Author(s): Laurel A. Kluber 1 , 2 , * , Sarah R. Carrino-Kyker 1 , 2 , Kaitlin P. Coyle 1 , Jared L. DeForest 3 , Charlotte R. Hewins 2 , Alanna N. Shaw 3 , Kurt A. Smemo 2 , 4 , David J. Burke 1 , 2
Although temperate forests are assumed to be nitrogen (N)-limited , current N deposition and possible ecosystem N saturation trends suggest that forests could become phosphorus (P)-limited , , , . This P limitation scenario is exacerbated in those forests with acidic soils or receiving significant anthropogenic acid deposition, such as mixed deciduous forests of the northeastern United States . When soil pH is below 5.5, Al 3+ becomes mobilized and binds to inorganic P, rendering it less available for plant uptake. Despite a biogeochemical expectation of P limitation, evidence for P limitation in hardwood forests has been lacking , , suggesting that ecosystems can compensate for reduced mineral P availability. However, several recent studies have found evidence for P limitation in some acidic hardwood forests ,  and have alluded to possible compensation mechanisms.
Although plants can utilize a variety of mechanisms to increase P uptake, the most efficient and widely utilized method is mycorrhizal colonization . Mycorrhizal fungi form a symbiotic relationship with plant roots where fungi provide plants with increased nutrient uptake in exchange for carbon (C) derived from photosynthesis. Mixed deciduous forests contain trees that form both arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) associations and there is ample evidence that these mycorrhizae increase P uptake and nutrition in plants, although the P uptake rate and efficiency can vary among mycorrhizal species , , , . Both AM and EcM fungi have the ability to produce extracellular enzymes to hydrolyze organic P; however, EcM have a much greater capacity to do so . Furthermore, some EcM are known to produce organic acids as a means to acquire P through mineral weathering , , . Although AM fungi have not been shown to produce organic acids to enhance P uptake , they are known to alleviate stress on plants growing in acidic soils  and immobilize Al3+ , potentially improving host plant P status.
Most previous studies examining the effects of soil pH on EcM fungi suggest that acidic conditions reduce EcM colonization and alter community structure , , although some have seen no influence of acidification on community structure . A number of liming experiments also reported EcM community shifts with increased soil pH , , . For example, Wallander et al.  increased pH from 4.5 to 5.8 and suggested that the EcM community shifted from one specialized for organic N acquisition to one that utilizes inorganic N, although P availability was not addressed. Likewise, Lilleskov et al.  observed that as soil N availability increased, EcM communities shifted towards taxa possibly specialized for nutrient uptake under acidic, P-limiting conditions. However, these studies did not measure or directly address P availability.
A number of studies have...