inorganic versus organic) also tend to vary predictably over gradients (Fig.  1 A,B).  Plant responses to mycorrhizal fungi also vary by plant species and grassland type ( Reinhart et al. 2017 ). Mycorrhizae can scavenge nutrients (i.e. orthophosphate, organic-P); however, if they cannot solubilize calcium bound P, then geochemical gradients may constrain plant-mycorrhizal interactions (Fig.  1 C).Calcareous soils are found in &gt;30% of soils and are common in arid, semiarid, and semi-humid systems (e.g. grasslands, shrublands). We used geochemical and biogeochemical experiments to test hypotheses on P limitation and acquisition (Lajtha and Schlesinger 1988). Hypothesis: an increase in soil calcium carbonate (CaCO 3 ) will 1) reduce soil P bioavailability (geochemical exp.), 2) reduce plant biomass and phosphorus uptake (biogeochemical exp.), and 3) shift P acquisition strategies (i.e. root mining, mycorrhizal scavenging) (biogeochemical exp., Fig.  1 C).Experiments had a replacement series (CaCO 3  to silica sand) geochemical treatment (CaCO 3  additions: 0, 0.02, 0.10, and 0.30 [g × 100 g -1 ]) to simulate soil properties over natural grassland gradients. The biogeochemistry experiment was a completely randomized design with the geochemical and mycorrhizal fungi treatments (+, -) (4 × 2 factorial), 8 plant species, and 8 replications.  Geochemical exp.’s response variables were soil extractable nutrients and pH. Biogeochemistry exp.’s response variables were nutrient uptake and plant performance.The geochemical experiment confirmed that increasing CaCO 3  increased pH water  of subsurface soil (7.6 to 8.5), reduced the intensity of available P (assessed by anion exchange membranes) in treated soils by as much as 57%, and had no obvious effect on accessible P (Olsen-P). In other words, metaphorical cups of P (with straws) had similar amounts of accessible P per cup; however, additions of CaCO 3  effectively reduced the straws’ diameter and limited (re-)supply of available P.On average, large additions of CaCO 3  reduced plant biomass by 20% and plant uptake of P by 15% for the biogeochemistry experiment. On average, mycorrhizal fungi increased plant biomass by 6%.  Rarely did mycorrhizal fungi and geochemical treatments interact and affect plant biomass. When they did interact, mycorrhizae benefited plants in pots with greater levels of P intensity (i.e. no or little added CaCO 3 ) thereby suggesting mycorrhizae may not help to solubilize calcium bound P.These findings confirm CaCO 3  in subsoils reduced the intensity of bioavailable P, which consequently decreased plant biomass and P uptake. However, we cannot rule out that CaCO 3  additions may have additionally impacted plants response due to changes in micronutrient solubility (i.e. zinc), etc. ( Lajtha and Schlesinger 1988 ). While plants tended to benefit from mycorrhizae, we found no evidence to support a prediction of the nutritional mutualisms hypothesis—that benefits are greatest under P-limitation. On the contrary, our data suggest that plants will rely more on root mining P acquisition strategies than mycorrhizal scavenging ( Albornoz et al. 2020 ) where the inorganic-P sub-cycle dominates and P solubility is limited due to calcium-phosphate formation (Fig.  1 C).

Dueling Plant Phosphorus Acquisition Strategies Over a Geochemical Gradient: A Biogeochemistry Experiment Approach