A symbolic Neanderthal accumulation of large herbivore crania.

This work examines the possible behaviour of Neanderthal groups at the Cueva Des-Cubierta (central Spain) via the analysis of the latter's archaeological assemblage. Alongside evidence of Mousterian lithic industry, Level 3 of the cave infill was found to contain an assemblage of mammalian bone remains dominated by the crania of large ungulates, some associated with small hearths. The scarcity of post-cranial elements, teeth, mandibles and maxillae, along with evidence of anthropogenic modification of the crania (cut and percussion marks), indicates that the carcasses of the corresponding animals were initially processed outside the cave, and the crania were later brought inside. A second round of processing then took place, possibly related to the removal of the brain. The continued presence of crania throughout Level 3 indicates that this behaviour was recurrent during this level's formation. This behaviour seems to have no subsistence-related purpose but to be more symbolic in its intent.

Soil bacterial communities remain altered after 30 years of agriculture abandonment in Pampa grasslands.

Old fields are spreading in the world because of agriculture abandonment, and they show a persistence of exotic plant species with little recovery towards the original vegetation composition. Soil biota may also differ between old fields and native grasslands, but were comparatively less studied than plant communities, despite their importance in biogeochemical processes. Here we compared soil bacterial communities of exotic-dominated old fields with those of remnants of native grasslands in the Inland Pampa, Argentina, using the 16S rRNA gene amplicon sequencing approach. We also characterized plant communities, soil physico-chemical properties, and soil respiration. We expected more diverse soil bacterial communities, with higher heterogeneity, in remnant grasslands than in old fields because of a more diverse and more heterogeneous plant community. However, our results showed that soil bacterial communities had higher Shannon diversity in old fields than in remnant grasslands, but richness was not significantly different. Also we found different bacterial community compositions between grasslands even at a low taxonomic level. On the other hand, old fields harbored less heterogeneous bacterial communities than remnants, and bacteria and plant beta diversity were correlated. Despite contrasting plant and bacterial composition between old fields and remnant grasslands, soil physico-chemical properties were quite similar between grasslands. Overall, our results showed that bacterial communities in grassland soils were associated with changes in plant communities after agricultural abandonment. Plant-microbial feedbacks might regulate plant and soil bacterial community assemblage in old fields, yet further research is needed to demonstrate this potential feedback mechanism.

Sunlight Doubles Aboveground Carbon Loss in a Seasonally Dry Woodland in Patagonia.

Photodegradation of aboveground senescent plant material (plant litter) due to exposure to solar radiation has been identified as a dominant control on carbon (C) loss in semi-arid ecosystems [1], upturning traditional models of C cycling based only on available moisture and litter quality. In addition to the photochemical mineralization of organic matter [1, 2], sunlight alters the chemistry of cell walls in plant litter [3, 4], making them more susceptible to subsequent biotic degradation [5-7]. Nevertheless, the interactive effects of sunlight exposure, climate seasonality, and biotic decomposition on C turnover remain unresolved in terrestrial ecosystems. We show here that exposure to sunlight accelerated litter decomposition in a Patagonian woodland with a marked dry summer season. Controls on initial decomposition varied seasonally from direct photochemical mineralization in the dry summer to biotic degradation in the wet winter. By manipulating sunlight received by plant litter using spectral filters that attenuated ultraviolet and short-wave visible light, we demonstrate that direct photodegradation and its legacy, associated with increased microbial access to labile carbohydrates, are responsible for the acceleration of aboveground C turnover in this Mediterranean-type climate. Across plant species and over a 2-year period, litter exposed to the full solar spectrum decomposed twice as fast as litter that received attenuated sunlight. Changes in vegetation cover or biodiversity due to projected increased drought and dry season length [8] will likely exacerbate C losses from aboveground litter due to sunlight exposure, negatively impacting the C balance in ecosystems that are particularly vulnerable to global change [9].

Plant, fungal, bacterial, and nitrogen interactions in the litter layer of a native Patagonian forest.

Plant-microbial interactions in the litter layer represent one of the most relevant interactions for biogeochemical cycling as litter decomposition is a key first step in carbon and nitrogen turnover. However, our understanding of these interactions in the litter layer remains elusive. In an old-growth mixed Nothofagus forest in Patagonia, we studied the effects of single tree species identity and the mixture of three tree species on the fungal and bacterial composition in the litter layer. We also evaluated the effects of nitrogen (N) addition on these plant-microbial interactions. In addition, we compared the magnitude of stimulation of litter decomposition due to home field advantage (HFA, decomposition occurs more rapidly when litter is placed beneath the plant species from which it had been derived than beneath a different plant species) and N addition that we previously demonstrated in this same forest, and used microbial information to interpret these results. Tree species identity had a strong and significant effect on the composition of fungal communities but not on the bacterial community of the litter layer. The microbial composition of the litter layer under the tree species mixture show an averaged contribution of each single tree species. N addition did not erase the plant species footprint on the fungal community, and neither altered the bacterial community. N addition stimulated litter decomposition as much as HFA for certain tree species, but the mechanisms behind N and HFA stimulation may have differed. Our results suggest that stimulation of decomposition from N addition might have occurred due to increased microbial activity without large changes in microbial community composition, while HFA may have resulted principally from plant species' effects on the litter fungal community. Together, our results suggest that plant-microbial interactions can be an unconsidered driver of litter decomposition in temperate forests.

Nonlinear responses in salt marsh functioning to increased nitrogen addition.

Salt marshes provide storm protection to shorelines, sequester carbon (C), and mitigate coastal eutrophication. These valuable coastal ecosystems are confronted with increasing nitrogen (N) inputs from anthropogenic sources, such as agricultural runoff, wastewater, and atmospheric deposition. To inform predictions of salt marsh functioning and sustainability in the future, we characterized the response of a variety of plant, microbial, and sediment responses to a seven-level gradient of N addition in three Californian salt marshes after 7 and 14 months of N addition. The marshes showed variable responses to the experimental N gradient that can be grouped as neutral (root biomass, sediment respiration, potential carbon mineralization, and potential net nitrification), linear (increasing methane flux, decreasing potential net N mineralization, and increasing sediment inorganic N), and nonlinear (saturating aboveground plant biomass and leaf N content, and exponentially increasing sediment inorganic and organic N). The three salt marshes showed quantitative differences in most ecosystem properties and processes rates; however, the form of the response curves to N addition were generally consistent across the three marshes, indicating that the responses observed may be applicable to other marshes in the region. Only for sediment properties (inorganic and organic N pool) did the shape of the response differ significantly between marshes. Overall, the study suggests salt marshes are limited in their ability to sequester C and N with future increases in N, even without further losses in marsh area.

There's no place like home? An exploration of the mechanisms behind plant litter-decomposer affinity in terrestrial ecosystems.

Litter decomposition in terrestrial ecosystems is an important first step for carbon and nutrient cycling, as senescent plant material is degraded and consequently incorporated, along with microbial products, into soil organic matter. The identification of litter affinity effects, whereby decomposition is accelerated in its home environment (home-field advantage, HFA), highlights the importance of plant-soil interactions that have consequences for biogeochemical cycling. While not universal, these affinity effects have been identified in a range of ecosystems, particularly in forests without disturbance. The optimization of the local decomposer community to degrade a particular combination of litter traits is the most oft-cited explanation for HFA effects, but the ways in which this specialized community can develop are only beginning to be understood. We explore ways in which HFA, or more broadly litter affinity effects, could arise in terrestrial ecosystems. Plant-herbivore interactions, microbial symbiosis, legacies from phyllosphere communities and attractors of specific soil fauna could contribute to spatially defined affinity effects for litter decomposition. Pyrosequencing soil communities and functional linkages of soil fauna provide great promise in advancing our mechanistic understanding of these interactions, and could lead to a greater appreciation of the role of litter-decomposer affinity in the maintenance of soil functional diversity.

The effect of nitrogen enrichment on c(1)-cycling microorganisms and methane flux in salt marsh sediments.

Methane (CH(4)) flux from ecosystems is driven by C(1)-cycling microorganisms - the methanogens and the methylotrophs. Little is understood about what regulates these communities, complicating predictions about how global change drivers such as nitrogen enrichment will affect methane cycling. Using a nitrogen addition gradient experiment in three Southern California salt marshes, we show that sediment CH(4) flux increased linearly with increasing nitrogen addition (1.23 µg CH(4) m(-2) day(-1) for each g N m(-2) year(-1) applied) after 7 months of fertilization. To test the reason behind this increased CH(4) flux, we conducted a microcosm experiment altering both nitrogen and carbon availability under aerobic and anaerobic conditions. Methanogenesis appeared to be both nitrogen and carbon (acetate) limited. N and C each increased methanogenesis by 18%, and together by 44%. In contrast, methanotrophy was stimulated by carbon (methane) addition (830%), but was unchanged by nitrogen addition. Sequence analysis of the sediment methylotroph community with the methanol dehydrogenase gene (mxaF) revealed three distinct clades that fall outside of known lineages. However, in agreement with the microcosm results, methylotroph abundance (assayed by qPCR) and composition (assayed by terminal restriction fragment length polymorphism analysis) did not vary across the experimental nitrogen gradient in the field. Together, these results suggest that nitrogen enrichment to salt marsh sediments increases methane flux by stimulating the methanogen community.

Intrinsic effects of species on leaf litter and root decomposition: a comparison of temperate grasses from North and South America.

Plant species affect natural ecosystems through interactions between environmental and genetic factors. The importance of plant species in controlling decomposition is now well-established through its influence on litter quality, which affects mass loss and nutrient release. At the same time, direct species effects are often confounded with indirect site effects due to the ecophysiological responses of plants to environmental variability. We evaluated the intrinsic effects of species on litter quality and decomposition, comparing 14 native perennial grass species from three different grassland ecosystems in North and South America. Plants were grown under controlled greenhouse conditions to eliminate any indirect effects of climate on litter quality, and senescent material of leaf litter and roots were collected. The initial litter nutrient quality and the carbon quality were assessed, and decomposition was determined over a period of one year by placing litterbags in a common grassland site. In spite of constant growth conditions, species' litter showed broad and significant differences in N, P and lignin concentration, as well as C:N ratio, with the greatest differences occurring between C(3) and C(4) species and leaf litter and root material. In addition, decomposition was significantly different among species and between leaf litter and roots within species, with constants (k) ranging from 1.50 to 3.65 year(-1) for leaf litter, and 0.51-1.82 year(-1) for roots. These results highlight the fact that, independent of climate or edaphic changes due to human activity, changes in plant species or in allocation patterns among plant organs in grassland ecosystems could have a large effect on carbon turnover. At the same time, the way in which intrinsic species characteristics affect decomposition demonstrates a large degree of functional convergence among species from grasslands of North and South America.

Plant litter decomposition in a semi-arid ecosystem controlled by photodegradation.

The carbon balance in terrestrial ecosystems is determined by the difference between inputs from primary production and the return of carbon to the atmosphere through decomposition of organic matter. Our understanding of the factors that control carbon turnover in water-limited ecosystems is limited, however, as studies of litter decomposition have shown contradictory results and only a modest correlation with precipitation. Here we evaluate the influence of solar radiation, soil biotic activity and soil resource availability on litter decomposition in the semi-arid Patagonian steppe using the results of manipulative experiments carried out under ambient conditions of rainfall and temperature. We show that intercepted solar radiation was the only factor that had a significant effect on the decomposition of organic matter, with attenuation of ultraviolet-B and total radiation causing a 33 and 60 per cent reduction in decomposition, respectively. We conclude that photodegradation is a dominant control on above-ground litter decomposition in this semi-arid ecosystem. Losses through photochemical mineralization may represent a short-circuit in the carbon cycle, with a substantial fraction of carbon fixed in plant biomass being lost directly to the atmosphere without cycling through soil organic matter pools. Furthermore, future changes in radiation interception due to decreased cloudiness, increased stratospheric ozone depletion, or reduced vegetative cover may have a more significant effect on the carbon balance in these water-limited ecosystems than changes in temperature or precipitation.