Soil Organic Phosphorus Transformations During Pedogenesis

Abstract Long-term changes in soil phosphorus influence ecosystem development and lead to a decline in the productivity of forests in undisturbed landscapes. Much of the soil phosphorus occurs in a series of organic compounds that differ in their availability to organisms, but changes in the relative abundance of these compounds during pedogenesis remain unknown. We used alkaline extraction and solution phosphorus-31 nuclear magnetic resonance spectroscopy to assess the chemical nature of soil organic phosphorus along a 120,000-year post-glacial chronosequence at Franz Josef, New Zealand. Inositol phosphates, DNA, phospholipids, and phosphonates accumulated rapidly during the first 500 years of soil development characterized by nitrogen limitation of biological productivity, but then declined slowly to low concentrations in older soils characterized by intense phosphorus limitation. However, the relative contribution of the various compounds to the total organic phosphorus varied along the sequence in dramatic and surprising ways. The proportion of inositol hexakisphosphate, conventionally considered to be relatively recalcitrant in the environment, declined markedly in older soils, apparently due to a corresponding decline in amorphous metal oxides, which weather to crystalline forms during pedogenesis. In contrast, the proportion of DNA, considered relatively bioavailable in soil, increased continually throughout the sequence, due apparently to incorporation within organic structures that provide protection from biological attack. The changes in soil organic phosphorus coincided with marked shifts in plant and microbial communities, suggesting that differences in the forms and bioavailability of soil organic phosphorus have ecological significance. Overall, the results strengthen our understanding of phosphorus transformations during pedogenesis and provide important insight into factors regulating the composition of soil organic phosphorus.     

Nowhere to run: semi-permeable barriers affect pronghorn space use

Animal movement can mediate the ecological consequences of fragmentation; however, barriers such as fences, roads, and railways are becoming a pervasive threat to wildlife. Pronghorn (Antilocapra americana) habitat in western North America has been fragmented by roads, railways, and fences. Although pronghorn are sensitive to barriers, neither the relative permeability of different barriers to crossing nor their influence on space use have been quantified. We used a large global positioning system (GPS)-collar dataset of pronghorn (n = 1,010 animal-years) in Wyoming, USA, to first quantify the likelihood that pronghorn cross each of 5 different anthropogenic barriers, including fences, county roads, railroads, state highways, and interstate highways (i.e., interstates). Next, we assessed how each barrier influenced pronghorn space use during the winter as indexed by the area occupied, and daily displacement relative to the density of barriers on an individual's winter range. The semi-permeability of the 5 barriers varied substantially, with the interstate being the most severe barrier to pronghorn movement. Pronghorn were >300 times less likely to cross interstates compared to state highways. Although pronghorn space use was rarely influenced by barriers within individual core winter ranges, pronghorn space use was constrained by barriers on the buffered periphery of individual winter ranges. Despite their different permeability to movement, the density of fences and combined interstates and railroads had similarly negative effects on pronghorn space use. Our results illustrate that the degree to which pronghorn avoid crossing barriers may scale up to affect access to habitat. Additionally, our results indicate that the effects of barriers on habitat access are not proportional to their permeability. Our results add to a growing consensus that effective management of mobile species depends on understanding how different kinds of semi-permeable barriers influence access and use of habitats.     

Climate change and range restriction of common salamanders in eastern Canada and the United States

The sensitivity of amphibian species to shifts in environmental conditions has been exhibited through long-term population studies and the projection of ecological niche models under expected conditions. Species in biodiversity hotspots have been the focus of ample predictive modeling studies, while, despite their significant ecological value, wide-ranging and common taxa have received less attention. We focused on predicting range restriction of the spotted salamander (Ambystoma maculatum), blue-spotted salamander (A. laterale), four-toed salamander (Hemidactylium scutatum), and red-backed salamander (Plethodon cinereus) under future climate scenarios. Using bias-corrected future climate data and biodiversity database records, we developed maximum entropy (MaxEnt) models under current conditions and for climate change projections in 2050 and 2070. We calculated positivity rates of species localities to represent proportions of habitat expected to remain climatically suitable with continued climate change. Models projected under future conditions predicted average positivity rates of 91% (89–93%) for the blue-spotted salamander, 23% (2–41%) for the spotted salamander, 4% (0.7–9%) for the four-toed salamander, and 61% (42–76%) for the red-backed salamander. Range restriction increased with time and greenhouse gas concentration for the spotted salamander, four-toed salamander, and red-backed salamander. Common, widespread taxa that often receive less conservation resources than other species are at risk of experiencing significant losses to their climatic ranges as climate change continues. Efforts to maintain populations of species should be focused on regions expected to experience fewer climatic shifts such as the interior and northern zones of species' distributions.     

Cattle grazing in CRP grasslands during the nesting season: effects on avian abundance and diversity

The Conservation Reserve Program (CRP) is a primary tool for restoring grassland in the United States, in part as wildlife habitat, which has benefited declining grassland bird populations. Among potential mid-contract management practices used to maintain early-successional CRP grasslands, cattle grazing had been prohibited and is currently disincentivized during the primary nesting season for birds (much of the growing season), despite the important role that large herbivores historically played in structuring grassland ecosystems. Conservative grazing of CRP grasslands could increase spatial heterogeneity in vegetation structure and plant diversity, potentially supporting higher densities of some grassland bird species and higher bird diversity. Our objective was to determine the effect of experimental cattle grazing on species-specific relative abundance and occupancy, species diversity, and community dissimilarity of grassland birds on CRP grasslands across the longitudinal extent of Kansas, USA (a 63.5-cm precipitation gradient) during the 2017–2019 avian breeding seasons. Fifty-three of 108 fields were grazed by cattle during the growing seasons of 2017 and 2018 and all fields were rested from grazing in 2019. For all analyses, we examined separate model sets for semiarid western versus more mesic eastern Kansas. Using data from line transect surveys, we modeled relative abundances of 5 songbird species: grasshopper sparrow (Ammodramus savannarum), dickcissel (Spiza americana), eastern meadowlark (Sturnella magna), western meadowlark (Sturnella neglecta), and brown-headed cowbird (Molothrus ater). Grazing had delayed yet positive effects on abundances of grasshopper sparrow in western Kansas, and eastern meadowlark in eastern Kansas, but negative effects on dickcissel abundance in western Kansas and especially on burned fields in eastern Kansas. Somewhat counterintuitively, brown-headed cowbirds in western Kansas were more abundant on ungrazed versus grazed fields in the years after grazing began. In addition, we modeled multi-season occupancy of 3 gamebird species (ring-necked pheasant [Phasianus colcicus], northern bobwhite [Colinus virginianus], mourning dove [Zenaida macroura]) and Henslow's sparrow (Centronyx henslowii); grazing did not affect occupancy of these species. In eastern Kansas, species diversity was highest in grazed, unburned fields. In western Kansas, bird communities in grazed and ungrazed fields were dissimilar, as determined from multivariate analysis. Though regionally variable, conservative stocking of cattle on CRP grasslands during the nesting season as a mid-contract management tool might increase bird species diversity by restructuring habitat that accommodates a greater variety of species and decreasing abundances of species associated with taller, denser stands of vegetation.     

Biodiversity associated with restored small-scale mussel habitats has restoration decision implications

The global loss of marine ecosystem engineers has caused an unprecedented decline in biodiversity. Although wild shellfish habitats have been shown to support biodiverse ecosystems, little is known about how biodiversity is altered by restored shellfish habitats, particularly mussels. To explore the biodiversity response to restored mussel habitats we deposited mussels on the seafloor in 1.5?×?1.5 m plots across a gradient of benthic environments. To understand a holistic community response, this study looks at the response of three faunal classifications over 1 year: infauna, epifauna, and pelagic fauna, compared with adjacent control plots (no mussels). The restored mussel habitats recorded 42 times more demersal fish than control areas, while macroalgae and mobile benthic invertebrates had over a twofold increase in abundance. Overall, the addition of mussels to the seafloor resulted in a general reduction of infaunal abundance and biodiversity, but an increase in epifaunal and pelagic faunal abundances, specifically from those species that benefit from benthic habitat complexity and an increase in food availability. From a management perspective, we highlight location-specific differences to consider for future restoration efforts, including environmental conditions and potential observed factors such as nearby sources of species, particularly predators, and relevant demersal fish ranges. Ultimately, measuring biodiversity responses in small-scale studies will serve as a valuable guide for larger scale restoration efforts and this study recommends considerations to enhance biodiversity outcomes in restored mussel habitats.

     

Induction of Photosynthetic Carbon Fixation in Anoxia Relies on Hydrogenase Activity and Proton-Gradient Regulation-Like1-Mediated Cyclic Electron Flow in Chlamydomonas reinhardtii

The model green microalga Chlamydomonas reinhardtii is frequently subject to periods of dark and anoxia in its natural environment. Here, by resorting to mutants defective in the maturation of the chloroplastic oxygen-sensitive hydrogenases or in Proton-Gradient Regulation-Like1 (PGRL1)-dependent cyclic electron flow around photosystem I (PSI-CEF), we demonstrate the sequential contribution of these alternative electron flows (AEFs) in the reactivation of photosynthetic carbon fixation during a shift from dark anoxia to light. At light onset, hydrogenase activity sustains a linear electron flow from photosystem II, which is followed by a transient PSI-CEF in the wild type. By promoting ATP synthesis without net generation of photosynthetic reductants, the two AEF are critical for restoration of the capacity for carbon dioxide fixation in the light. Our data also suggest that the decrease in hydrogen evolution with time of illumination might be due to competition for reduced ferredoxins between ferredoxin-NADP⁺ oxidoreductase and hydrogenases, rather than due to the sensitivity of hydrogenase activity to oxygen. Finally, the absence of the two alternative pathways in a double mutant pgrl1 hydrogenase maturation factor G-2 is detrimental for photosynthesis and growth and cannot be compensated by any other AEF or anoxic metabolic responses. This highlights the role of hydrogenase activity and PSI-CEF in the ecological success of microalgae in low-oxygen environments.Unicellular photosynthetic organisms such as the green alga Chlamydomonas reinhardtii frequently experience anoxic conditions in their natural habitat, especially during the night when the microbial community consumes the available oxygen. Under anoxia, lack of ATP synthesis by F₁F_O ATP synthase (EC 3.6.3.14) due to the absence of mitochondrial respiration is compensated by the activity of various plant- and bacterial-type fermentative enzymes that drive a sustained glycolytic activity (Mus et al., 2007; Terashima et al., 2010; Grossman et al., 2011; Yang et al., 2014). In C. reinhardtii, upstream glycolytic enzymes, including the reversible glyceraldehyde 3-P dehydrogenase, are located in the chloroplast (Johnson and Alric, 2012). This last enzyme is shared by the glycolysis (oxidative activity) and the Calvin-Benson-Bassham (CBB) cycle (reductive activity; Johnson and Alric, 2013). In dark anoxic conditions, the CBB cycle is inactive, thus avoiding wasteful using up of available ATP and depletion of the required intermediates for glycolysis. On the other side, ability of microalgae to perform photosynthetic carbon fixation when transferred from dark to light in the absence of oxygen might also be critical for adaptation to their environment. In such conditions, not only the linear electron flow (LEF) to Rubisco, but also alternative electron flow (AEF) toward oxygen (chlororespiration, Mehler reaction, and mitochondrial respiration; for review, see Miyake, 2010; Peltier et al., 2010; Cardol et al., 2011) is impaired. Thus, cells need to circumvent a paradoxical situation: the activity of the CBB cycle requires the restoration of the cellular ATP, but the chloroplastic F₁F_O ATP synthase activity is compromised by the impairment of most of the photosynthetic electron flows that usually generate the proton motive force in oxic conditions. Other AEFs, specific to anoxic conditions, should therefore be involved to promote ATP synthesis without net synthesis of NADPH and explain the light-induced restoration of CBB cycle activity.Among enzymes expressed in anoxia, the oxygen-sensitive hydrogenases (HYDA1 and HYDA2 in C. reinhardtii) catalyze the reversible reduction of protons into molecular hydrogen from the oxidation of reduced ferredoxins (FDXs; Florin et al., 2001). Although hydrogen metabolism in microalgae has been largely studied in the last 15 years in perspective of promising future renewable energy carriers (Melis et al., 2000; Kruse et al., 2005; Ghirardi et al., 2009), the physiological role of such an oxygen-sensitive enzyme linked to the photosynthetic pathway has been poorly considered. The 40-year-old proposal that H₂ evolution by hydrogenase is involved in induction of photosynthetic electron transfer after anoxic incubation (Kessler, 1973; Schreiber and Vidaver, 1974) has been only recently demonstrated in C. reinhardtii. Gas exchange measurements showed that H₂ evolution occurs prior to CO₂ fixation upon illumination (Cournac et al., 2002). At light onset after a prolonged period in dark anoxic conditions, the photosynthetic electron flow is mainly a LEF toward hydrogenase (Godaux et al., 2013), and lack of hydrogenase activity in hydrogenase maturation factor EF (hydEF) mutant strain deficient in hydrogenases maturation (Posewitz et al., 2004) induces a lag in induction of PSII activity (Ghysels et al., 2013). In cyanobacteria, the bidirectional Ni-Fe hydrogenase might also work as an electron valve for disposal of electrons generated at the onset of illumination of cells (Cournac et al., 2004) or when excess electrons are generated during photosynthesis, preventing the slowing of the electron transport chain under stress conditions (Appel et al., 2000; Carrieri et al., 2011). The bidirectional Ni-Fe hydrogenase could also dispose of excess of reducing equivalents during fermentation in dark anaerobic conditions, helping to generate ATP and maintaining homeostasis (Barz et al., 2010). A similar role for hydrogenase in setting the redox poise in the chloroplast of C. reinhardtii in anoxia has been recently uncovered (Clowez et al., 2015).Still, the physiological and evolutionary advantages of hydrogenase activity have not been demonstrated so far, and the mechanism responsible for the cessation of hydrogen evolution remains unclear. In this respect, at least three hypotheses have been formulated: (1) the inhibition of hydrogenase by O₂ produced by water photolysis (Ghirardi et al., 1997; Cohen et al., 2005), (2) the competition between ferredoxin-NADP⁺ oxidoreductase (FNR) and hydrogenase activity for reduced FDX (Yacoby et al., 2011), and (3) the inhibition of electron supply to hydrogenases by the proton gradient generated by another AEF, the cyclic electron flow around PSI (PSI-CEF; Tolleter et al., 2011). First described by Arnon (1955), PSI-CEF consists in a reinjection of electrons from reduced FDX or NADPH pool in the plastoquinone (PQ) pool. By generating an additional transthylakoidal proton gradient without producing reducing power, this AEF thus contributes to adjust the ATP/NADPH ratio for carbon fixation in various energetic unfavorable conditions including anoxia (Tolleter et al., 2011; Alric, 2014), high light (Tolleter et al., 2011; Johnson et al., 2014), or low CO₂ (Lucker and Kramer, 2013). In C. reinhardtii, two pathways have been suggested to be involved in PSI-CEF: (1) a type II NAD(P)H dehydrogenase (NDA2; Jans et al., 2008) driving the electrons from NAD(P)H to the PQ pool and (2) a pathway involving Proton Gradient Regulation (PGR) proteins where electrons from reduced FDXs return to the PQ pool or cytochrome b₆f. Not fully understood, this latter pathway comprises at least Proton Gradient Regulation5 (PGR5) and Proton-Gradient Regulation-Like1 (PGRL1) proteins (Iwai et al., 2010; Tolleter et al., 2011; Johnson et al., 2014) and is the major route for PSI-CEF in C. reinhardtii cells placed in anoxia (Alric, 2014).In this work, we took advantage of specific C. reinhardtii mutants defective in hydrogenase activity and PSI-CEF to study photosynthetic electron transfer after a period of dark anoxic conditions. Based on biophysical and physiological complementary studies, we demonstrate that at least hydrogenase activity or PSI-CEF is compulsory for the activity of the CBB cycle and for the survival of the cells submitted to anoxic conditions in their natural habitat.     

Effects of nutrients and warming on Planktothrix dynamics and diversity: a palaeolimnological view based on sedimentary DNA and RNA

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