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1. Primary productivity as a control over soil microbial diversity along environmental gradients in a polar desert ecosystem

   https://doi.org/10.7717/peerj.3377  

   Geyer, Kevin M.; Takacs-Vesbach, Cristina D.; Gooseff, Michael N.; Barrett, John E. (January 2017, PeerJ)

   Primary production is the fundamental source of energy to foodwebs and ecosystems, and is thus an important constraint on soil communities. This coupling is particularly evident in polar terrestrial ecosystems where biological diversity and activity is tightly constrained by edaphic gradients of productivity (e.g., soil moisture, organic carbon availability) and geochemical severity (e.g., pH, electrical conductivity). In the McMurdo Dry Valleys of Antarctica, environmental gradients determine numerous properties of soil communities and yet relatively few estimates of gross or net primary productivity (GPP, NPP) exist for this region. Here we describe a survey utilizing pulse amplitude modulation (PAM) fluorometry to estimate rates of GPP across a broad environmental gradient along with belowground microbial diversity and decomposition. PAM estimates of GPP ranged from an average of 0.27 μmol O 2 /m 2 /s in the most arid soils to an average of 6.97 μmol O 2 /m 2 /s in the most productive soils, the latter equivalent to 217 g C/m 2 /y in annual NPP assuming a 60 day growing season. A diversity index of four carbon-acquiring enzyme activities also increased with soil productivity, suggesting that the diversity of organic substrates in mesic environments may be an additional driver of microbial diversity. Overall, soil productivity was a stronger predictor of microbial diversity and enzymatic activity than any estimate of geochemical severity. These results highlight the fundamental role of environmental gradients to control community diversity and the dynamics of ecosystem-scale carbon pools in arid systems. 

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2. Belowground responses to altered precipitation regimes in two semi-arid grasslands

   https://doi.org/10.6073/pasta/2d233f4ba9af3a13b564a1c6364704c7  

   Holguin, Jennifer; Collins, Scott L; McLaren, Jennie R (January 2022, Environmental Data Initiative)

   Predicted climate change extremes, such as severe and prolonged drought, may profoundly impact biogeochemical processes like carbon and nitrogen cycling in water-limited ecosystems. To increase our understanding of how extreme climate events impact belowground ecosystem processes, we investigated the effects of five years of severe growing season drought and two-month delay in monsoon precipitation on belowground productivity and biogeochemical processes in two semi-arid grasslands. This experiment takes place during the fifth year of the Extreme Drought in Grassland Experiment (EDGE) at the Sevilleta National Wildlife Refuge (SNWR), a Long-Term Ecological Research in central New Mexico, USA. The two grassland sites a Chihuahuan Desert grassland dominated by Bouteloua eriopoda and Great Plains grassland dominated by B. gracilis are ~5km apart in the SWNR. The EDGE platform was established in the spring of 2012 (pre-treatment). Each site contains three treatments (ten replicates): ambient rainfall, extreme growing season drought, and delayed monsoon. The extreme drought treatment reduces growing season rainfall (April through September) each year by 66%, which equates to a 50% reduction of annual precipitation while maintaining natural precipitation patterns. There are 10 replicates per treatment within each site. All plots are 3 x 4 m in size and are paired spatially into blocks with treatments assigned randomly within a block. We measured an array of belowground and biogeochemical variables. Each variable was measured either once, twice, or three times (specific information on sampling scheme for each measured variable in methods section). Belowground net primary productivity, standing crop root biomass, total organic carbon, and total nitrogen were measured once. Extractable organic carbon, extractable total nitrogen, microbial biomass carbon, microbial biomass nitrogen and extracellular enzymes were measured twice. Available soil nitrate, available soil ammonium, and available soil phosphate were measured three times. 

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3. Stronger fertilization effects on aboveground versus belowground plant properties across nine U.S. grasslands

   https://doi.org/10.1002/ecy.3891  

   Keller, Adrienne B.; Walter, Christopher A.; Blumenthal, Dana M.; Borer, Elizabeth T.; Collins, Scott L.; DeLancey, Lang C.; Fay, Philip A.; Hofmockel, Kirsten S.; Knops, Johannes M.; Leakey, Andrew D.; et al (October 2022, Ecology)

   Increased nutrient inputs due to anthropogenic activity are expected to increase primary productivity across terrestrial ecosystems, but changes in allocation aboveground versus belowground with nutrient addition have different implications for soil carbon (C) storage. Thus, given that roots are major contributors to soil C storage, understanding belowground net primary productivity (BNPP) and biomass responses to changes in nutrient availability is essential to predicting carbon–climate feedbacks in the context of interacting global environmental changes. To address this knowledge gap, we tested whether a decade of nitrogen (N) and phosphorus (P) fertilization consistently influenced aboveground and belowground biomass and productivity at nine grassland sites spanning a wide range of climatic and edaphic conditions in the continental United States. Fertilization effects were strong aboveground, with both N and P addition stimulating aboveground biomass at nearly all sites (by 30% and 36%, respectively, on average). P addition consistently increased root production (by 15% on average), whereas other belowground responses to fertilization were more variable, ranging from positive to negative across sites. Site-specific responses to P were not predicted by the measured covariates. Atmospheric N deposition mediated the effect of N fertilization on root biomass and turnover. Specifically, atmospheric N deposition was positively correlated with root turnover rates, and this relationship was amplified with N addition. Nitrogen addition increased root biomass at sites with low N deposition but decreased it at sites with high N deposition. Overall, these results suggest that the effects of nutrient supply on belowground plant properties are context dependent, particularly with regard to background N supply rates, demonstrating that site conditions must be considered when predicting how grassland ecosystems will respond to increased nutrient loading from anthropogenic activity. 

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4. Biome Transition Along Elevational Gradients in New Mexico (SEON) Study: Flux Tower Net Primary Productivity (NPP) Quadrat Study at the Sevilleta National Wildlife Refuge, New Mexico

   https://doi.org/10.6073/pasta/3e38dae78feeb17cbe5b8666ace0f1ab  

   Baur, Lauren; Collins, Scott; Muldavin, Esteban; Rudgers, Jennifer A; Pockman, William T. (January 2021, Environmental Data Initiative)

   {"Abstract":["The varied topography and large elevation gradients that\n characterize the arid and semi-arid Southwest create a wide range of\n climatic conditions - and associated biomes - within relatively\n short distances. This creates an ideal experimental system in which\n to study the effects of climate on ecosystems. Such studies are\n critical given that the Southwestern U.S. has already experienced\n changes in climate that have altered precipitation patterns (Mote et\n al. 2005), and stands to experience dramatic climate change in the\n coming decades (Seager et al. 2007; Ting et al. 2007). Climate\n models currently predict an imminent transition to a warmer, more\n arid climate in the Southwest (Seager et al. 2007; Ting et al.\n 2007). Thus, high elevation ecosystems, which currently experience\n relatively cool and mesic climates, will likely resemble their lower\n elevation counterparts, which experience a hotter and drier climate.\n In order to predict regional changes in carbon storage, hydrologic\n partitioning and water resources in response to these potential\n shifts, it is critical to understand how both temperature and soil\n moisture affect processes such as evaportranspiration (ET), total\n carbon uptake through gross primary production (GPP), ecosystem\n respiration (Reco), and net ecosystem exchange of carbon, water and\n energy across elevational gradients. We are using a sequence of six\n widespread biomes along an elevational gradient in New Mexico --\n ranging from hot, arid ecosystems at low elevations to cool, mesic\n ecosystems at high elevation to test specific hypotheses related to\n how climatic controls over ecosystem processes change across this\n gradient. We have an eddy covariance tower and associated\n meteorological instruments in each biome which we are using to\n directly measure the exchange of carbon, water and energy between\n the ecosystem and the atmosphere. This gradient offers us a unique\n opportunity to test the interactive effects of temperature and soil\n moisture on ecosystem processes, as temperature decreases and soil\n moisture increases markedly along the gradient and varies through\n time within sites. This dataset examines how different stages of\n burn affects above-ground biomass production (ANPP) in a mixed\n desert-grassland. Net primary production is a fundamental ecological\n variable that quantifies rates of carbon consumption and fixation.\n Estimates of NPP are important in understanding energy flow at a\n community level as well as spatial and temporal responses to a range\n of ecological processes. Above-ground net primary production is the\n change in plant biomass, represented by stems, flowers, fruit and\n foliage, over time and incorporates growth as well as loss to death\n and decomposition. To measure this change the vegetation variables\n in this dataset, including species composition and the cover and\n height of individuals, are sampled twice yearly (spring and fall) at\n permanent 1m x 1m plots. The data from these plots is used to build\n regressions correlating biomass and volume via weights of select\n harvested species obtained in SEV157, "Net Primary Productivity\n (NPP) Weight Data." This biomass data is included in SEV292,\n "Flux Tower Seasonal Biomass and Seasonal and Annual NPP\n Data.""]} 

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5. Sensitivity of grassland carbon pools to plant diversity, elevated CO ₂ , and soil nitrogen addition over 19 years

   https://doi.org/10.1073/pnas.2016965118  

   Pastore, Melissa A.; Hobbie, Sarah E.; Reich, Peter B. (April 2021, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Whether the terrestrial biosphere will continue to act as a net carbon (C) sink in the face of multiple global changes is questionable. A key uncertainty is whether increases in plant C fixation under elevated carbon dioxide (CO 2 ) will translate into decades-long C storage and whether this depends on other concurrently changing factors. We investigated how manipulations of CO 2 , soil nitrogen (N) supply, and plant species richness influenced total ecosystem (plant + soil to 60 cm) C storage over 19 y in a free-air CO 2 enrichment grassland experiment (BioCON) in Minnesota. On average, after 19 y of treatments, increasing species richness from 1 to 4, 9, or 16 enhanced total ecosystem C storage by 22 to 32%, whereas N addition of 4 g N m −2 ⋅ y −1 and elevated CO 2 of +180 ppm had only modest effects (increasing C stores by less than 5%). While all treatments increased net primary productivity, only increasing species richness enhanced net primary productivity sufficiently to more than offset enhanced C losses and substantially increase ecosystem C pools. Effects of the three global change treatments were generally additive, and we did not observe any interactions between CO 2 and N. Overall, our results call into question whether elevated CO 2 will increase the soil C sink in grassland ecosystems, helping to slow climate change, and suggest that losses of biodiversity may influence C storage as much as or more than increasing CO 2 or high rates of N deposition in perennial grassland systems. 

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