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This article is a Commentary on Moyes et al., 210: 657–668.

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.

Plant diversity has long been assumed to predict soil microbial diversity. However, contradictory results have been found when examining their relationships, particularly at broad spatial scales. To address this issue, we conducted a meta‐analysis to evaluate the patterns in the correlation between plant diversity and soil microbial diversity and the underlying factors driving the relationship. We collected correlation data from 84 studies covering more than 3900 natural terrestrial samples globally. Using the hierarchical mixed‐effects model, we investigated factors including targeted taxonomic group, microbial examination method, sampling extent, biome type, soil type, and environmental factors to assess the patterns of the plant–microbial correlation and the determinants of their variations. We found that microbial richness displayed a modest but positive correlation with plant diversity (r = 0.333, CI = 0.220–0.437). In spite of variability among taxonomic groups and their relationship with plant diversity, positive correlations were more pronounced in the intermediate sampling extent of latitude and elevation coverage, and tropical forests. Among examined environmental factors, soil pH was negatively associated with the plant and soil microbial relationships at large spatial scales. The plant–microbial correlation appears more sensitive to edaphic factor variation in the poor nutrients and soil less compact systems. Collectively, our results point to key differences across taxonomic groups, spatial scales and biomes, and the modulating effects of climate and soil. The findings shed light on our deep understanding in plant–microbial diversity relationships at broad spatial scales and ecosystem sensitivity to biodiversity loss and environmental change.

We maintained a factorial nitrogen (N), phosphorus (P), and potassium (K) addition experiment for 11 years in a humid lowland forest growing on a relatively fertile soil in Panama to evaluate potential nutrient limitation of tree growth rates, fine-litter production, and fine-root biomass. We replicated the eight factorial treatments four times using 32 plots of 40 ×× 40 m each. The addition of K was associated with significant decreases in stand-level fine-root biomass and, in a companion study of seedlings, decreases in allocation to roots and increases in height growth rates. The addition of K and N together was associated with significant increases in growth rates of saplings and poles (1-–10 cm in diameter at breast height) and a further marginally significant decrease in stand-level fine-root biomass. The addition of P was associated with a marginally significant ( P == 0.058) increase in fine-litter production that was consistent across all litter fractions. Our experiment provides evidence that N, P, and K all limit forest plants growing on a relatively fertile soil in the lowland tropics, with the strongest evidence for limitation by K among seedlings, saplings, and poles.

1. Relationships between mycorrhizal plants and soil nitrogen (N) have led to the speculation that the chemistry of plant litter and the saprotrophy of mycorrhizal symbionts can function together to closely couple the N cycle between plants and soils. We hypothesized that a tannin-rich, ericoid mycorrhizal (ERM) plant promotes the retention of protein-tannin N in soil, and that this N source is accessible to saprotrophic ERM symbionts and their hosts, but remains less available to co-occurring ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) symbionts and their hosts. 2. We tested this feedback hypothesis in a southern Appalachian forest community composed of two microsites: a hardwood microsite with ECM and AM trees in the overstorey and understorey, and an AM herb layer; and a rhododendron microsite where the understorey and herb layer are replaced by ERM rhododendron. We synthesized ¹⁵N-enriched protein-tannin complexes from leaf litter extracts representing each forest microsite and examined the fate of ¹⁵N in soil volumes 3 months and 1 year after the complexes were placed in the field. 3. Protein-tannin complexes derived from the rhododendron microsite led to a higher retention of ¹⁵N in soil organic matter and a lower recovery in dissolved N pools than those from the hardwood microsite, supporting the hypothesis that rhododendron tannins create stable complexes that increase organic N retention in soils. 4. Rhododendron complexes led to greater ¹⁵N-enrichment in ERM roots than in AM and ECM roots, supporting the hypothesis that rhododendron can better access the N complexed by its own litter tannins than can co-occurring forest trees and plants. Our results suggest that both fungal saprotrophy and a high specific root length contribute to the ability of ERM roots to acquire N from complex organic sources. 5. Synthesis. This study provides evidence of an intricate N feedback where plant litter chemistry influences the cycle of N to maximize N acquisition by the host's mycorrhizal roots, while hindering N acquisition by mycorrhizal roots of co-occurring plants. Feedback processes such as these have the potential to drive patterns in nitrogen cycling and productivity in many terrestrial ecosystems.

Long-term fire exclusion may weaken ecosystem resistance to the return of fire. We investigated how a surface wildfire that occurred after several decades of fire exclusion affected a southern Appalachian forest transitioning from a fire-adapted to a fire-intolerant state. Tree traits associated with fire adaptation often co-occur with traits for nutrient conservation, including the ectomycorrhizal (ECM) association. In the absence of fire, the ECM association may facilitate the accumulation of organic matter, which becomes colonized by fine roots that then become vulnerable to consumption or damage by fire. Therefore, a deeper organic horizon might make stands of fire-adapted, ECM trees less resistant to a surface wildfire than stands of arbuscular mycorrhizal (AM), fire-intolerant trees. To test this hypothesis, we established plots in stands that fall along a gradient of mycorrhizal tree relative abundance both inside and outside the perimeter of the 2016 Rock Mountain wildfire. With increasing relative abundance of ECM trees, we found increasing organic horizon depth and mass and slower rates of decay, even for litter of ECM tree species. We calculated a major (73–83%) reduction in fine root biomass and length in the organic horizon following the wildfire. Over three years post-fire, we observed a higher probability of crown decline, basal sprouting and aboveground biomass mortality with increasing abundance of ECM trees. We propose that the biogeochemistry of mycorrhizal associations can help explain why fire exclusion makes stands of fireadapted trees less resistant to a surface wildfire than those with fire-intolerant trees.

1. Anthropogenic nitrogen (N) deposition is exposing plants and their arbuscular mycorrhizal fungi (AMFs) to elevated N availability, often leading to shifts in communities of AMF. However, physiological trade-offs among AMF taxa in their response to N enrichment vs the ability to acquire other soil nutrients could have negative effects on plant and ecosystem productivity. It follows that information on the functional traits of AMF taxa can be used to generate predictions of their potential role in mediating ecosystem responses to N enrichment. 2. Arbuscular mycorrhizal fungi taxa that produce extensive networks of external hyphae should forage for N and phosphorus (P) more effectively, but these services incur greater carbon (C) costs to the plant. If N enrichment ameliorates plant nutrient limitation, then plants may reduce C available for AMF, which in turn could eliminate AMF taxa with large extensive external hyphae from the soil community. As a result, the remaining AMF taxa may confer less benefit to their host plants. 3. Using a synthesis of data from the literature, we found that the ability of a taxon to persist in the face of increasing soil N availability was particularly high in isolates from the genus Glomus, but especially low among the Gigasporaceae. Across AMF genera, our data support the prediction that AMF with a tolerance for high soil N may confer a lower P benefit to their host plant. Relationships between high N tolerance and production of external hyphae were mixed. 4. Synthesis. If the relationship between N tolerance and plant P benefit is widespread then shifts in arbuscular mycorrhizal fungi communities associated with N deposition could have negative consequences for the ability of plants to acquire P and possibly other nutrients via a mycorrhizal pathway. Based on this relationship, we predict that arbuscular mycorrhizal fungi responses could constrain net primary productivity in P-limited ecosystems exposed to N enrichment. This prediction could be tested in future empirical and modelling studies.

Climate change is increasing drought frequency, which may affect symbiotic N2 fixation (SNF), a process that facilitates ecosystem recovery from disturbance. Here, we assessed the effect of drought frequency on the ecophysiology and SNF rate of a common N2-fixing tree in eastern US forests. We grew Robinia pseudoacacia seedlings under the same mean soil moisture, but with different drought frequency caused by wet–dry cycles of varying periodicity. We found no effect of drought frequency on final biomass or mean SNF rate. However, seedlings responded differently to wet and dry phases depending on drought frequency. Under low-frequency droughts, plants fixed carbon (C) and nitrogen (N) at similar rates during wet and dry phases. Conversely, under high-frequency droughts, plants fixed C and N at low rates during dry phases and at high rates during wet phases. Our findings suggest that R. pseudoacacia growth is resistant to increased drought frequency because it employs two strategies – drought tolerance or drought avoidance, followed by compensation. SNF may play a role in both by supplying N to leaf tissues for acclimation and by facilitating compensatory growth following drought. Our findings point to SNF as a mechanism for plants and ecosystems to cope with drought.

Invasive plants have the potential to affect decomposition both directly, by introducing novel substrates that may differ from native species in key structural or chemical properties, and indirectly through changes to soil properties and microbial communities. The relative importance of these two mechanisms is unclear, especially with regard to wood decomposition. To explore these questions, we used a novel method to rapidly assess the wood decay rates of 11 native and 11 invasive non-native angiosperm species. The study was repeated at three pairs of sites, each consisting of an invaded and a relatively uninvaded forest. The invaded sites had either been colonized by a non-native grass (Microstegium vimineum (Trin.) A. Camus), a non-native woody shrub (Ligustrum sinense Lour.) or by multiple invasive species. After one year in the field, mass loss varied more than two-fold among the 22 wood species (24.2–52.3%). Wood origin (i.e., native or non-native) was only important at the Microstegium sites, with non-native species decomposing marginally faster than native species. Wood decomposed faster at both the Ligustrum-invaded and multiply-invaded sites than in their respective uninvaded sites but there were no differences between sites invaded or not by Microstegium. We detected positive relationships overall between mass loss and pH, K, P and NO3−, but invasion had no consistent effects on these soil properties. The results from this study show that the differences in wood decay rates between native and non-native species and the effects of invasion are highly idiosyncratic, with effects depending greatly on species and ecological context.