Microbially mediated mechanisms underlie soil carbon accrual by conservation agriculture under decade-long warming.

Increasing soil organic carbon (SOC) in croplands by switching from conventional to conservation management may be hampered by stimulated microbial decomposition under warming. Here, we test the interactive effects of agricultural management and warming on SOC persistence and underlying microbial mechanisms in a decade-long controlled experiment on a wheat-maize cropping system. Warming increased SOC content and accelerated fungal community temporal turnover under conservation agriculture (no tillage, chopped crop residue), but not under conventional agriculture (annual tillage, crop residue removed). Microbial carbon use efficiency (CUE) and growth increased linearly over time, with stronger positive warming effects after 5 years under conservation agriculture. According to structural equation models, these increases arose from greater carbon inputs from the crops, which indirectly controlled microbial CUE via changes in fungal communities. As a result, fungal necromass increased from 28 to 53%, emerging as the strongest predictor of SOC content. Collectively, our results demonstrate how management and climatic factors can interact to alter microbial community composition, physiology and functions and, in turn, SOC formation and accrual in croplands.

A restatement of the natural science evidence base concerning grassland management, grazing livestock and soil carbon storage.

Approximately a third of all annual greenhouse gas emissions globally are directly or indirectly associated with the food system, and over a half of these are linked to livestock production. In temperate oceanic regions, such as the UK, most meat and dairy is produced in extensive systems based on pasture. There is much interest in the extent to which such grassland may be able to sequester and store more carbon to partially or completely mitigate other greenhouse gas emissions in the system. However, answering this question is difficult due to context-specificity and a complex and sometimes inconsistent evidence base. This paper describes a project that set out to summarize the natural science evidence base relevant to grassland management, grazing livestock and soil carbon storage potential in as policy-neutral terms as possible. It is based on expert appraisal of a systematically assembled evidence base, followed by a wide stakeholders engagement. A series of evidence statements (in the appendix of this paper) are listed and categorized according to the nature of the underlying information, and an annotated bibliography is provided in the electronic supplementary material.

Microbial necromass in cropland soils: A global meta-analysis of management effects.

Microbial necromass is a large and persistent component of soil organic carbon (SOC), especially under croplands. The effects of cropland management on microbial necromass accumulation and its contribution to SOC have been measured in individual studies but have not yet been summarized on the global scale. We conducted a meta-analysis of 481-paired measurements from cropland soils to examine the management effects on microbial necromass and identify the optimal conditions for its accumulation. Nitrogen fertilization increased total microbial necromass C by 12%, cover crops by 14%, no or reduced tillage (NT/RT) by 20%, manure by 21%, and straw amendment by 21%. Microbial necromass accumulation was independent of biochar addition. NT/RT and straw amendment increased fungal necromass and its contribution to SOC more than bacterial necromass. Manure increased bacterial necromass higher than fungal, leading to decreased ratio of fungal-to-bacterial necromass. Greater microbial necromass increases after straw amendments were common under semi-arid and in cool climates in soils with pH <8, and were proportional to the amount of straw input. In contrast, NT/RT increased microbial necromass mainly under warm and humid climates. Manure application increased microbial necromass irrespective of soil properties and climate. Management effects were especially strong when applied during medium (3-10 years) to long (10+ years) periods to soils with larger initial SOC contents, but were absent in sandy soils. Close positive links between microbial biomass, necromass and SOC indicate the important role of stabilized microbial products for C accrual. Microbial necromass contribution to SOC increment (accumulation efficiency) under NT/RT, cover crops, manure and straw amendment ranged from 45% to 52%, which was 9%-16% larger than under N fertilization. In summary, long-term cropland management increases SOC by enhancing microbial necromass accumulation, and optimizing microbial necromass accumulation and its contribution to SOC sequestration requires site-specific management.

Mechanisms of nitrogen transfer in a model clover-ryegrass pasture: a 15N-tracer approach.

Purpose: Nitrogen (N) transfer from white clover (Trifolium repens cv.) to ryegrass (Lolium perenne cv.) has the potential to meet ryegrass N requirements. This study aimed to quantify N transfer in a mixed pasture and investigate the influence of the microbial community and land management on N transfer. Methods: Split root 15N-labelling of clover quantified N transfer to ryegrass via exudation, microbial assimilation, decomposition, defoliation and soil biota. Incorporation into the microbial protein pool was determined using compound-specific 15N-stable isotope probing approaches. Results: N transfer to ryegrass and soil microbial protein in the model system was relatively small, with one-third arising from root exudation. N transfer to ryegrass increased with no microbial competition but soil microbes also increased N transfer via shoot decomposition. Addition of mycorrhizal fungi did not alter N transfer, due to the source-sink nature of this pathway, whilst weevil grazing on roots decreased microbial N transfer. N transfer was bidirectional, and comparable on a short-term scale. Conclusions: N transfer was low in a model young pasture established from soil from a permanent grassland with long-term N fertilisation. Root exudation and decomposition were major N transfer pathways. N transfer was influenced by soil biota (weevils, mycorrhizae) and land management (e.g. grazing). Previous land management and the role of the microbial community in N transfer must be considered when determining the potential for N transfer to ryegrass. Supplementary Information: The online version contains supplementary material available at 10.1007/s11104-022-05585-0.

The "Regulator" Function of Viruses on Ecosystem Carbon Cycling in the Anthropocene.

Viruses act as "regulators" of the global carbon cycle because they impact the material cycles and energy flows of food webs and the microbial loop. The average contribution of viruses to the Earth ecosystem carbon cycle is 8.6‰, of which its contribution to marine ecosystems (1.4‰) is less than its contribution to terrestrial (6.7‰) and freshwater (17.8‰) ecosystems. Over the past 2,000 years, anthropogenic activities and climate change have gradually altered the regulatory role of viruses in ecosystem carbon cycling processes. This has been particularly conspicuous over the past 200 years due to rapid industrialization and attendant population growth. The progressive acceleration of the spread and reproduction of viruses may subsequently accelerate the global C cycle.

Soil burial reduces decomposition and offsets erosion-induced soil carbon losses in the Indian Himalaya.

The extent to which soil erosion is a net source or sink of carbon globally remains unresolved but has the potential to play a key role in determining the magnitude of CO2 emissions from land-use change in rapidly eroding landscapes. The effects of soil erosion on carbon storage in low-input agricultural systems, in acknowledged global soil erosion hotspots in developing countries, are especially poorly understood. Working in one such hotspot, the Indian Himalaya, we measured and modelled field-scale soil budgets, to quantify erosion-induced changes in soil carbon storage. In addition, we used long-term (1-year) incubations of separate and mixed soil horizons to better understand the mechanisms controlling erosion-induced changes in soil carbon cycling. We demonstrate that high rates of soil erosion did not promote a net carbon loss to the atmosphere at the field scale. Furthermore, our experiments showed that rates of decomposition in the organic matter-rich subsoil layers in depositional areas were lower per unit of soil carbon than from other landscape positions; however, these rates could be increased by mixing with topsoils. The results indicate that, the burial of soil carbon, and separation from fresh carbon inputs, led to reduced rates of decomposition offsetting potential carbon losses during soil erosion and transport within the cultivated fields. We conclude that the high rates of erosion experienced in these Himalayan soils do not, in isolation, drive substantial emissions of organic carbon, and there is the potential to promote carbon storage through sustainable agricultural practice.

Microbial metabolic response to winter warming stabilizes soil carbon.

Current consensus on global climate change predicts warming trends with more pronounced temperature changes in winter than summer in the Northern Hemisphere at high latitudes. Moderate increases in soil temperature are generally related to faster rates of soil organic carbon (SOC) decomposition in Northern ecosystems, but there is evidence that SOC stocks have remained remarkably stable or even increased on the Tibetan Plateau under these conditions. This intriguing observation points to altered soil microbial mediation of carbon-cycling feedbacks in this region that might be related to seasonal warming. This study investigated the unexplained SOC stabilization observed on the Tibetan Plateau by quantifying microbial responses to experimental seasonal warming in a typical alpine meadow. Ecosystem respiration was reduced by 17%-38% under winter warming compared with year-round warming or no warming and coincided with decreased abundances of fungi and functional genes that control labile and stable organic carbon decomposition. Compared with year-round warming, winter warming slowed macroaggregate turnover rates by 1.6 times, increased fine intra-aggregate particulate organic matter content by 75%, and increased carbon stabilized in microaggregates within stable macroaggregates by 56%. Larger bacterial "necromass" (amino sugars) concentrations in soil under winter warming coincided with a 12% increase in carboxyl-C. These results indicate the enhanced physical preservation of SOC under winter warming and emphasize the role of soil microorganisms in aggregate life cycles. In summary, the divergent responses of SOC persistence in soils exposed to winter warming compared to year-round warming are explained by the slowing of microbial decomposition but increasing physical protection of microbially derived organic compounds. Consequently, the soil microbial response to winter warming on the Tibetan Plateau may cause negative feedbacks to global climate change and should be considered in Earth system models.

Contribution of soil microbial necromass to SOC stocks during vegetation recovery in a subtropical karst ecosystem.

Carbon sequestration is a key soil function, and an increase in soil organic carbon (SOC) is an indicator of ecosystem recovery because it underpins other ecosystem services by acting as a substrate for the soil microbial community. The soil microbial community constitutes the active pool of SOC, and its necromass (microbial residue carbon, MRC) contributes strongly to the stable SOC pool. Therefore, we propose that the potential for restoration of degraded karst ecosystems lies in the abundance of soil microbial community and the persistence of its necromass, and may be measured by changes in its contribution to the active and stable SOC pools during recovery. We investigated changes in SOC stocks using an established space-for-time chronosequence along a perturbation gradient in the subtropical karst ecosystem: sloping cropland < abandoned cropland < shrubland < secondary forest < primary forest. Microbial biomarkers were extracted from soil profiles from surface to bedrock and used to measure the contributions of the soil microbial community composition (using phospholipid fatty acids, PLFAs) and MRC (using amino sugars) to SOC stocks at each recovery stage. The results showed that the SOC stocks ranged from 10.53 to 31.77 kg m-2 and increased with recovery stage, with total MRC accounting for 17-28% of SOC. Increasing PLFAs and MRC abundances were positively correlated with improved soil structure (decreased bulk density) and organic carbon, nitrogen and phosphorus nutrient. Bacterial MRC contributes more to SOC stocks than fungal residue carbon during vegetation recovery. The PLFA analysis indicated that Gram positive bacteria were the largest microbial group and were relatively more abundant in deeper soils, and biomarkers for saprophytic and ectomycorrhizal fungi were more abundant in soils under woody vegetation. In conclusion, this study suggests that the soil microbial community in karst soils have the potential to adapt to changing soil conditions and contribute substantially to building SOC stocks after abandonment of agriculture in degraded karst landscapes.

The persistence of bacterial diversity and ecosystem multifunctionality along a disturbance intensity gradient in karst soil.

Extensive, progressive rock emergence causes localized variations in soil biogeochemical and microbial properties that may influence the capacity for the regeneration of degraded karst ecosystems. It is likely that karst ecosystem recovery relies on the persistence of soil functions at the microbial scale, and we aimed to explored the role of interactions between soil bacterial taxa and identify keystone species that deliver key biogeochemical functions, i.e. carbon (C) and nutrient (nitrogen, N and phosphorus, P) cycling. We applied high-throughput sequencing and phylogenetic molecular ecological network approaches to topsoils sampled at rock-soil interfaces and adjacent bulk soil along an established gradient of land-use intensity in the Chinese Karst Critical Zone Observatory. Bacterial α-diversity was greater under increased perturbation and at the rock-soil interface compared to bulk soils under intensive cultivation. However, bacterial ecological networks were less intricate and connected fewer keystone taxa as human disturbance increased and at the rock-soil interface. Co-occurrence within the bacterial community in natural primary forest soils was 13% larger than cultivated soils. The relative abundances of keystone taxa Acidobacteria, Bacteroidetes and Chloroflexi increased with land-use intensity, while Proteobacteria, Actinobacteria and Verrucomicrobia decreased by up to 6%. In general, Bacteroidetes, Verrucomicrobia and Chlorobi were related to C-cycling, Proteobacteria, Actinobacteria and Chloroflexi were related to N-cycling, and Actinobacteria and Nitrospirae were related to both N- and P-cycling. Proteobacteria and Chlorobi affected C-cycling and multiple functionality indexes in the abandoned land. We conclude that increasing land-use intensity changed the soil bacterial community structure and decreased bacterial interactions. However, increases in α-diversity at the rock-soil interface in cultivated soils indicated that major soil functions related to biogeochemical cycling were maintained within keystone taxa in this microenvironment. Our study provides foundations to test the success of different regeneration practices in restoring soil microbial diversity and the multifunctionality of karst ecosystems.

Lactoferrin quantification in cattle faeces by ELISA.

BACKGROUND: Promoting and maintaining health is critical to ruminant welfare and productivity. Within human medicine, faecal lactoferrin is quantified for routine assessment of various gastrointestinal illnesses avoiding the need for blood sampling. This approach might also be adapted and applied for non-invasive health assessments in animals. METHODS: In this proof-of-concept study, a bovine lactoferrin enzyme-linked immunosorbent assays (ELISA), designed for serum and milk, was applied to a faecal supernatant to assess its potential for quantifying lactoferrin in the faeces of cattle. Faecal lactoferrin concentrations were compared to background levels to assess the viability of the technique. A comparison was then made against serum lactoferrin levels to determine if they were or were not reflective of one another. RESULTS: The optical densities of faecal samples were significantly greater than background readings, supporting the hypothesis that the assay was effective in quantifying faecal lactoferrin (T 13, 115 = 11.99, p < 0.0005). The mean faecal lactoferrin concentration was 0.269 µg mL-1 (S.E. 0.031) and the mean serum concentration 0.074 µg mL-1 (S.E. 0.005). Lactoferrin concentrations of faecal and serum samples, taken from the same animals on the same day, were significantly different (T 21 = 2.20, p = 0.039) and did not correlate (r = 0.2699, p = 0.238). CONCLUSION: Results support the hypothesis that lactoferrin can be quantified in cattle faeces by ELISA. Whilst further research is required to determine the physiological source of the lactoferrin, this highlights the potential of the method for non-invasive assessment of cattle immunology and pathology.

A Previously Undescribed Helotialean Fungus That Is Superabundant in Soil Under Maritime Antarctic Higher Plants.

We report a previously undescribed member of the Helotiales that is superabundant in soils at two maritime Antarctic islands under Antarctic Hairgrass (Deschampsia antarctica Desv.). High throughput sequencing showed that up to 92% of DNA reads, and 68% of RNA reads, in soils from the islands were accounted for by the fungus. Sequencing of the large subunit region of ribosomal (r)DNA places the fungus close to the Pezizellaceae, Porodiplodiaceae, and Sclerotiniaceae, with analyses of internal transcribed spacer regions of rDNA indicating that it has affinities to previously unnamed soil and root fungi from alpine, cool temperate and Low Arctic regions. The fungus was found to be most frequent in soils containing C aged to 1,000-1,200 years before present. The relative abundances of its DNA and RNA reads were positively associated with soil carbon and nitrogen concentrations and δ13C values, with the relative abundance of its DNA being negatively associated with soil pH value. An isolate of the fungus produces flask-shaped phialides with a pronounced venter bearing masses of conidia measuring 4.5-6(7) × 1.8-2.5 µm, suggestive of anamorphic Chalara. Enzymatic studies indicate that the isolate strongly synthesizes the extracellular enzyme acid phosphatase, and also exhibits alkaline phosphatase and naphthol-AS-BI-phosphohydrolase activities. Ecophysiological measurements indicate optimal hyphal growth of the isolate at a pH of 4.2-4.5 and a water potential of -0.66 MPa. The isolate is a psychrotroph, exhibiting measureable hyphal growth at -2°C, optimal hyphal extension rate at 15°C and negligible growth at 25°C. It is proposed that the rising temperatures that are predicted to occur in maritime Antarctica later this century will increase the growth rate of the fungus, with the potential loss of ancient C from soils. Analyses using the GlobalFungi Database indicate that the fungus is present in cold, acidic soils on all continents. We advocate further studies to identify whether it is superabundant in soils under D. antarctica elsewhere in maritime Antarctica, and for further isolates to be obtained so that the species can be formally described.

Long-term nitrogen addition modifies microbial composition and functions for slow carbon cycling and increased sequestration in tropical forest soil.

Nitrogen (N) deposition is a component of global change that has considerable impact on belowground carbon (C) dynamics. Plant growth stimulation and alterations of fungal community composition and functions are the main mechanisms driving soil C gains following N deposition in N-limited temperate forests. In N-rich tropical forests, however, N deposition generally has minor effects on plant growth; consequently, C storage in soil may strongly depend on the microbial processes that drive litter and soil organic matter decomposition. Here, we investigated how microbial functions in old-growth tropical forest soil responded to 13 years of N addition at four rates: 0 (Control), 50 (Low-N), 100 (Medium-N), and 150 (High-N) kg N ha-1  year-1 . Soil organic carbon (SOC) content increased under High-N, corresponding to a 33% decrease in CO2 efflux, and reductions in relative abundances of bacteria as well as genes responsible for cellulose and chitin degradation. A 113% increase in N2 O emission was positively correlated with soil acidification and an increase in the relative abundances of denitrification genes (narG and norB). Soil acidification induced by N addition decreased available P concentrations, and was associated with reductions in the relative abundance of phytase. The decreased relative abundance of bacteria and key functional gene groups for C degradation were related to slower SOC decomposition, indicating the key mechanisms driving SOC accumulation in the tropical forest soil subjected to High-N addition. However, changes in microbial functional groups associated with N and P cycling led to coincidentally large increases in N2 O emissions, and exacerbated soil P deficiency. These two factors partially offset the perceived beneficial effects of N addition on SOC storage in tropical forest soils. These findings suggest a potential to incorporate microbial community and functions into Earth system models considering their effects on greenhouse gas emission, biogeochemical processes, and biodiversity of tropical ecosystems.

Migration and leaching characteristics of base cation: indicating environmental effects on soil alkalinity in a karst area.

In karst areas, rock dissolution often results in the development of underground networks, which act as subterranean pathways for rapid water and nutrient (and possibly soil) loss during precipitation events. Loss of soluble nutrients degrades surface soils and decreases net primary productivity, so it is important to establish flow pathways and quantify nutrient loss during rainfall events of different magnitudes. We conducted a simulated rainfall experiment in karst and nonkarst areas to compare the concentration of nutrients in surface and subsurface flow water and effects on soil alkalinity in three lithologic soil formations under five different rainfall intensity treatments. Compared with the nonkarst area, the runoff in subsurface flows and the proportion of nutrient loss in the subsurface flow are larger in the karst area and less affected by rain intensity. The maximum loss loads of calcium (Ca2+) and magnesium (Mg2+) ions were 32.9 and 19.8 kg ha-1, respectively. With the estimate of base cation loss loads in the China southern karst area under the rainfall intensity of 45 mm h-1, more than 80% of the base cation loss load occurred in the limestone karst area. Although the alkalinity leaching value in nonkarst was similar to that in the karst area under simulated rainfall conditions, its impact on the ecological environment was quite different.

Root imaging showing comparisons in root distribution and ontogeny in novel Festulolium populations and closely related perennial ryegrass varieties.

The incorporation of new sophisticated phenotyping technologies within a crop improvement program allows for a plant breeding strategy that can include selections for major root traits previously inaccessible due to the challenges in their phenotype assessment. High-throughput precision phenotyping technology is employed to evaluate root ontogeny and progressive changes to root architecture of both novel amphiploid and introgression lines of Festulolium over four consecutive months of the growing season and these compared under the same time frame to that of closely related perennial ryegrass (L. perenne) varieties. Root imaging using conventional photography and assembled multiple merged images was used to compare frequencies in root number, their distribution within 0-20 and 20-40 cm depths within soil columns, and progressive changes over time. The Festulolium hybrids had more extensive root systems in comparison with L. perenne, and this was especially evident at depth. It was shown that the acquisition of extensive root systems in Festulolium hybrids was not dependent on the presence of an entire Festuca genome. On the contrary, the most pronounced effect on root development within the four Festulolium populations studied was observed in the introgression line Bx509, where a single small genome sequence from F. arundinacea had been previously transferred onto its homoeologous site on the long arm of chromosome 3 of an otherwise complete L. perenne genome. This demonstrates that a targeted introgression-breeding approach may be sufficient to confer a significant improvement in the root morphology in Lolium without a significant compromise to its genome integrity. The forage production of Bx509 was either higher (months 1-3) or equivalent to (month 4) that of its L. perenne parent control demonstrating that the enhanced root development achieved by the introgression line was without compromise to its agronomic performance.

Nitrogen loss from karst area in China in recent 50 years: An in-situ simulated rainfall experiment's assessment.

Karst topography covers more than 1/3 of the People's Republic of China in area. The porous, fissured, and soluble nature of the underlying karst bedrock (primarily dolomite and limestone) leads to the formation of underground drainage systems. Karst conduit networks dominate this system, and rainfall takes a crucial role on water cycle at China karst area. Nitrogen loss from the karst system is of particular concern, with regard to nutrient use efficiency as well as water quality, as much of the karst system, including steeply sloping terrain, is used for intensive agriculture. We use simulated rainfall experiments to determine the relationship between rainfall and nitrogen loss at typical karst slope land and then estimate nitrogen loss from the karst soil. The results show that both surface runoff and subsurface runoff have a significant linear correlation with rainfall at all studied sites. Subsurface runoff is larger than surface runoff at two karst sites, while the opposite is true at the non-karst site. Exponential function satisfactorily described the correlation between rainfall and nitrogen concentrations in runoff. Nitrates accounted for 60%-95% of the dissolved nitrogen loss (DN, an index of N-loss in this research). The estimated annual N-loss load varies between 1.05 and 1.67 Tg N/year in the whole karst regions of China from 1961 to 2014. Approximately, 90% of the N-loss load occurred during the wet season, and 90% of that passed through the subsurface. Understanding the processes and estimating N-loss is highly valuable in determining long-term soil security and sustainability in karst regions.

Modelling the impact of targeted anthelmintic treatment of cattle on dung fauna.

The insecticidal properties of many anthelmintics pose a risk to dung fauna through the effects of drug residues in dung on the activity, oviposition and development of dung-dwelling invertebrates. Reductions in dung fauna numbers can inhibit dung degradation, which may impact biodiversity and nutrient cycling on farms. A simulation model was created to predict the impact of antiparasitic drugs on cattle dung fauna, and calibrated using published data on the dung-breeding fly Scathophaga stercoraria. This model was then tested under different effective dung drug concentrations (EC) and proportions of treated cattle (PT) to determine the impact under different application regimens. EC accounted for 12.9% of the observed variation in S. stercoraria population size, whilst PT accounted for 54.9%. The model outputs indicate that the tendency within veterinary medicine for targeted selective treatments (TST), in order to attenuate selection for drug resistance in parasite populations, will decrease the negative impacts of treatments on dung fauna populations by providing population refugia. This provides novel evidence for the benefits of TST regimens on local food webs, relative to whole-herd treatments. The model outputs were used to create a risk graph for stakeholders to use to estimate risk of anthelminthic toxicity to dung fauna.

Landscape-scale assessments of stable carbon isotopes in soil under diverse vegetation classes in East Africa: application of near-infrared spectroscopy.

AIMS: Stable carbon isotopes are important tracers used to understand ecological food web processes and vegetation shifts over time. However, gaps exist in understanding soil and plant processes that influence δ13C values, particularly across smallholder farming systems in sub-Saharan Africa. This study aimed to develop predictive models for δ13C values in soil using near infrared spectroscopy (NIRS) to increase overall sample size. In addition, this study aimed to assess the δ13C values between five vegetation classes. METHODS: The Land Degradation Surveillance Framework (LDSF) was used to collect a stratified random set of soil samples and to classify vegetation. A total of 154 topsoil and 186 subsoil samples were collected and analyzed using NIRS, organic carbon (OC) and stable carbon isotopes. RESULTS: Forested plots had the most negative average δ13C values, -26.1‰; followed by woodland, -21.9‰; cropland, -19.0‰; shrubland, -16.5‰; and grassland, -13.9‰. Prediction models were developed for δ13C using partial least squares (PLS) regression and random forest (RF) models. Model performance was acceptable and similar with both models. The root mean square error of prediction (RMSEP) values for the three independent validation runs for δ13C using PLS ranged from 1.91 to 2.03 compared to 1.52 to 1.98 using RF. CONCLUSIONS: This model performance indicates that NIR can be used to predict δ13C in soil, which will allow for landscape-scale assessments to better understand carbon dynamics.

Base cations and micronutrients in soil aggregates as affected by enhanced nitrogen and water inputs in a semi-arid steppe grassland.

The intensification of grassland management by nitrogen (N) fertilization and irrigation may threaten the future integrity of fragile semi-arid steppe ecosystems by affecting the concentrations of base cation and micronutrient in soils. We extracted base cations of exchangeable calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na) and extractable micronutrients of iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) from three soil aggregate sizes classes (microaggregates, <0.25mm; small macroaggregates, 0.25-2mm; large macroaggregates, >2mm) from a 9-yearN and water field manipulation study. There were significantly more base cations (but not micronutrients) in microaggregates compared to macroaggregates which was related to greater soil organic matter and clay contents. Nitrogen addition significantly decreased exchangeable Ca by up to 33% in large and small macroaggregates and exchangeable Mg by up to 27% in three aggregates but significantly increased extractable Fe, Mn and Cu concentrations (by up to 262%, 150%, and 55%, respectively) in all aggregate size classes. However, water addition only increased exchangeable Na, while available Fe and Mn were decreased by water addition when averaging across all N treatments and aggregate classes. The loss of exchangeable Ca and Mg under N addition and extractable Fe and Mn in soil aggregates under water addition might potentially constrain the productivity of this semi-arid grassland ecosystem.

The Role of Microbial Community Composition in Controlling Soil Respiration Responses to Temperature.

Rising global temperatures may increase the rates of soil organic matter decomposition by heterotrophic microorganisms, potentially accelerating climate change further by releasing additional carbon dioxide (CO2) to the atmosphere. However, the possibility that microbial community responses to prolonged warming may modify the temperature sensitivity of soil respiration creates large uncertainty in the strength of this positive feedback. Both compensatory responses (decreasing temperature sensitivity of soil respiration in the long-term) and enhancing responses (increasing temperature sensitivity) have been reported, but the mechanisms underlying these responses are poorly understood. In this study, microbial biomass, community structure and the activities of dehydrogenase and ß-glucosidase enzymes were determined for 18 soils that had previously demonstrated either no response or varying magnitude of enhancing or compensatory responses of temperature sensitivity of heterotrophic microbial respiration to prolonged cooling. The soil cooling approach, in contrast to warming experiments, discriminates between microbial community responses and the consequences of substrate depletion, by minimising changes in substrate availability. The initial microbial community composition, determined by molecular analysis of soils showing contrasting respiration responses to cooling, provided evidence that the magnitude of enhancing responses was partly related to microbial community composition. There was also evidence that higher relative abundance of saprophytic Basidiomycota may explain the compensatory response observed in one soil, but neither microbial biomass nor enzymatic capacity were significantly affected by cooling. Our findings emphasise the key importance of soil microbial community responses for feedbacks to global change, but also highlight important areas where our understanding remains limited.

Not poles apart: Antarctic soil fungal communities show similarities to those of the distant Arctic.

Antarctica's extreme environment and geographical isolation offers a useful platform for testing the relative roles of environmental selection and dispersal barriers influencing fungal communities. The former process should lead to convergence in community composition with other cold environments, such as those in the Arctic. Alternatively, dispersal limitations should minimise similarity between Antarctica and distant northern landmasses. Using high-throughput sequencing, we show that Antarctica shares significantly more fungi with the Arctic, and more fungi display a bipolar distribution, than would be expected in the absence of environmental filtering. In contrast to temperate and tropical regions, there is relatively little endemism, and a strongly bimodal distribution of range sizes. Increasing southerly latitude is associated with lower endemism and communities increasingly dominated by fungi with widespread ranges. These results suggest that micro-organisms with well-developed dispersal capabilities can inhabit opposite poles of the Earth, and dominate extreme environments over specialised local species.