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1. Global impacts of fertilization and herbivore removal on soil net nitrogen mineralization are modulated by local climate and soil properties

   https://doi.org/10.1111/gcb.15308  

   Risch, Anita C. ; Zimmermann, Stefan ; Moser, Barbara ; Schütz, Martin ; Hagedorn, Frank ; Firn, Jennifer ; Fay, Philip A. ; Adler, Peter B. ; Biederman, Lori A. ; Blair, John M. ; et al ( July 2020 , Global Change Biology)

   Soil nitrogen (N) availability is critical for grassland functioning. However, human activities have increased the supply of biologically-limiting nutrients, and changed the density and identity of mammalian herbivores. These anthropogenic changes may alter net soil N mineralization (soil net Nmin), i.e., the net balance between N mineralization and immobilization, which could severely impact grassland structure and functioning. Yet, to date, little is known about how fertilization and herbivore removal individually, or jointly, affect soil net Nmin across a wide range of grasslands that vary in soil and climatic properties. Here, we collected data from 22 grasslands on five continents, all part of a globally replicated experiment, to assess how fertilization and herbivore removal affected potential (laboratory-based) and realized (field-based) soil net Nmin. Herbivore removal in the absence of fertilization did not alter potential and realized soil net Nmin. However, fertilization alone and in combination with herbivore removal consistently increased potential soil net Nmin. Realized soil net Nmin, in contrast, significantly decreased in fertilized plots where herbivores were removed. Treatment effects on potential and realized soil net Nmin were contingent on site-specific soil and climatic properties. Fertilization effects on potential soil net Nmin were larger at sites with higher mean annual precipitation (MAP) and temperature of the wettest quarter (T.q.wet). Reciprocally, realized soil net Nmin declined most strongly with fertilization and herbivore removal at sites with lower MAP and higher T.q.wet. In summary, our findings show that anthropogenic nutrient enrichment, herbivore exclusion, and alterations in future climatic conditions can negatively impact soil net Nmin across global grasslands under realistic field conditions. This is important context-dependent knowledge for grassland management worldwide. 

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2. Nitrogen Fertilizer, Arbuscular Mycorrhizal Fungi, and Soil Nematodes Affect Lignin Quality and Quantity in Switchgrass (Panicum virgatum L.)

   https://doi.org/10.1007/s12155-021-10284-2  

   Basyal, Binod ; Foster, Cliff ; Gross, Katherine L. ; Emery, Sarah M. ( April 2021 , BioEnergy Research)

   null (Ed.)

   which limit cell wall digestibility and efficiency of cellulose conversion to bioethanol, can be influenced by belowground biotic and abiotic factors. Switchgrass (Panicum virgatum L.) is a leading lignocellulosic biofuel crop and forms strong belowground associations with arbuscular mycorrhizal fungi (AMF), is susceptible to belowground plant-parasitic nematodes (PPN), and when grown in monoculture generally requires nitrogen (N) fertilization. The main objectives of the study were to investigate the effects of N fertilizer and belowground organisms on lignin content and composition of switchgrass. Leaf, stem, and root tissues were evaluated separately to test whether these factors had varying belowground (local) or aboveground (systemic) effects on plants. These factors were manipulated in a field study in 2017 using biocide applications to reduce soil fungi and nematodes. Combined biocide application reduced p-hydroxyphenyl (H) unit abundance in the leaves by 14% and increased the syringyl:guaiacyl (S:G) ratio in stems by 2%. Application of fungicide alone increased stem syringyl (S) unit by 12.4% as compared with control plots, and 11.1% as compared with nematicide plots. Overall, fertilizer increased total stem lignin by 3%, stem S unit by 6.7%, and stem S:G ratio by 10%, whereas it reduced the amount of H-unit in the roots by 11%. While the effects of N fertilizer were more pronounced in this study, changes to soil organisms had similar magnitudes of effect for some measures of lignin, indicating that these belowground interactions may be important for growers to consider in the future. 

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3. Diversity‐dependent soil acidification under nitrogen enrichment constrains biomass productivity

   https://doi.org/10.1111/gcb.15329  

   Kimmel, Kaitlin ; Furey, George N. ; Hobbie, Sarah E. ; Isbell, Forest ; Tilman, David ; Reich, Peter B. ( September 2020 , Global Change Biology)

   In most plant communities, the net effect of nitrogen enrichment is an increase in plant productivity. However, nitrogen enrichment also has been shown to decrease species richness and to acidify soils, each of which may diminish the long‐term impact of nutrient enrichment on productivity. Here we use a long‐term (20 year) grassland plant diversity by nitrogen enrichment experiment in Minnesota, United States (a subexperiment within the BioCON experiment) to quantify the net impacts of nitrogen enrichment on productivity, including its potential indirect effects on productivity via changes in species richness and soil pH over an experimental diversity gradient. Overall, we found that nitrogen enrichment led to an immediate positive increment in productivity, but that this effect became nonsignificant over later years of the experiment, with the difference in productivity between fertilized and unfertilized plots decreasing in proportion to nitrogen addition‐dependent declines in soil pH and losses of plant diversity. The net effect of nitrogen enrichment on productivity could have been 14.5% more on average over 20 years in monocultures if not for nitrogen‐induced decreases in pH and about 28.5% more on average over 20 years in 16 species communities if not for nitrogen‐induced species richness losses. Together, these results suggest that the positive effects of nutrient enrichment on biomass production can diminish in their magnitude over time, especially because of soil acidification in low diversity communities and especially because of plant diversity loss in initially high diversity communities.

    

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4. Disturbance to biocrusts decreased cyanobacteria, N ‐fixer abundance, and grass leaf N but increased fungal abundance

   https://doi.org/10.1002/ecy.3656  

   Adelizzi, Rose ; O'Brien, E_A ; Hoellrich, Mikaela ; Rudgers, Jennifer_A ; Mann, Michael ; Fernandes, Vanessa_Moreira_Camara ; Darrouzet‐Nardi, Anthony ; Stricker, Eva ( March 2022 , Ecology)

   Interactions between plants and soil microbes influence plant nutrient transformations, including nitrogen (N) fixation, nutrient mineralization, and resource exchanges through fungal networks. Physical disturbances to soils can disrupt soil microbes and associated processes that support plant and microbial productivity. In low resource drylands, biological soil crusts (“biocrusts”) occupy surface soils and house key autotrophic and diazotrophic bacteria, non‐vascular plants, or lichens. Interactions among biocrusts, plants, and fungal networks between them are hypothesized to drive carbon and nutrient dynamics; however, comparisons across ecosystems are needed to generalize how soil disturbances alter microbial communities and their contributions to N pools and transformations. To evaluate linkages among plants, fungi, and biocrusts, we disturbed all unvegetated surfaces with human foot trampling twice yearly from 2013–2019 in dry conditions in cyanobacteria‐dominated biocrusts in the Chihuahuan Desert grassland and shrubland ecosystems. After 5 years, disturbance decreased the abundances of cyanobacteria (especiallyMicrocoleus steenstrupiiclade) and N‐fixers (Scytonemasp., andSchizothrixsp.) by >77% and chlorophyllaby up to 55% but, conversely, increased soil fungal abundance by 50% compared with controls. Responses of root‐associated fungi differed between the two dominant plant species and ecosystem types, with a maximum of 80% more aseptate hyphae in disturbed than in control plots. Although disturbance did not affect¹⁵N tracer transfer from biocrusts to the dominant grass,Bouteloua eriopoda, disturbance increased available soil N by 65% in the shrubland, and decreased leaf N ofB. eriopodaby up to 16%, suggesting that, although rapid N transfer during peak production was not affected by disturbance, over the long‐term plant nutrient content was disrupted. Altogether, the shrubland may be more resilient to detrimental changes due to disturbance than grassland, and these results demonstrated that disturbances to soil microbial communities have the potential to cause substantial changes in N pools by reducing and reordering biocrust taxa.

    

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5. Three Decades of Divergent Land Use and Plant Community Change Alters Soil C and N Content in Tallgrass Prairie

   https://doi.org/10.1029/2020JG005723  

   Connell, R. Kent ; Nippert, Jesse B. ; Blair, John M. ( August 2020 , Journal of Geophysical Research: Biogeosciences)

   Frequent fire and grazing by megafauna are important determinants of tallgrass prairie plant community structure. However, fire suppression and removal of native grazers have altered these natural disturbance regimes and changed grassland plant communities with potential long‐term consequences for soil carbon (C) and nitrogen (N) storage. We investigated multidecade changes in soil C and N pools in response to contrasting long‐term burning and grazing treatments. Fire suppression with or without grazers and exclusion of grazers in annually burned prairie increased soil C content and shifted the δ¹³C signature of soil C over time, concomitant with changes in plant community composition. Soil δ¹³C values indicated that increased soil C content was associated with an increased contribution from plants using a C₃photosynthetic pathway (i.e., woody shrubs) under fire suppression. Soil N content also increased when fire was suppressed, relative to frequently burned grassland, but the rate of increase was slower when grazers were present. Additionally, changes in δ¹⁵N values suggested that grazing increased the openness of the N cycle, presumably due to greater N losses. By coupling long‐term fire and grazing treatments with plant community data and soil samples archived over three decades, we demonstrate that human‐caused changes to natural disturbance regimes in a tallgrass prairie significantly alter soil C and N cycles through belowground changes associated with shifts in the plant community. Since natural disturbance regimes have been altered in grasslands across the world, our results are relevant for understanding the long‐term biogeochemical consequences of these ongoing land use changes.

    

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