Extracellular enzymes synthesized by soil microbes play a central role in the biogeochemical cycling of nutrients in the environment. The pH optima of eight hydrolytic enzymes involved in the cycles ...of carbon, nitrogen, phosphorus, and sulfur, were assessed in a series of tropical forest soils of contrasting pH values from the Republic of Panama. Assays were conducted using 4-methylumbelliferone-linked fluorogenic substrates in modified universal buffer. Optimum pH values differed markedly among enzymes and soils. Enzymes were grouped into three classes based on their pH optima: (i) enzymes with acidic pH optima that were consistent among soils (cellobiohydrolase, β-xylanase, and arylsulfatase), (ii) enzymes with acidic pH optima that varied systematically with soil pH, with the most acidic pH optima in the most acidic soils (α-glucosidase, β-glucosidase, and N-acetyl-β-glucosaminidase), and (iii) enzymes with an optimum pH in either the acid range or the alkaline range depending on soil pH (phosphomonoesterase and phosphodiesterase). The optimum pH values of phosphomonoesterase were consistent among soils, being 4 to 5 for acid phosphomonoesterase and 10 to 11 for alkaline phosphomonoesterase. In contrast, the optimum pH for phosphodiesterase activity varied systematically with soil pH, with the most acidic pH optima (3.0) in the most acidic soils and the most alkaline pH optima (pH 10) in near-neutral soils. Arylsulfatase activity had a very acidic optimum pH in all soils (pH less-than or equal to3.0) irrespective of soil pH. The differences in pH optima may be linked to the origins of the enzymes and/or the degree of stabilization on solid surfaces. The results have important implications for the interpretation of hydrolytic enzyme assays using fluorogenic substrates.
Tropical soils contain one-third of the carbon stored in soils globally
, so destabilization of soil organic matter caused by the warming predicted for tropical regions this century
could accelerate ...climate change by releasing additional carbon dioxide (CO
) to the atmosphere
. Theory predicts that warming should cause only modest carbon loss from tropical soils relative to those at higher latitudes
, but there have been no warming experiments in tropical forests to test this
. 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
emissions. Two years of warming of the whole soil profile by four degrees Celsius increased CO
emissions by 55 per cent compared to soils at ambient temperature. The additional CO
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.
Soil contains more carbon than the atmosphere and vegetation combined. Understanding the mechanisms controlling the accumulation and stability of soil carbon is critical to predicting the Earth's ...future climate. Recent studies suggest that decomposition of soil organic matter is often limited by nitrogen availability to microbes and that plants, via their fungal symbionts, compete directly with free-living decomposers for nitrogen. Ectomycorrhizal and ericoid mycorrhizal (EEM) fungi produce nitrogen-degrading enzymes, allowing them greater access to organic nitrogen sources than arbuscular mycorrhizal (AM) fungi. 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. 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.
Phosphorus availability is widely assumed to limit primary productivity in tropical forests, but support for this paradigm is equivocal. Although biogeochemical theory predicts that phosphorus ...limitation should be prevalent on old, strongly weathered soils, experimental manipulations have failed to detect a consistent response to phosphorus addition in species-rich lowland tropical forests. Here we show, by quantifying the growth of 541 tropical tree species across a steep natural phosphorus gradient in Panama, that phosphorus limitation is widespread at the level of individual species and strengthens markedly below a threshold of two parts per million exchangeable soil phosphate. However, this pervasive species-specific phosphorus limitation does not translate into a community-wide response, because some species grow rapidly on infertile soils despite extremely low phosphorus availability. These results redefine our understanding of nutrient limitation in diverse plant communities and have important implications for attempts to predict the response of tropical forests to environmental change.
Tropical forest vegetation is shaped by climate and by soil, but understanding how the distributions of individual tree species respond to specific resources has been hindered by high diversity and ...consequent rarity. To study species over an entire community, we surveyed trees and measured soil chemistry across climatic and geological gradients in central Panama and then used a unique hierarchical model of species occurrence as a function of rainfall and soil chemistry to circumvent analytical difficulties posed by rare species. The results are a quantitative assessment of the responses of 550 tree species to eight environmental factors, providing a measure of the importance of each factor across the entire tree community. Dry-season intensity and soil phosphorus were the strongest predictors, each affecting the distribution of more than half of the species. Although we anticipated clear-cut responses to dry-season intensity, the finding that many species have pronounced associations with either high or low phosphorus reveals a previously unquantified role for this nutrient in limiting tropical tree distributions. The results provide the data necessary for understanding distributional limits of tree species and predicting future changes in forest composition.
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.
Ecological succession in a changing world Chang, Cynthia C.; Turner, Benjamin L.; Bardgett, Richard
The Journal of ecology,
March 2019, 2019-03-00, 20190301, Volume:
107, Issue:
2
Journal Article
Peer reviewed
Open access
Ecological succession – how biological communities re‐assemble and change over time following natural or anthropogenic disturbance – has been studied since the birth of ecology, and the resulting ...theoretical framework underpins many aspects of the discipline. Recently, the mechanistic basis of classic succession theory has been advanced by studies of plant and microbial interactions, functional traits, and retrogressive stages of ecosystem development. This special issue brings together a series of papers that highlight these contemporary novel approaches and how our understanding of ecological succession has advanced.
Four key themes emerge from the issue: (a) generalizations about succession, (b) the influence of dispersal and habitat size on successional trajectories, (c) changes in plant functional traits during succession, and (d) belowground community interactions during long term during ecosystem development.
Synthesis. The articles in the special issue highlight novel perspectives on succession theory, revealing the importance of historical contingency, disturbance severity, dispersal limitation, functional traits, and belowground community processes in determining patterns of ecosystem development. Together, they reinforce the importance of ecological succession in understanding the response of plant and microbial communities to disturbance in a changing world.
The articles in the special issue highlight novel perspectives on succession theory, revealing the importance of historical contingency, disturbance severity, dispersal limitation, functional traits, and belowground community processes in determining patterns of ecosystem development. Together, they reinforce the importance of ecological succession in understanding the response of plant and microbial communities to disturbance in a changing world.
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