Apatite (a calcium phosphate mineral present in most soils) is the most important source of phosphorus to most natural ecosystems. In intensively managed forests where large amounts of phosphorus are removed by harvesting, knowing the rate at which phosphorus is supplied by apatite weathering is critical to determining whether such harvesting is sustainable over the long term without fertilization. It has been suggested that when biological demand for phosphorus or calcium exceeds the supply, ectomycorrhizal fungi that grow in a mutualistic relationship with tree roots may be able to accelerate the weathering of apatite and other soil minerals by releasing organic acids into the soil. The process has been demonstrated in controlled laboratory experiments, but is difficult to measure in the field. Thus, the pattern and importance of ectomycorrhizal weathering in nutrient-stressed ecosystems is poorly understood.
This project will use the elevated concentrations of rare Earth elements (REEs) and the distinctive isotopic ratios of lead (Pb) in apatite to trace its weathering products. REEs and Pb isotope concentrations will be measured in apatite extracted from soil samples and also in the mushrooms produced by ectomycorrhizal fungi. Soils and mushrooms will be collected at eight northern hardwood forest stands where nitrogen and phosphorus fertilization experiments have recently begun and which differ in soil apatite abundance and stand age. These measurements across stands with contrasting natural and manipulated nutrient demand and availability will improve our understanding of the ecosystem-scale controls on ectomycorrhizal weathering, and the species of fungi responsible. The work will ultimately lead to improved estimates of long-term nutrient supply under different harvest regimes and soil mineralogies, and therefore improve our ability to manage such forests sustainably. Additional broader impacts will include involving undergraduate students, international students, and middle school teachers in research activities.
This project was intended to test whether rare earth elements and stable isotopes of lead could provide information about differences in the weathering rate of primary phosphate minerals in soil, including apatite. Tools for understanding the controls on soil weathering rates may allow us to better manage forests for sustainable provision of timber and energy resources without depleting soil nutrient availability over the long term. In a greenhouse experiment on birch seedlings grown with access to crushed granite, we found a coherent signature of apatite tracers (including high rare earth element concentrations and radiogenic Pb isotope ratios) in pots with birches relative to those without, and in mycorrhizal birches relative to non-mycorrhizal birches. These results suggest that the effect of biological systems on apatite dissolution is in principle detectable using these techniques. However, in mycorrhizal mushroom samples taken from six forested study sites, we did not observe clear differences in these tracers among stands that we hypothesized would differ in biological nutrient demand and hence allocation to mycorrhizal fungi capable of weathering nutrient-rich minerals. Additional analyses and application beyond our study plots will be required to demonstrate the utility of these tracers to provide management-relevant data in real-world forest ecosystems.
