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Tropical soils contain one-third of the carbon stored in soils globally 1 , so destabilization of soil organic matter caused by the warming predicted for tropical regions this century 2 could accelerate climate change by releasing additional carbon dioxide (CO 2 ) to the atmosphere 3 – 6 . Theory predicts that warming should cause only modest carbon loss from tropical soils relative to those at higher latitudes 5 , 7 , but there have been no warming experiments in tropical forests to test this 8 . Here we show that in situ experimental warming of a lowland tropical forest soil on Barro Colorado Island, Panama, caused an unexpectedly large increase in soil CO 2 emissions. Two years of warming of the whole soil profile by four degrees Celsius increased CO 2 emissions by 55 per cent compared to soils at ambient temperature. The additional CO 2 originated from heterotrophic rather than autotrophic sources, and equated to a loss of 8.2 ± 4.2 (one standard error) tonnes of carbon per hectare per year from the breakdown of soil organic matter. During this time, we detected no acclimation of respiration rates, no thermal compensation or change in the temperature sensitivity of enzyme activities, and no change in microbial carbon-use efficiency. These results demonstrate that soil carbon in tropical forests is highly sensitive to warming, creating a potentially substantial positive feedback to climate change. When tropical forest soils are warmed in situ, they release more CO 2 than predicted by theory, creating a potentially substantial positive feedback to climate change.

Ecosystem mycorrhizal type is shown to have a stronger effect on soil carbon storage than temperature, precipitation, clay content and primary production; ecosystems dominated by ectomycorrhizal and ericoid mycorrhizal fungi contain 70% more soil carbon per unit nitrogen than do ecosystems dominated by arbuscular mycorrhizal fungi. Root fungi key to soil carbon Ecosystems differ in the type of plant-associated mycorrhizal fungi (root symbionts associated with nearly all land plants) that dominate. Ectomycorrhiza and ericoid mycorrhizal (EEM) fungi produce nitrogen-degrading enzymes, whereas arbuscular mycorrhiza do not, leading to the prediction that plants in the EEM ecosystems will compete with decomposers for soil nitrogen and therefore increase soil carbon storage. These authors assemble a global data set to show that this is indeed the case, with 70% more carbon storage in EEM ecosystems than in ecosystems dominated by arbuscular mycorrhiza, and that mycorrhizal type is more important than other determinants of soil carbon storage levels. Soil contains more carbon than the atmosphere and vegetation combined 1 . Understanding the mechanisms controlling the accumulation and stability of soil carbon is critical to predicting the Earth’s future climate 2 , 3 . Recent studies suggest that decomposition of soil organic matter is often limited by nitrogen availability to microbes 4 , 5 , 6 and that plants, via their fungal symbionts, compete directly with free-living decomposers for nitrogen 6 , 7 . Ectomycorrhizal and ericoid mycorrhizal (EEM) fungi produce nitrogen-degrading enzymes, allowing them greater access to organic nitrogen sources than arbuscular mycorrhizal (AM) fungi 8 , 9 , 10 . This leads to the theoretical prediction that soil carbon storage is greater in ecosystems dominated by EEM fungi than in those dominated by AM fungi 11 . Using global data sets, we show that soil in ecosystems dominated by EEM-associated plants contains 70% more carbon per unit nitrogen than soil in ecosystems dominated by AM-associated plants. The effect of mycorrhizal type on soil carbon is independent of, and of far larger consequence than, the effects of net primary production, temperature, precipitation and soil clay content. Hence the effect of mycorrhizal type on soil carbon content holds at the global scale. This finding links the functional traits of mycorrhizal fungi to carbon storage at ecosystem-to-global scales, suggesting that plant–decomposer competition for nutrients exerts a fundamental control over the terrestrial carbon cycle.

1. Organic phosphorus is abundant in soil and its turnover can supply a considerable fraction of the phosphorus taken up by natural vegetation. Despite this, the ecological significance of organic phosphorus remains poorly understood, which is remarkable given the biological importance of phosphorus in terrestrial environments. 2. Of particular interest is the possibility that coexisting plant species partition soil organic phosphorus to reduce competition. This seems likely given the large number of biologically available phosphorus compounds that occur in soil and the variety of mechanisms by which plants can utilize them. 3. Here I propose a conceptual model of resource partitioning for soil phosphorus. The model describes a hypothetical example of four coexisting plant species that differ in their ability to access soil organic phosphorus compounds, which are grouped to form a gradient of biological availability based on the processes involved in their utilization by plants. 4. Synthesis: Resource partitioning for soil phosphorus could provide an additional mechanism to explain the coexistence and distribution of plant species. It is likely to occur widely in terrestrial environments, but should have greatest ecological significance wherever productivity is limited by the availability of soil phosphorus. This includes freshwater wetlands, super-humid temperate regions and ecosystems developed on strongly-weathered soils that cover vast areas of ancient landscapes in Africa, Australia and South America.

Terrestrial ecosystem carbon (C) sequestration plays an important role in ameliorating global climate change. While tropical forests exert a disproportionately large influence on global C cycling, there remains an open question on changes in below-ground soil C stocks with global increases in nitrogen (N) deposition, because N supply often does not constrain the growth of tropical forests. We quantified soil C sequestration through more than a decade of continuous N addition experiment in an N-rich primary tropical forest. Results showed that long-term N additions increased soil C stocks by 7 to 21%, mainly arising from decreased C output fluxes and physical protection mechanisms without changes in the chemical composition of organic matter. A meta-analysis further verified that soil C sequestration induced by excess N inputs is a general phenomenon in tropical forests. Notably, soil N sequestration can keep pace with soil C, based on consistent C/N ratios under N additions. These findings provide empirical evidence that below-ground C sequestration can be stimulated in mature tropical forests under excess N deposition, which has important implications for predicting future terrestrial sinks for both elevated anthropogenic CO₂ and N deposition. We further developed a conceptual model hypothesis depicting how soil C sequestration happens under chronic N deposition in N-limited and N-rich ecosystems, suggesting a direction to incorporate N deposition and N cycling into terrestrial C cycle models to improve the predictability on C sink strength as enhanced N deposition spreads from temperate into tropical systems.

The mechanisms that shape plant diversity along resource gradients remain unresolved because competing theories have been evaluated in isolation. By testing multiple theories simultaneously across a >2-million-year dune chronosequence in an Australian biodiversity hotspot, we show that variation in plant diversity is not explained by local resource heterogeneity, resource partitioning, nutrient stoichiometry, or soil fertility along this strong resource gradient. Rather, our results suggest that diversity is determined by environmental filtering from the regional flora, driven by soil acidification during long-term pedogenesis. This finding challenges the prevailing view that resource competition controls local plant diversity along resource gradients, and instead reflects processes shaping species pools over evolutionary time scales.

Long‐term pedogenesis leads to important changes in the availability of soil nutrients, especially nitrogen (N) and phosphorus (P). Changes in the availability of micronutrients can also occur, but are less well understood. We explored whether changes in leaf nutrient concentrations and resorption were consistent with a shift from N to P limitation of plant productivity with soil age along a > 2‐million‐year dune chronosequence in south‐western Australia. We also compared these traits among plants of contrasting nutrient‐acquisition strategies, focusing on N, P and micronutrients. The range in leaf [P] for individual species along the chronosequence was exceptionally large for both green (103–3000 μg P g⁻¹) and senesced (19–5600 μg P g⁻¹) leaves, almost equalling that found globally. From the youngest to the oldest soil, cover‐weighted mean leaf [P] declined from 1840 to 228 μg P g⁻¹, while P‐resorption efficiency increased from 0% to 79%. All species converged towards a highly conservative P‐use strategy on the oldest soils. Declines in cover‐weighted mean leaf [N] with soil age were less strong than for leaf [P], ranging from 13.4 mg N g⁻¹ on the youngest soil to 9.5 mg N g⁻¹ on the oldest soil. However, mean leaf N‐resorption efficiency was greatest (45%) on the youngest, N‐poor soils. Leaf N:P ratio increased from 8 on the youngest soil to 42 on the oldest soil. Leaf zinc (Zn) concentrations were low across all chronosequence stages, but mean Zn‐resorption efficiency was greatest (55–74%) on the youngest calcareous dunes, reflecting low Zn availability at high pH. N₂‐fixing species had high leaf [N] compared with other species. Non‐mycorrhizal species had very low leaf [P] and accumulated Mn across all soils. We surmise that this reflects Mn solubilization by organic acids released for P acquisition. Synthesis. Our results show community‐wide variation in leaf nutrient concentrations and resorption that is consistent with a shift from N to P limitation during long‐term ecosystem development. High Zn resorption on young calcareous dunes supports the possibility of micronutrient co‐limitation. High leaf [Mn] on older dunes suggests the importance of carboxylate release for P acquisition. Our results show a strong effect of soil nutrient availability on nutrient‐use efficiency and reveal considerable differences among plants of contrasting nutrient‐acquisition strategies.

Soil chronosequences provide valuable model systems to investigate pedogenesis and associated effects of nutrient availability on biological communities. However, long-term chronosequences occurring under seasonally dry climates remain scarce. We assessed soil development and nutrient dynamics along the Jurien Bay chronosequence, a 2 million-year sequence of coastal dunes in southwestern Australia. The chronosequence is significant because it occurs in a Mediterranean climate and supports hyperdiverse shrublands within a global biodiversity hotspot. Young soils formed during the Holocene (<6,500 years old) are strongly alkaline and contain abundant carbonate, which is leached from the profile within a few thousand years. Middle Pleistocene soils (ca 120,000–500,000 years old) are yellow decalcified sands with residual iron oxide coatings on quartz grains over a petrocalcic horizon that occurs at increasing depth as soils age. Early Pleistocene soils (>2,000,000 years old) are completely leached of iron oxides and consist of bleached quartz sand several meters deep. Changes in soil organic matter and nutrient status along the Jurien Bay chronosequence are consistent with patterns observed along other long-term chronosequences and correspond closely to expectations of the Walker and Syers (1976) model of biogeochemical change during pedogenesis. Organic carbon and nitrogen (N) accumulate rapidly to maximum amounts in intermediate-aged Holocene dunes and then decline as soils age. In contrast, total phosphorus (P) declines continuously along the chronosequence to extremely low levels after 2 million years of pedogenesis, eventually representing some of the lowest P soils globally. Ratios of soil organic carbon to P and N to P increase continuously along the chronosequence, consistent with a shift from N limitation on young soils to extreme P limitation on old soils. Phosphorus fractionation by sequential extraction reveals a rapid decline in primary and non-occluded phosphate and an increase in organic and occluded P as soils age. Concentrations of extractable (that is, readily bioavailable) N and P, as well as exchangeable cations, are greatest in Holocene dunes and decline to low levels in Pleistocene dunes. Extractable micronutrient concentrations were generally very low and varied little across the chronosequence. We conclude that the Jurien Bay chronosequence is an important example of changing patterns of nutrient limitation linked to long-term soil and ecosystem development under a Mediterranean climate.

Soil biota influence plant performance through plant-soil feedback, but it is unclear whether the strength of such feedback depends on plant traits and whether plant-soil feedback drives local plant diversity. We grew 16 co-occurring plant species with contrasting nutrient-acquisition strategies from hyperdiverse Australian shrublands and exposed them to soil biota from under their own or other plant species. Plant responses to soil biota varied according to their nutrient-acquisition strategy, including positive feedback for ectomycorrhizal plants and negative feedback for nitrogen-fixing and nonmycorrhizal plants. Simulations revealed that such strategy-dependent feedback is sufficient to maintain the high taxonomic and functional diversity characterizing these Mediterranean-climate shrublands. Our study identifies nutrient-acquisition strategy as a key trait explaining how different plant responses to soil biota promote local plant diversity.

Phosphorus (P) is an essential nutrient that is often in limited supply, with P availability constraining biomass production in many terrestrial ecosystems. Despite decades of work on plant responses to P deficiency and the importance of soil microbes to terrestrial ecosystem processes, how soil microbes respond to, and cope with, P deficiencies remains poorly understood. We studied 583 soils from two independent sample sets that each span broad natural gradients in extractable soil P and collectively represent diverse biomes, including tropical forests, temperate grasslands, and arid shrublands. Phosphorus (P) is an essential nutrient that is often in limited supply, with P availability constraining biomass production in many terrestrial ecosystems. Despite decades of work on plant responses to P deficiency and the importance of soil microbes to terrestrial ecosystem processes, how soil microbes respond to, and cope with, P deficiencies remains poorly understood. We studied 583 soils from two independent sample sets that each span broad natural gradients in extractable soil P and collectively represent diverse biomes, including tropical forests, temperate grasslands, and arid shrublands. We paired marker gene and shotgun metagenomic analyses to determine how soil bacterial and archaeal communities respond to differences in soil P availability and to detect corresponding shifts in functional attributes. We identified microbial taxa that are consistently responsive to extractable soil P, with those taxa found in low P soils being more likely to have traits typical of oligotrophic life history strategies. Using environmental niche modeling of genes and gene pathways, we found an enriched abundance of key genes in low P soils linked to the carbon-phosphorus (C-P) lyase and phosphonotase degradation pathways, along with key components of the high-affinity phosphate-specific transporter (Pst) and phosphate regulon (Pho) systems. Taken together, these analyses suggest that catabolism of phosphonates is an important strategy used by bacteria to scavenge phosphate in P-limited soils. Surprisingly, these same pathways are important for bacterial growth in P-limited marine waters, highlighting the shared metabolic strategies used by both terrestrial and marine microbes to cope with P limitation.

Humid tropical forests contain some of the largest soil organic carbon (SOC) stocks on Earth. Much of this SOC is in subsoil, yet variation in the distribution of SOC through the soil profile remains poorly characterized across tropical forests. We used a correlative approach to quantify relationships among depth distributions of SOC, fine root biomass, nutrients and texture to 1 m depths across 43 lowland tropical forests in Panama. The sites span rainfall and soil fertility gradients, and these are largely uncorrelated for these sites. We used fitted β parameters to characterize depth distributions, where β is a numerical index based on an asymptotic relationship, such that larger β values indicate greater concentrations of root biomass or SOC at depth in the profile. Root β values ranged from 0.82 to 0.95 and were best predicted by soil pH and extractable potassium (K) stocks. For example, the three most acidic (pH < 4) and K-poor (< 20 g K m⁻²) soils contained 76 ± 5% of fine root biomass from 0 to 10 cm depth, while the three least acidic (pH > 6.0) and most K-rich (> 50 g K m⁻²) soils contained only 41 ± 9% of fine root biomass at this depth. Root β and SOC β values were inversely related, such that a large fine root biomass in surface soils corresponded to large SOC stocks in subsoils (50–100 cm). SOC β values were best predicted by soil pH and base cation stocks, with the three most base-poor soils containing 34 ± 8% of SOC from 50 to 100 cm depth, and the three most base-rich soils containing just 9 ± 2% of SOC at this depth. Nutrient depth distributions were not related to Root β or SOC β values. These data show that large surface root biomass stocks are associated with large subsoil C stocks in strongly weathered tropical soils. Further studies are required to evaluate why this occurs, and whether changes in surface root biomass, as may occur with global change, could in turn influence SOC storage in tropical forest subsoils.