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As global change shifts the species composition of forests, we need to understand which species characteristics affect soil organic matter cycling to predict future soil carbon (C) storage. Recently, whether a tree species forms a symbiosis with arbuscular (AM) versus ectomycorrhizal (EcM) fungi has been suggested as a strong predictor of soil carbon storage, but there is wide variability within EcM systems. In this study, we investigated how mycorrhizal associations and the species composition of canopy trees and mycorrhizal fungi relate to the proportion of soil C and nitrogen (N) in mineral-associations and soil C:N across four sites representing distinct climates and tree communities in the Eastern U.S. broadleaf forest biome. In two of our sites, we found the expected relationship of declining mineral-associated C and N and increasing soil C:N ratios as the basal area of EcM-associating trees increased. However, across all sites these soil properties strongly correlated with canopy tree and fungal species composition. Sites where the expected pattern with EcM basal area was observed were 1) dominated by trees with lower quality litter in the Pinaceae and Fagaceae families and 2) dominated by EcM fungi with medium distance exploration type hyphae, melanized tissues, and the potential to produce peroxidases. This observational study demonstrates that differences in soil organic matter between AM andEcM systems are dependent on the taxa of trees and EcM fungi involved. Important information is lost when the rich mycorrhizal symbiosis is reduced to two categories. This article is protected by copyright. All rights reserved.

Most land plants acquire nitrogen (N) through associations with arbuscular (AM) and ectomycorrhizal (ECM) fungi, but these symbionts employ contrasting strategies for N acquisition, which may lead to different stocks of soil carbon (C). We experimentally test this hypothesis with a mesocosm system where AM and ECM tree seedling roots, or their hyphae only, could access mineral soils with 13 C- and 15 N-enriched organic matter. We quantified loss of soil C and N, plant uptake of N and new inputs of plant C to soil. We found that AM, but not ECM, seedlings reduced soil C relative to controls. Soil C loss was greater in the presence of roots relative to hyphae only for both AM and ECM seedlings, but was correlated with plant N uptake for AM seedlings only. While new plant C inputs stimulated soil C loss in both symbioses, we detected plant C inputs more frequently and measured higher rates of decomposer activity in soils colonized by AM relative to ECM seedlings. Our study experimentally demonstrates how mycorrhizal strategies for N can affect soil C and C:N, even at the scale of an individual plant. Such effects may contribute to broad patterns in soil C across terrestrial ecosystems.

Increased drought intensity and frequency due to climate change may reduce the abundance and activity of nitrogen (N2 )-fixing plants, which supply new N to terrestrial ecosystems. As a result, drought may indirectly reduce ecosystem productivity through its effect on the N cycle. Here, we manipulated growing season net rainfall across a series of plots in an early successional mesic deciduous forest to understand how drought affects the aboveground productivity of the N2 -fixing tree Robinia pseudoacacia and three co-occurring nonfixing tree species. We found that lower soil moisture was associated with reduced productivity of R. pseudoacacia but not of nonfixing trees. As a result, the relative biomass and density of R. pseudoacacia declined in drier soils over time. Greater aboveground biomass of R. pseudoacacia was also associated with greater total soil N, extractable inorganic N, N mineralization rates, and productivity of nonfixing trees. These soil N effects may reflect current N2 fixation by R. pseudoacacia saplings, or the legacy effect of former trees in the same location. Our results suggest that R. pseudoacacia promotes the growth of nonfixing trees in early succession through its effect on the N cycle. However, the sensitivity of R. pseudoacacia to dry soils may reduce N2 fixation under scenarios of increasing drought intensity and frequency, demonstrating a mechanism by which drought may indirectly diminish potential forest productivity and recovery rate from disturbance.

Fire is a critical force in structuring ecosystems, but it also removes substantial amounts of nitrogen (N), which can limit plant growth. Biological N fixation (BNF) may alleviate fire-induced N deficiencies that inhibit ecosystem recovery, yet if and how BNF achieves this under frequent fire is unclear. This problem is further complicated in the context of modern human influences (such as land-use history and atmospheric N deposition), which may confound the relationship between fire and fixation. Here, we investigate whether BNF supplies the N necessary to replace fire-induced N losses in restored longleaf pine savannas, and, if so, what factors control fixation. We established 54 1-ha plots of longleaf pine capturing 227 yr of forest recovery and a broad gradient of fire return interval (1.5-20 yr) at two sites in the southeastern United States. We quantified N fixation from three functional groups (herbaceous legumes, soil crusts, and asymbiotic N fixing bacteria), N losses from individual fire events and ecosystem dynamics of N supply and demand. We found that BNF rates were low but sustained over stand age but were substantially below estimated rates of atmospheric N deposition. While fire temporarily stimulated BNF from herbaceous legumes, neither BNF nor atmospheric N deposition were sufficient to balance N losses from fire and soil N stocks declined over stand age. However, rates of N mineralization were surprisingly high and tree productivity was unrelated to N availability, questioning the importance of N limitation in these temperate savannas. While it is possible that progressive N losses signal a decline in ecosystem resiliency, N enrichment from multiple land-use transitions and anthropogenic N deposition may suppress rates of BNF or diminish its importance as a long-term N balancing source in these pyrogenic ecosystems. In this case, fire may be acting as relief mechanism, critical for returning the modern longleaf pine landscape to its historical oligotrophic condition.

Questions remain as to which soil nutrients limit primary production in tropical forests. Phosphorus (P) has long been considered the primary limiting element in lowland forests, but recent evidence demonstrates substantial heterogeneity in response to nutrient addition, highlighting a need to understand and diagnose nutrient limitation across diverse forests. Fine-root characteristics including their abundance, functional traits, and mycorrhizal symbionts can be highly responsive to changes in soil nutrients and may help to diagnose nutrient limitation. Here, we document the response of fine roots to long-term nitrogen (N), P, and potassium (K) fertilization in a lowland forest in Panama. Because this experiment has demonstrated that N and K together limit tree growth and P limits fine litter production, we hypothesized that fine roots would also respond to nutrient addition. Specifically we hypothesized that N, P, and K addition would reduce the biomass, diameter, tissue density, and mycorrhizal colonization of fine roots, and increase nutrient concentration in root tissue. Most morphological root traits responded to the single addition of K and the paired addition of N and P, with the greatest response to all three nutrients combined. The addition of N, P, and K together reduced fine-root biomass, length, and tissue density, and increased specific root length, whereas root diameter remained unchanged. Nitrogen addition did not alter root N concentration, but P and K addition increased root P and K concentration, respectively. Mycorrhizal colonization of fine roots declined with N, increased with P, and was unresponsive to K addition. Although plant species composition remains unchanged after 14 years of fertilization, fine-root characteristics responded to N, P, and K addition, providing some of the strongest stand-level responses in this experiment. Multiple soil nutrients regulate fine-root abundance, morphological and chemical traits, and their association with mycorrhizal fungi in a species-rich lowland tropical forest.