Sequestration of P fractions in the soils of an incipient ferralisation chronosequence on a humid tropical volcanic island

Background Phosphorus (P) is the limiting nutrient in many mature tropical forests. The ecological significance of declining P stocks as soils age is exacerbated by much of the remaining P being progressively sequestered. However, the details of how and where P is sequestered during the ageing in tropical forest soils remains unclear. Results We examined the relationships between various forms of the Fe and Al sesquioxides and the Hedley fractions of P in soils of an incipient ferralitic chronosequence on an altitudinal series of gently sloping benches on Green Island, off the southeastern coast of Taiwan. These soils contain limited amounts of easily exchangeable P. Of the sesquioxide variables, only Fe and Al crystallinities increased significantly with bench altitude/soil age, indicating that the ferralisation trend is weak. The bulk of the soil P was in the NaOH and residual extractable fractions, and of low lability. The P fractions that correlated best with the sesquioxides were the organic components of the NaHCO3 and NaOH extracts. Conclusions The amorphous sesquioxides, Feo and Alo, were the forms that correlated best with the P fractions. A substantial proportion of the labile P appears to be organic and to be associated with Alo in organic-aluminium complexes. The progression of P sequestration appears to be slightly slower than the chemical and mineralogical indicators of ferralisation.

A multiple model machine learning approach for soil classification from cone penetration test data

The most popular methods for soil classification from cone penetration test (CPT) data are based on examining two-dimensional charts. In the last years, several authors have dedicated efforts on replicating and discussing these methods using machine learning techniques. Nonetheless, most of them apply few techniques, include only one dataset and do not explore more than three input features. This work circumvents these issues by: (i) comparing five different machine learning techniques, which are also combined in an ensemble; (ii) using three distinct CPT datasets, one composed of 111 soundings from different countries, one composed of 38 soundings with information of soil age and the third composed of 64 soundings taken from the city of São Paulo, Brazil; and (iii) testing combinations of five input features. Results show that, in most cases, the ensemble of multiple models achieves better predictive performance than any technique isolated. Accuracies close to the maximum were obtained in some cases without the need of pore pressure information, which is costly to measure in geotechnical practice.

Vegetation structure determines cyanobacterial communities during soil development across global biomes

Soil cyanobacteria play essential ecological roles and are known to experience large changes in their diversity and abundance throughout early succession. However, much less is known about how and why soil cyanobacterial communities change as soil develops from centuries to millennia, and the effects of aboveground vegetation on these communities. We combined an extensive field survey including 16 global soil chronosequences across contrasting ecosystems (from deserts to tropical forests) with molecular analyses to investigate how the diversity and abundance of soil cyanobacteria under vegetation change during soil development from hundreds to thousands of years. We show that, in most chronosequences, the abundance, species richness and community composition of soil cyanobacteria were relatively stable as soil develops (from centuries to millennia). Regardless of soil age, forest chronosequences were consistently dominated by non-photosynthetic cyanobacteria (Vampirovibrionia), while grasslands and shrublands were dominated by photosynthetic cyanobacteria. Chronosequences undergoing drastic vegetation shifts during soil development (e.g. transitions from grasslands to forests) experienced significant changes in the composition of soil cyanobacteria communities. Our results advance our understanding of the ecology of cyanobacterial classes, specially the understudied non-photosynthetic ones and highlight the key role of vegetation as a major driver of their temporal dynamics as soil develops.

Soil age and soil organic carbon content shape biochemical responses to multiple freeze–thaw events in soils along a postmining agricultural chronosequence

Freeze–thaw (FT) events exert a great physiological stress on the soil microbial community and thus significantly impact soil biogeochemical processes. Studies often show ambiguous and contradicting results, because a multitude of environmental factors affect biogeochemical responses to FT. Thus, a better understanding of the factors driving and regulating microbial responses to FT events is required. Soil chronosequences allow more focused comparisons among soils with initially similar start conditions. We therefore exposed four soils with contrasting organic carbon contents and opposing soil age (i.e., years after restoration) from a postmining agricultural chronosequence to three consecutive FT events and evaluated soil biochgeoemical responses after thawing. The major microbial biomass carbon losses occurred after the first FT event, while microbial biomass N decreased more steadily with subsequent FT cycles. This led to an immediate and lasting decoupling of microbial biomass carbon:nitrogen stoichiometry. After the first FT event, basal respiration and the metabolic quotient (i.e., respiration per microbial biomass unit) were above pre-freezing values and thereafter decreased with subsequent FT cycles, demonstrating initially high dissimilatory carbon losses and less and less microbial metabolic activity with each iterative FT cycle. As a consequence, dissolved organic carbon and total dissolved nitrogen increased in soil solution after the first FT event, while a substantial part of the liberated nitrogen was likely lost through gaseous emissions. Overall, high-carbon soils were more vulnerable to microbial biomass losses than low-carbon soils. Surprisingly, soil age explained more variation in soil chemical and microbial responses than soil organic carbon content. Further studies are needed to dissect the factors associated with soil age and its influence on soil biochemical responses to FT events.

Microbial phylogenetic relatedness links to distinct successional patterns of bacterial and fungal communities

Development of soil microbial communities along ecological succession is crucial for ecosystem functioning and maintenance. However, ecological processes mediating microbial community assembly and microbial co-occurrence patterns along ecological succession remain unclear. Here, we explored community phylogenetic structures, ecological processes driving community phylogenetic turnover, and taxa co-occurrence patterns in bacterial and fungal communities across a well-established chronosequence of post-mining lands spanning 54 years of recovery. Meanwhile, by synthesizing prior studies of microbial phylogeny in community assembly, we proposed two conceptual models to better explain our results. At early successional stages, the significantly increasing phylogenetic clustering of bacterial communities with soil age was co-determined by the environmental selection from soil vegetation cover and by bacterial heterogeneous responses that less phylogenetically similar bacteria differently expanded their population in response to the increasing resource availability in soil along succession. At later successional stages, bacterial community phylogenetic structures displayed progressively lower variability. The fungal community phylogenetic structures varied relatively less and were independent of soil age, soil properties and vegetation cover, which was attributed to the dominance of stochastic processes in community structure turnover along succession. Network analysis revealed a decrease in bacterial co-occurrence complexity along succession, which aligned with a decrease in average pairwise phylogenetic distances between co-occurring bacteria. These patterns together implied a decrease in potential bacterial cooperation that was probably mediated by increasing resource availability along succession. The increased complexity of fungal co-occurrence along succession was independent of the phylogeny between co-occurring fungi. This study provides new sights into ecological processes and mechanisms underlying bacterial and fungal community dynamics along ecological succession, thereby boosting our understanding of the interactions between microbial community assembly and soil environment gradients.

Agriculture effects on geochemical soil properties and stability of soil organic carbon on tropical Andosols

<p>The impact of soil age on geochemical properties and carbon cycling has been studied via chronosequences. However, only few studies have addressed how land-use and soil age might interactively shape properties of Andosols and in turn their capability to retain organic carbon (OC). Geochemical soil analyses and laboratory incubation experiments were carried out to assess soil characteristics and mineralization of soil organic carbon (SOC) in Indonesian soils with two contrasting land uses, viz. pine forest and horticulture. Both of these land uses are the results of conversion of primary forest which had similar parent materials, soil age, as well as weathering intensity. Results showed that intensive agricultural practices (+ 40-50 years) did not result in a significant loss of SOC or the increase of bulk density compared to forest. On the other hand, they were found to increase pH, exchangeable cations, base saturation, and most strikingly non-crystalline materials (i.e. Al<sub>o</sub> + ½ Fe<sub>o</sub>) leading to phenotype formation in agricultural soils. Positive correlations were found between non-crystalline materials with properties such as soil specific surface area and micropores volume, and it was also positively correlated with SOC, particularly in the subsoil. This study highlighted the resilience of Andosols to soil degradation under agricultural practices and its ability to stabilize SOC.</p>

Long-term silicon dynamics in terrestrial ecosystems: insights from 2-million years soil chronosequences

<p>Silicon (Si) is widely recognized as an important regulator of the global carbon (C) cycle via its effect on diatom productivity in oceans and the weathering of silicate minerals on continents. Si is also a beneficial plant nutrient, improving resistance to herbivory and pathogens and mitigating the negative effects of several abiotic stresses, including nutrient limitation. However, changes in Si sources and cycling during long-term development of terrestrial ecosystems remain poorly understood. We studied Si in soils and plants along two 2-Ma coastal dune chronosequences in southwestern Australia (Jurien Bay and Guilderton). Soil development along these chronosequences includes carbonate leaching in Holocene soils, formation of secondary Si-bearing minerals in Mid-Pleistocene soils, followed by their loss via dissolution, to yield quartz-rich soils of Early-Pleistocene age. The chronosequences also exhibit an extreme gradient of soil fertility in terms of rock-derived nutrients, and shifts from nitrogen (N) to phosphorus (P) limitation of plant productivity as soils age. Along each chronosequence, we quantified the pools of reactive Si-bearing phases and plant-available Si in the soils, and physically extracted soil phytoliths (amorphous silica formed in plant tissues). We also quantified Si, macronutrients and total phenols in the most abundant plants growing along the best-studied of the two chronosequences (Jurien Bay). We found that plant-available Si was lowest in young and carbonate-rich soils, because carbonates weathering reduces the weathering of silicate minerals by consuming protons, and Si is strongly sorbed by secondary minerals in alkaline soils. Plant-available Si increased in intermediate-age soils during the formation of secondary minerals (kaolinite), and finally decreased in old, quartz-rich soils, due to continuous desilication. As pedogenic Si pools became depleted with increasing soil age, Si availability was increasingly determined by soil phytoliths. At Jurien Bay, foliar Si increased continuously as soils aged, in contrast with foliar macronutrients that declined markedly in strongly weathered soils. Finally, foliar phenol concentrations declined with increasing soil age and were negatively correlated with foliar Si at the community and individual species level, suggesting a tradeoff between these two leaf defense strategies. Our results highlight a nonlinear response of plant-available Si to long-term pedogenesis, with an increase during carbonate loss and a decrease in the silicates weathering domain. They also demonstrate that the retention of Si by plants during ecosystem retrogression sustains its terrestrial cycling by leveraging the high reactivity of soil phytoliths compared with soil-derived aluminosilicates. Moreover, the continuous increase of plant Si concentrations as rock-derived nutrients are depleted suggests important plant benefits associated with Si in P-impoverished environments. This is in line with the resource availability hypothesis, which predicts that plants adapted to infertile soils have high levels of anti-herbivore leaf defenses. In particular, old and P-depleted soils increased the relative expression of Si-based defenses, while young soils where plant productivity is limited by N promoted leaf phenol accumulation. Overall, our results demonstrate that long-term ecosystem and soil development strongly influence soil-plant Si dynamics, with cascading effects on plant ecology and global Si and C biogeochemistry.</p>

Diversity of putative ericoid mycorrhizal fungi increases with soil age and progressive phosphorus limitation across a 4.1 million-year chronosequence

Ericaceous plants rely on ericoid mycorrhizal fungi for nutrient acquisition. However, the factors that affect the composition and structure of fungal communities associated with the roots of ericaceous plants remain largely unknown. Here, we use a 4.1-myr soil chronosequence in Hawaii to test the hypothesis that changes in nutrient availability with soil age determine the diversity and species composition of fungi associated with ericoid roots. We sampled roots of a native Hawaiian plant, Vaccinium calycinum, and used DNA metabarcoding to quantify changes in fungal diversity and community composition. We also used a fertilization experiment at the youngest and oldest sites to assess the importance of nutrient limitation. We found an increase in diversity and a clear pattern of species turnover across the chronosequence, driven largely by putative ericoid mycorrhizal fungi. Fertilization with nitrogen at the youngest site and phosphorus at the oldest site reduced fungal diversity, suggesting a direct role of nutrient limitation. Our results also reveal the presence of novel fungal species associated with Hawaiian Ericaceae and suggest a greater importance of phosphorus availability for communities of ericoid mycorrhizal fungi than is generally assumed.