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.
Phytate (myo-inositol hexakisphosphate salts) can constitute a large fraction of the organic P in soils. As a more recalcitrant form of soil organic P, up to 51 million metric tons of phytate ...accumulate in soils annually, corresponding to ∼65% of the P fertilizer application. However, the availability of phytate is limited due to its strong binding to soils via its highly-phosphorylated inositol structure, with sorption capacity being ∼4 times that of orthophosphate in soils. Phosphorus (P) is one of the most limiting macronutrients for agricultural productivity. Given that phosphate rock is a finite resource, coupled with the increasing difficulty in its extraction and geopolitical fragility in supply, it is anticipated that both economic and environmental costs of P fertilizer will greatly increase. Therefore, optimizing the use of soil phytate-P can potentially enhance the economic and environmental sustainability of agriculture production. To increase phytate-P availability in the rhizosphere, plants and microbes have developed strategies to improve phytate solubility and mineralization by secreting mobilizing agents including organic acids and hydrolyzing enzymes including various phytases. Though we have some understanding of phytate availability and phytase activity in soils, the limiting steps for phytate-P acquisition by plants proposed two decades ago remain elusive. Besides, the relative contribution of plant- and microbe-derived phytases, including those from mycorrhizas, in improving phytate-P utilization is poorly understood. Hence, it is important to understand the processes that influence phytate-P acquisition by plants, thereby developing effective molecular biotechnologies to enhance the dynamics of phytate in soil. However, from a practical view, phytate-P acquisition by plants competes with soil P fixation, so the ability of plants to access stable phytate must be evaluated from both a plant and soil perspective. Here, we summarize information on phytate availability in soils and phytate-P acquisition by plants. In addition, agronomic approaches and biotechnological strategies to improve soil phytate-P utilization by plants are discussed, and questions that need further investigation are raised. The information helps to better improve phytate-P utilization by plants, thereby reducing P resource inputs and pollution risks to the wider environment.
Nutrient availability is widely considered to constrain primary productivity in lowland tropical forests, yet there is little comparable information for the soil microbial biomass. We assessed ...microbial nutrient limitation by quantifying soil microbial biomass and hydrolytic enzyme activities in a long-term nutrient addition experiment in lowland tropical rain forest in central Panama. Multiple measurements were made over an annual cycle in plots that had received a decade of nitrogen, phosphorus, potassium, and micronutrient addition. Phosphorus addition increased soil microbial carbon (13 %), nitrogen (21 %), and phosphorus (49 %), decreased phosphatase activity by ~65 % and N-acetyl β-glucosaminidase activity by 24 %, but did not affect β-glucosidase activity. In contrast, addition of nitrogen, potassium, or micronutrients did not significantly affect microbial biomass or the activity of any enzyme. Microbial nutrients and hydrolytic enzyme activities all declined markedly in the dry season, with the change in microbial biomass equivalent to or greater than the annual nutrient flux in fine litter fall. Although multiple nutrients limit tree productivity at this site, we conclude that phosphorus limits microbial biomass in this strongly-weathered lowland tropical forest soil. This finding indicates that efforts to include enzymes in biogeochemical models must account for the disproportionate microbial investment in phosphorus acquisition in strongly-weathered soils.
Background
Tropical tree species can maintain high growth rates on low-phosphorus (P) soils. However, the physiological basis of the high growth rates of tropical tree species remains unknown.
Scope
...Here, we examine how traits related to P uptake and use efficiency might account for this phenomenon. Based on a comparison of plant physiological responses to P and nitrogen (N) limitation, we hypothesize that distinct evolutionary processes have occurred on strongly weathered tropical soils characterized by low P availability relative to weakly and moderately weathered temperate soils characterized by low N availability. Efficient P-use arises through the synthesis of galactolipids rather than phospholipids, small genome size, preferential and flexible P allocation to leaves, efficient P resorption from wood and leaves, tissue longevity, and a decrease in P allocation to reproduction. Efficient P uptake mechanisms include synthesis of phosphatase in roots to acquire organic P from phosphodiesters and phytate, association with mycorrhizal fungi efficient at acquiring P, secretion of organic anions from roots to mobilize soil P, increased mass flow, and modifications of root depth distribution and structure.
Conclusions
Despite the prevalence of low P soils throughout the tropics, few studies have explored P-use efficiency and acquisition mechanisms in tropical trees. We predict that the wide range of mechanisms by which plants can efficiently acquire and use P maintains productivity and promotes species diversity on low P soils in the tropics and elsewhere.
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.
The end-Cretaceous event was catastrophic for terrestrial communities worldwide, yet its long-lasting effect on tropical forests remains largely unknown. We quantified plant extinction and ecological ...change in tropical forests resulting from the end-Cretaceous event using fossil pollen (>50,000 occurrences) and leaves (>6000 specimens) from localities in Colombia. Late Cretaceous (Maastrichtian) rainforests were characterized by an open canopy and diverse plant-insect interactions. Plant diversity declined by 45% at the Cretaceous-Paleogene boundary and did not recover for ~6 million years. Paleocene forests resembled modern Neotropical rainforests, with a closed canopy and multistratal structure dominated by angiosperms. The end-Cretaceous event triggered a long interval of low plant diversity in the Neotropics and the evolutionary assembly of today's most diverse terrestrial ecosystem.
Forest soils store large amounts of carbon (C) and nitrogen (N), yet how predicted shifts in forest composition will impact long‐term C and N persistence remains poorly understood. A recent ...hypothesis predicts that soils under trees associated with arbuscular mycorrhizas (AM) store less C than soils dominated by trees associated with ectomycorrhizas (ECM), due to slower decomposition in ECM‐dominated forests. However, an incipient hypothesis predicts that systems with rapid decomposition—e.g. most AM‐dominated forests—enhance soil organic matter (SOM) stabilization by accelerating the production of microbial residues. To address these contrasting predictions, we quantified soil C and N to 1 m depth across gradients of ECM‐dominance in three temperate forests. By focusing on sites where AM‐ and ECM‐plants co‐occur, our analysis controls for climatic factors that covary with mycorrhizal dominance across broad scales. We found that while ECM stands contain more SOM in topsoil, AM stands contain more SOM when subsoil to 1 m depth is included. Biomarkers and soil fractionations reveal that these patterns are driven by an accumulation of microbial residues in AM‐dominated soils. Collectively, our results support emerging theory on SOM formation, demonstrate the importance of subsurface soils in mediating plant effects on soil C and N, and indicate that shifts in the mycorrhizal composition of temperate forests may alter the stabilization of SOM.
We quantified soil carbon (C) and nitrogen (N) to 1‐m depth across gradients of ectomycorrhizal (ECM)‐ vs. arbuscular mycorrhizal (AM)‐associated tree dominance within three temperate broadleaf forests. Contrary to previous hypotheses, we found that AM‐dominated soils store more C and N overall, and more C and N in the putatively most stable pools—deep and mineral‐associated soil organic matter. Our data support the emerging hypothesis that systems with fast decomposition should store more stable soil organic matter.
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 CO
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.
Soil phosphorus (P) availability in lowland tropical rainforests influences the distribution and growth of tropical tree species. Determining the P-acquisition strategies of tropical tree species ...could therefore yield insight into patterns of tree β-diversity across edaphic gradients. In particular, the synthesis of root phosphatases is likely to be of significance given that organic P represents a large pool of potentially available P in tropical forest soils. It has also been suggested that a high root phosphatase activity in putative nitrogen (N) -fixing legumes might explain their high abundance in lowland neotropical forests under low P supply. Here, we measured phosphomonoesterase (PME) activity on the first three root orders of co-occurring tropical tree species differing in their N-fixation capacity, growing on soils of contrasting P availability in Panama. Our results show that root PME activity was higher on average in P-poor than in P-rich soils, but that local variation in PME activity among co-occurring species within a site was larger than that explained by differences in soil P across sites. Legumes expressed higher PME activity than nonlegumes, but nodulated legumes (i.e., actively fixing nitrogen) did not differ from legumes without nodules, indicating that PME activity is unrelated to N fixation. Finally, PME activity declined with increasing root order, but the magnitude of the decline varied markedly among species, highlighting the importance of classifying fine roots into functional groups prior to measuring root traits. Our results support the hypothesis that low-P promotes a high root PME activity, although the high local variation in this trait among co-occurring species points toward a high functional diversity in P-acquisition strategies within an individual community.
Tropical forests are renowned for their high diversity, yet in many sites a single tree species accounts for the majority of the individuals in a stand. An explanation for these monodominant forests ...remains elusive, but may be linked to mycorrhizal symbioses. We tested three hypotheses by which ectomycorrhizas might facilitate the dominance of the tree, Oreomunnea mexicana, in montane tropical forest in Panama. We tested whether access to ectomycorrhizal networks improved growth and survival of seedlings, evaluated whether ectomycorrhizal fungi promote seedling growth via positive plant–soil feedback, and measured whether Oreomunnea reduced inorganic nitrogen availability. We found no evidence that Oreomunnea benefits from ectomycorrhizal networks or plant–soil feedback. However, we found three‐fold higher soil nitrate and ammonium concentrations outside than inside Oreomunnea‐dominated forest and a correlation between soil nitrate and Oreomunnea abundance in plots. Ectomycorrhizal effects on nitrogen cycling might therefore provide an explanation for the monodominance of ectomycorrhizal tree species worldwide.
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