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Abstract

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 in dry conditions from 2013-2018 in cyanobacteria-dominated biocrusts in Chihuahuan Desert grassland and shrubland ecosystems. Our study included microbial communities and N pools sampled at different time points in the disturbance treatments at one or both sites. We began our sampling after observations in April 2018 that the chlorophyll a content was at least double in control than disturbed plots in both ecosystems (Chung et al. 2019). Stomping occurred in May, and we collected soil and plant samples in June 2018 for N pools and soil and root fungal abundance. We collected additional soil samples in September 2018 and conducted the 15N tracer experiment to observe rates of N transfer from biocrust to plants before the fall stomp treatment in October. We collected chlorophyll a samples and soils for sequencing bacteria in September of 2019, also before the fall stomp treatment.
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Abstract

Dryland ecosystems are increasing in geographic extent and contribute greatly to interannual variability in global carbon dynamics. Disentangling interactions among dominant primary producers can help partition their contributions to dryland C dynamics. We measured the δ13C signatures of biological soil crusts and dominant plant species (C3 and C4) across a regional scale in the southwestern USA to determine if biocrust cyanobacteria were coupled to plant productivity (using plant-derived C mixotrophically), or independent of plant activity (and therefore purely autotrophic).

Abstract

Conceptual context: Species interactions may couple the resource dynamics of different primary producers and may enhance productivity by reducing loss from the system. In low-resource systems, this biotic control may be especially important for maintaining productivity. In drylands, the activities of vascular plants and biological soil crusts can be decoupled in space because biocrusts grow on the soil surface but plant roots are underground, and decoupled in time due to biocrusts activating with smaller precipitation events than plants. Soil fungi are hypothesized to functionally couple the plants and biocrusts by transporting nutrients. We studied whether disrupting fungi between biocrusts and plants reduces nitrogen transfer and retention and decreases primary production as predicted by the fungal loop hypothesis. Additionally, we compared varying precipitation regimes that can drive different timing and depth of biological activities. Methodological approach: We used field mesocosms in which the potential for fungal connections between biocrusts and roots remained intact or were impeded by mesh. We imposed a precipitation regime of small, frequent or large, infrequent rain events. We used 15N to track fungal-mediated nitrogen (N) transfer. We quantified microbial carbon use efficiency and plant and biocrust production and N content.

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 notmore »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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It is a critical time to reflect on the National Ecological Observatory Network (NEON) science to date as well as envision what research can be done right now with NEON (and other) data and what training is needed to enable a diverse user community. NEON became fully operational in May 2019 and has pivoted from planning and construction to operation and maintenance. In this overview, the history of and foundational thinking around NEON are discussed. A framework of open science is described with a discussion of how NEON can be situated as part of a larger data constellation—across existing networks and different suites of ecological measurements and sensors. Next, a synthesis of early NEON science, based on >100 existing publications, funded proposal efforts, and emergent science at the very first NEON Science Summit (hosted by Earth Lab at the University of Colorado Boulder in October 2019) is provided. Key questions that the ecology community will address with NEON data in the next 10 yr are outlined, from understanding drivers of biodiversity across spatial and temporal scales to defining complex feedback mechanisms in human–environmental systems. Last, the essential elements needed to engage and support a diverse and inclusive NEON user communitymore »are highlighted: training resources and tools that are openly available, funding for broad community engagement initiatives, and a mechanism to share and advertise those opportunities. NEON users require both the skills to work with NEON data and the ecological or environmental science domain knowledge to understand and interpret them. This paper synthesizes early directions in the community’s use of NEON data, and opportunities for the next 10 yr of NEON operations in emergent science themes, open science best practices, education and training, and community building.

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