Soil nitrogen mineralisation (N
), the conversion of organic into inorganic N, is important for productivity and nutrient cycling. The balance between mineralisation and immobilisation (net N
) ...varies with soil properties and climate. However, because most global-scale assessments of net N
are laboratory-based, its regulation under field-conditions and implications for real-world soil functioning remain uncertain. Here, we explore the drivers of realised (field) and potential (laboratory) soil net N
across 30 grasslands worldwide. We find that realised N
is largely explained by temperature of the wettest quarter, microbial biomass, clay content and bulk density. Potential N
only weakly correlates with realised N
, but contributes to explain realised net N
when combined with soil and climatic variables. We provide novel insights of global realised soil net N
and show that potential soil net N
data available in the literature could be parameterised with soil and climate data to better predict realised N
.
Coupling experiments at small spatial scales with large-scale surveys can help to generalize experimental results across large spatial scales. The goal of the present study was to evaluate patterns ...of crab herbivory within and, at a larger scale, between many southwestern (SW) Atlantic salt marshes. Hence, we conducted experiments in an Argentinean salt marsh to elucidate the effects of crabChasmagnathus granulatusgrazing on the cordgrassSpartina densifloraand the factors that can affect this interaction, and then examined the potential generality of these results across SW Atlantic salt marshes from Brazil to Argentina (15 marshes, range ≈ 2000 km). Experimental examination of the effects of crabs (control and exclusion) on marsh grass transplants, and factors that can affect them, i.e. the presence or absence of plant neighbors and marsh height (middle and low), revealed that crab herbivory decreased plant biomass and increased stem mortality; however, herbivory effects were significantly diminished in the presence of plant neighbors and with increasing marsh elevation. Our geographical survey showed that crab herbivory is common in SW Atlantic salt marshes, with more than 20% of leaves damaged in most marshes and with greater consumption at marshes with higher crab densities. In addition, plants at the lower edge of marshes were generally the most consumed (max. >60% leaves consumed) and crabs preferredS. alternifloraoverS. densiflora. Over a regional spatial scale, our results suggest that herbivory may affect plant production at some marshes and can also play a role in limiting the lower tidal elevation limit of low-marsh plants.
The stress-gradient hypothesis predicts that interactions among plants are context dependent, shifting from facilitation to competition as environmental stress decreases. Although restricted to ...facilitation/competition, the mechanistic model behind the hypothesis is easily modified to include other negative interactions that are as important as competition in structuring natural communities, e.g., herbivory. To evaluate this hypothesis we experimentally tested if the balance between the facilitative and trophic effect of an intertidal, burrowing, herbivorous crab in marsh plants is context dependent and shifts from positive to negative as stress decreases. By sampling salt marshes differing in sediment size characteristics, we show that sites with larger sediment particle size had less stressful oxygen levels than sites with fine sediment particles, and that the level of stress was reduced by the presence of crab burrows. We then conducted a factorial experiment manipulating sediment size and crab presence. Results show that, by decreasing soil anoxic stress, crabs increase plant growth in stressful zones, but their ecological importance as herbivores increases in more benign zones. Our findings suggest that the balance between positive and negative interactions along stress gradients is more important than previously perceived and also applies to facilitation and herbivory between animals and plants.
The stress gradient hypothesis states that positive interactions become more important as physical stress increases.Organisms living in Patagonian intertidal ecosystems are subjected to high ...desiccation stress due to elevated temperatures and strong winds. In coastal environments, sponges and macroalgae are common benthic organisms, and they occur from the subtidal to intertidal zones. As human pressure has increased in these systems, both sponges and macroalgae have become more abundant, which has led to more frequent interactions between them. To evaluate the interaction between Hymeniacidon perlevis and Ulva lactuca in 2 contrasting environments, we sampled their abundance and conducted field factorial experiments consisting of sponge transplants and manipulation of algal presence. Our results show that in harsher environments, U. lactuca facilitates H. perlevis likely through the reduction of direct solar radiation or high substrate temperature beneath its canopy, whereas in milder environments, U. lactuca negatively affects H. perlevis throughout other physical or chemical mechanisms (e.g. abrasion, feeding interference through changing current dynamics, depletion of oxygen, allelochemical compound production).
Changes in rainfall patterns caused by anthropogenic global climate change or planetary-scale events, such as the El Niño Southern Oscillation, can significantly affect the abundance and distribution ...of organisms. Despite the evidence of such effects on marine and terrestrial systems, ecological consequences of rainfall fluctuations in coastal marine ecosystems remain poorly understood. Here we evaluate the effects of rainfall intensity on the interaction between the cordgrass Spartina densiflora and Azara’s grass mouse Akodon azarae in a southwestern Atlantic salt marsh (Mar Chiquita coastal lagoon, Argentina). Field surveys showed that the abundance of A. azarae increased during rainy summers (i.e. El Niño 2005 and 2007) and had lowest values during dry summers (i.e. La Niña 2008). Salt content in sediment and plant tissue were negatively related with rainfall. In addition, field experiments showed that increased sediment salinity resulted in increased salt content in plant tissues. Elevated soil salinity also increased the proportion of senescent S. densiflora tissues and reduced plant growth. The consumption of S. densiflora leaves by A. azarae also decreased with increased soil salinity. The proportion of S. densiflora in feces collected during the driest summer was very small. Therefore, changes in the abundance of A. azarae could be mediated by plant–rodent trophic interaction or by plant cover changes. In conclusion, rainfall fluctuations changed the abiotic environment (i.e. salinity), decreasing primary production and indirectly modifying habitat use by the omnivore A. azarae and its trophic interaction with S. densiflora. The present study provides evidence that rainfall can modify ecological processes that affect the structure and dynamics of coastal marine ecosystems.
Salt marshes are among the most productive systems of the world, with plant primary production limited by soil oxygen deficiency and nutrient availability. Nevertheless, root adaptations to anoxia ...and nutrient acquisition are different and often incompatible. The SW Atlantic salt marshes are characterized by high densities of the deep (up to 1 m) burrowing crab
Neohelice granulata (Dana, 1851) that may change soil physical and chemical characteristics by burrow construction. In this work, we experimentally evaluated the hypothesis that crab burrowing can enhance soil oxygenation, causing changes in
Spartina densiflora Brongniart below ground tissues from structures adapted to anoxia to systems adapted for nutrient acquisition. This response, in turn, would enhance plant productivity. Results from field observations show that oxygen availability is higher in zones with high burrow densities. As burrow densities increased, the plant root distribution changed from shallow (associated to low oxygen availability) to deeper and vertically homogeneous, with a positive correlation between burrow density and plant aboveground biomass. Experimental exclusion of crabs shows that they induce changes of root strategies from anoxia toleration to nutrient efficient acquisition, with increasing plant productivity. The invasive success that this plant shows in other parts of the world is likely to be due to their ability to tolerate harsh environmental conditions. Our results suggest that the morphological plasticity of
S. densiflora is also important in their native zone given the characteristics of their specific habitat.
Colonization of bare patches is a key process during community development given that pioneers usually have positive and negative effects on the forthcoming species. A variety of biotic and abiotic ...factors influence the process of colonization of bare patches. In salt marshes, however, the emphasis has been on abiotic factors and plant-plant interactions, while comparatively little attention has been paid to the role of plant-animal interactions in the successful colonization by pioneer plants. Thus, the goal of the present study was to evaluate whether bioturbation and herbivory by the burrowing crab Neohelice (Chasmagnathus) granulata affected the number of seedlings of the pioneer plant Sarcocornia perennis in SW Atlantic salt marshes. To evaluate this, we conducted an experiment using exclosures deployed at different times (post-dispersal and post-germination). The results showed that post-germination exclosures had 62% fewer seedlings than post-dispersal exclosures but 8 times more than plots always accessible to crabs. We also used glass beads to experimentally evaluate the potential effect of crab bioturbation on seed availability, and we used 1 yr old transplants to evaluate whether herbivory could explain post-germination mortality. Crab bioturbation reduced the number of glass beads on the surface by 56%, and transplants were highly consumed when crabs were present. These results suggest that seed burial by bioturbation exerts a pre-germination control while herbivory exerts a post-germination control. The results also highlight the importance of considering biotic factors when analyzing the success of marsh plants colonizing bare surfaces.
The Río de la Plata, one of the most important South American estuarine environments, is characterized by a bottom salinity front that generates an ecotone between the river and the estuary. Based on ...bottom trawls and costal sampling we describe the distribution, types, and amount of debris found in the bottom and shoreline across this front. Plastics and plastic bags were the main debris types in both areas. Concentrations of total debris upriver the front were always significantly higher than downriver the front showing that the front acts as a barrier accumulating debris. Moreover, a large part of debris end ups accumulated in the coastal area upriver the frontal position. This area is particularly sensitive because the coastline encompasses an UNESCO Man and the Biosphere Reserve and a Ramsar site, and due to the ecological significance of the front for many valuable species.
Some studies have shown that the balance between top-down and bottom-up processes is context dependent, but few have tested how biotic interactions can affect this balance. We quantified the attack ...frequencies by the stem borer mothHaimbachiasp. nov. on the cordgrassesSpartina densifloraandS. alterniflorain 5 marshes of the southwestern Atlantic coastline, located between 36° S and 41° S. We examined whether ecosystem engineering by burrowing crabsNeohelice(Chasmagnathus)granulata, which improves plant performance, increases herbivory incidence by moths. The results show that moths attack an important proportion ofSpartinaspp. stems (4% to 26%). Moth attack frequencies were higher on low elevation than on high elevation marsh plants, probably due to variations in plant performance across the physical stress gradient. Within marshes, burrowing crab densities and moth attack frequencies were positively correlated, suggesting that ecosystem engineering by crabs may increase moth attacks. Field experiments confirmed this prediction and suggest that the effect of crabs on moth attacks may be driven by nutrient availability. Our results show that stem-boring herbivores can cause significant plant mortality inSpartinaspp. marshes, and that ecosystem engineering by burrowing crabs increases this effect. Thus, in this system, biotic factors that improve plant performance lead to an increase in herbivory control.
The role of positive interactions is often crucial in communities with intense abiotic stress such as intertidal environments. Grasses acting as ecosystem engineers, for example, may ameliorate ...intertidal harsh physical conditions and modify the community structure. The mud snails
Heleobia australis d'Orbigny frequently inhabit the SW Atlantic marshes, mainly associated to intertidal marsh plants (mainly the smooth cordgrass
Spartina alterniflora Loisel) probably due to the plant indirect effects. The purpose of this work was to investigate the magnitude of these association and the processes that generate the pattern. Samples of the snail abundance in six SW Atlantic coastal marshes show that
H.
australis is associated to coastal areas of low energy and low or none freshwater input. This result is important because this species is being used as bioindicator of coastal estuarine systems during the Holocene. Thus the paleontological interpretation based on this species should be revised. Within the studied areas, snails are associated to intertidal marsh plants. However, stable isotope analysis shows that neither plant nor their epiphytes are their main food sources. Field experiments show that snails actively select areas with plants, although tethering experiments show that plants do not provide shelter from predators. However, plants do buffer physical stress factors such as temperature, which generate important mortality outside plants covered areas. These positive interactions have large effects on
H.
australis distributions in marsh communities; increasing the habitats available for colonization and affecting their local distribution.
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