Experimental evidence that poor soil phosphorus (P) solubility typical of drylands due to calcium co-precipitation favors autonomous plant P acquisition over collaboration with mycorrhizal fungi

Abstract

Calcareous dryland soils are rich in precipitated phosphate and represent >30% of Earth's land, yet the relative importance of phosphorus-acquisition strategies (P_AS) among plant species in these systems is not well known. No experiment has investigated potential interactions between varying amounts of added calcium carbonate (CaCO₃) and arbuscular mycorrhizal fungi (AMF) on plant performance and P_AS which could test for potential mechanisms without the limitations of in-situ comparisons along natural gradients. To fill this knowledge gap, we conducted an experiment with CaCO₃ addition, AMF inoculation, and three invasive and five native grassland plants. We expected an increase in soil [CaCO₃] of an alkaline subsoil to 1) reduce soil-available phosphorus (P), 2) reduce plant biomass and P uptake, and 3) shift P_AS toward increased root mining as Ca-bound P increases. The largest CaCO₃ addition reduced available P by as much as 57%. On average, the largest addition of CaCO₃ reduced total biomass of plants by 19% and plant uptake of P by 15%. The P_AS seemed to have changed, and CaCO₃ additions tended to increase an indicator of root exudation (shoot [Mn]) to mobilize Ca-bound P, suggesting plasticity for some inducible root mining, especially Artemisia frigida and Poa secunda. However, CaCO₃ and plant species interacted to affect shoot [Mn]. The invasive grass Bromus tectorum was superior at acquiring P (> P uptake, > shoot [Mn]) and thus tolerating low soluble P conditions. Rarely did AMF and CaCO₃ interact to affect plant biomass. When they did, mycorrhizal responsiveness did not increase where P was less available, suggesting AMF become less beneficial upon P and Ca coprecipitation. In dryland soils with less soluble P, plants are thus likely to rely more on root mining P_AS than mycorrhizal scavenging, except for the invasive forb Euphorbia esula that always benefitted from AMF inoculation.