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1. 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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2. Effects of Altered Precipitation on Biological Soil Crusts, Fungi, and Soil Nitrogen Availability at the Monsoon Rainfall Manipulation Experiment (MRME) in the Sevilleta National Wildlife Refuge, New Mexico (2016)

   https://doi.org/10.6073/pasta/573848e6e65f686686884670de506465  

   Kwiecinski, Jarek V (January 2019, Environmental Data Initiative)

   Microbial activity in drylands is mediated by the magnitude and frequency of growing season rain events that will shift as climate change progresses. Nitrogen is often co-limiting with water availability to dryland plants, and thus we investigated how microbes important to the nitrogen (N) cycle and soil N availability varied temporally and spatially in the context of a long-term rainfall variability experiment in the northern Chihuahuan Desert. Specifically, we assessed biological soil crust (biocrust) chlorophyll content, fungal abundance, and inorganic N in soils adjacent to individuals of the grassland foundation species, Bouteloua eriopoda, and in the unvegetated interspace at multiple time points associated with an experimental monsoon rain treatment. Treatments included small weekly (5 mm) or large monthly (20 mm) rain events, which had been applied during the summer monsoon for nine years prior to our sampling. Additionally, we evaluated target plant C:N ratios and added 15 N-glutamate to biocrusts to determine potential for nutrient transport to B. eriopoda. Biocrust chlorophyll was up to 67% higher in the small weekly or large monthly rainfall regimes compared to ambient controls. Fungal biomass was 57% lower in soil interspaces than adjacent to plants but did not respond to rainfall regime treatments. Ammonium and nitrate concentrations near plants declined through the sampling period but varied little in soil interspaces. There was limited movement of 15 N from interspace biocrusts to leaves but high 15 N retention in the soils even after additional ambient and experimental rain events. Plant C:N ratio was unaffected by rainfall treatments. The long-term alteration in rainfall regime in this experiment did not change how short-term microbial abundance or N availability responded to the magnitude or frequency of events, suggesting a limited response of N availability to future climate change. 

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3. Climate legacies determine grassland responses to future rainfall regimes

   https://doi.org/10.1111/gcb.16084  

   Broderick, Caitlin M.; Wilkins, Kate; Smith, Melinda D.; Blair, John M. (January 2022, Global Change Biology)

   Abstract Climate variability and periodic droughts have complex effects on carbon (C) fluxes, with uncertain implications for ecosystem C balance under a changing climate. Responses to climate change can be modulated by persistent effects of climate history on plant communities, soil microbial activity, and nutrient cycling (i.e., legacies). To assess how legacies of past precipitation regimes influence tallgrass prairie C cycling under new precipitation regimes, we modified a long‐term irrigation experiment that simulated a wetter climate for >25 years. We reversed irrigated and control (ambient precipitation) treatments in some plots and imposed an experimental drought in plots with a history of irrigation or ambient precipitation to assess how climate legacies affect aboveground net primary productivity (ANPP), soil respiration, and selected soil C pools. Legacy effects of elevated precipitation (irrigation) included higher C fluxes and altered labile soil C pools, and in some cases altered sensitivity to new climate treatments. Indeed, decades of irrigation reduced the sensitivity of both ANPP and soil respiration to drought compared with controls. Positive legacy effects of irrigation on ANPP persisted for at least 3 years following treatment reversal, were apparent in both wet and dry years, and were associated with altered plant functional composition. In contrast, legacy effects on soil respiration were comparatively short‐lived and did not manifest under natural or experimentally‐imposed “wet years,” suggesting that legacy effects on CO₂efflux are contingent on current conditions. Although total soil C remained similar across treatments, long‐term irrigation increased labile soil C and the sensitivity of microbial biomass C to drought. Importantly, the magnitude of legacy effects for all response variables varied with topography, suggesting that landscape can modulate the strength and direction of climate legacies. Our results demonstrate the role of climate history as an important determinant of terrestrial C cycling responses to future climate changes. 

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4. Climate Legacy Effects Shape Tallgrass Prairie Nitrogen Cycling

   https://doi.org/10.1029/2022JG006972  

   Broderick, Caitlin_M; Freeman, Kiona_M; Zeglin, Lydia_H; Blair, John_M (September 2022, Journal of Geophysical Research: Biogeosciences)

   Abstract Climate change is expected to shift precipitation regimes in the North American Central Plains with likely impacts on ecosystem functioning. In tallgrass prairies, water and nitrogen (N) can co‐limit ecosystem processes, so changes in precipitation may have complex effects on carbon (C) and N cycling. Rates of N supply such as N mineralization and nitrification respond differently to short‐ and long‐term patterns in water availability, and previous climate patterns may exert legacy effects on current N cycling that could alter ecosystem sensitivity to current precipitation regimes. We used a long‐term precipitation manipulation at Konza Prairie (Kansas, USA) to assess how previous and current precipitation influence tallgrass prairie N cycling. Supplemental irrigation was applied across upland and lowland prairie for ∼25 years to reduce water deficits; in 2017, we reversed some of these treatments and added a reduced rainfall treatment across both historic rainfall regimes, allowing us to assess how previous climate and current rainfall patterns interact to shape N cycling. In lowland prairie, previous irrigation doubled N mineralization and nitrification rates the year following cessation of irrigation. Reduced microbial C:N ratio and lower relative investment in N‐acquiring enzymes in previously irrigated lowlands suggested that a wetter climate created a legacy of increased N availability for microbes. Internal plant N resorption increased under short‐term irrigation but recovered to ambient levels following previous irrigation. Together, these results suggest that a history of wetter conditions can create a legacy of accelerated N cycling, with consequences for both plant and microbial functioning. 

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5. Repeated extreme droughts decrease root production, but not the potential for post‐drought recovery of root production, in a mesic grassland

   https://doi.org/10.1111/oik.08899  

   Slette, Ingrid J.; Hoover, David L.; Smith, Melinda D.; Knapp, Alan K. (January 2023, Oikos)

   Global climate change is expected to cause more frequent extreme droughts in many parts of the world. Despite the crucial role of roots in water acquisition and plant survival, our understanding of ecosystem vulnerability to drought is primarily based on aboveground impacts. As return intervals between droughts decrease, root responses to one drought might alter responses to subsequent droughts, but this remains unresolved. We conducted a seven‐year experiment that imposed extreme drought (growing season precipitation reduced 66%) in a mesic grassland. Plots were droughted during years 1–2 (‘Drought 1'), or years 5–6 (‘Drought 2') or both. We quantified root production during year 6 (final year of Drought 2) and year 7 (first year after Drought 2), when all plots received ambient precipitation. We found that repeated drought decreased root mass production more than twice as much as a single drought (−63% versus −27%, respectively, relative to ambient precipitation). Root mass production of the dominant C₄grassAndropogon gerardiidid not decrease significantly with either one or two droughts.A. gerardiiroot traits differed from subdominant species on average across all treatments, but drought did not alter root traits of eitherA. gerardiior the subdominant species (collectively). In year 6, root production in plots droughted 4 years ago had not recovered (−21% versus control), but root production recovered in all formerly droughted plots in year 7, when precipitation was above average. Our results highlight the complexity of root responses to drought. Drought‐induced reductions in root production can persist for years after drought and repeated drought can reduce production even further, but this does not preclude rapid recovery of root production in a wet year. 

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