Extreme drought impacts have been underestimated in grasslands and shrublands globally.

Climate change is increasing the frequency and severity of short-term (~1 y) drought events-the most common duration of drought-globally. Yet the impact of this intensification of drought on ecosystem functioning remains poorly resolved. This is due in part to the widely disparate approaches ecologists have employed to study drought, variation in the severity and duration of drought studied, and differences among ecosystems in vegetation, edaphic and climatic attributes that can mediate drought impacts. To overcome these problems and better identify the factors that modulate drought responses, we used a coordinated distributed experiment to quantify the impact of short-term drought on grassland and shrubland ecosystems. With a standardized approach, we imposed ~a single year of drought at 100 sites on six continents. Here we show that loss of a foundational ecosystem function-aboveground net primary production (ANPP)-was 60% greater at sites that experienced statistically extreme drought (1-in-100-y event) vs. those sites where drought was nominal (historically more common) in magnitude (35% vs. 21%, respectively). This reduction in a key carbon cycle process with a single year of extreme drought greatly exceeds previously reported losses for grasslands and shrublands. Our global experiment also revealed high variability in drought response but that relative reductions in ANPP were greater in drier ecosystems and those with fewer plant species. Overall, our results demonstrate with unprecedented rigor that the global impacts of projected increases in drought severity have been significantly underestimated and that drier and less diverse sites are likely to be most vulnerable to extreme drought.

The positive effect of plant diversity on soil carbon depends on climate.

Little is currently known about how climate modulates the relationship between plant diversity and soil organic carbon and the mechanisms involved. Yet, this knowledge is of crucial importance in times of climate change and biodiversity loss. Here, we show that plant diversity is positively correlated with soil carbon content and soil carbon-to-nitrogen ratio across 84 grasslands on six continents that span wide climate gradients. The relationships between plant diversity and soil carbon as well as plant diversity and soil organic matter quality (carbon-to-nitrogen ratio) are particularly strong in warm and arid climates. While plant biomass is positively correlated with soil carbon, plant biomass is not significantly correlated with plant diversity. Our results indicate that plant diversity influences soil carbon storage not via the quantity of organic matter (plant biomass) inputs to soil, but through the quality of organic matter. The study implies that ecosystem management that restores plant diversity likely enhances soil carbon sequestration, particularly in warm and arid climates.

Environmental factors regulate soil microbial attributes and their response to drought in rangeland ecosystems.

In ecosystems, soil microbial variables characterization are used to determine soil biological health and the response of soils to environmental stress. Although there are strong associations between plants and soil microorganisms, they may respond asynchronously to environmental factors and severe droughts. We aimed to: I) evaluate the special variation of soil microbiome such as microbial biomass carbon (MBC) and nitrogen (MBN), soil basal respiration (SBR) and microbial indexes in eight rangeland sites located across an aridity gradient (distributed from arid to mesic climates); II) analyze the relative importance of main environmental factors (climate, soils, and plants) and their relationships with microbial variables in the rangelands; and III) assess the effect of drought on microbial and plant variables in field-based manipulative experiments. First, we found significant changes of microbial variables along a precipitation and temperature gradient. The responses of MBC and MBN were strongly dependent on soil pH, soil nitrogen (N), soil organic carbon (SOC), C:N ratio and vegetation cover. In contrast, SBR was influenced by the aridity index (AI), the mean annual precipitation (MAP), the soil pH and vegetation cover. MBC, MBN and SBR were negatively related with soil pH compared to the other factors (C, N, C:N, vegetation cover, MAP and AI) that had a positive relationship. Second, we found a stronger soil microbial variables response to drought in arid sites compared to humid rangelands. Third, the responses of MBC, MBN, and SBR to drought showed positive relationships with vegetation cover and aboveground biomass, but with different regression slopes, this suggest that plant and microbial communities responded differently to drought. The results from this study improve our understanding about the microbial response to drought in different rangelands, and may facilitate the development of predictive models on responses of soil microorganisms in carbon cycle under global change scenarios.

Are service crops an alternative for mitigating N2 O emissions in soybean crops in the Argentinian Pampas?

Service crops (or cover crops) play an important role in simplified agricultural systems. Service crops reduce agricultural external inputs and increase ecosystem services but their ability to mitigate nitrous oxide (N2 O) emissions is still uncertain. The main objective of this study was to evaluate N2 O emissions in soybean-soybean (Glycine max [L.] Merr) rotations that included different service crops. Treatments included continuous soybean with winter fallow and soybean with three service crops: oat (Avena sativa L.), vetch (Vicia villosa Roth.), and a mixture of oat and vetch in a randomized complete block design. Service crops were sown 2 months after soybean harvest and were terminated 2 months before soybean planting. Nitrous oxide emissions were determined during the fourth year of the field experiment. We found that service crops did not significantly affect overall mean N2 O emission rates, with mean emission rates from the fallow, oat, vetch, and oat-vetch treatments of 1.82 ± 0.35, 1.95 ± 0.34, 2.71 ± 0.43, and 2.42 ± 0.42 kg N2 O-N ha-1 per year, respectively. Service crops with low C/N ratios (vetch and oat-vetch mixtures) significantly increased N2 O emissions in spring, after their termination. Overall, soil inorganic N content (NO3 - or NH4 + ) was the main driver that explained the N2 O emissions from different treatments, whereas water-filled pore space controlled the temporal variability of emissions. Our results suggest that service crops with a very short growing season may increase soil N availability for cash crops, but do not reduce N2 O emissions due to long periods of high N availability without crops.

Evolutionary history of grazing and resources determine herbivore exclusion effects on plant diversity.

Ecological models predict that the effects of mammalian herbivore exclusion on plant diversity depend on resource availability and plant exposure to ungulate grazing over evolutionary time. Using an experiment replicated in 57 grasslands on six continents, with contrasting evolutionary history of grazing, we tested how resources (mean annual precipitation and soil nutrients) determine herbivore exclusion effects on plant diversity, richness and evenness. Here we show that at sites with a long history of ungulate grazing, herbivore exclusion reduced plant diversity by reducing both richness and evenness and the responses of richness and diversity to herbivore exclusion decreased with mean annual precipitation. At sites with a short history of grazing, the effects of herbivore exclusion were not related to precipitation but differed for native and exotic plant richness. Thus, plant species' evolutionary history of grazing continues to shape the response of the world's grasslands to changing mammalian herbivory.

Richness, not evenness, varies across water availability gradients in grassy biomes on five continents.

We sought to understand the role that water availability (expressed as an aridity index) plays in determining regional and global patterns of richness and evenness, and in turn how these water availability-diversity relationships may result in different richness-evenness relationships at regional and global scales. We examined relationships between water availability, richness and evenness for eight grassy biomes spanning broad water availability gradients on five continents. Our study found that relationships between richness and water availability switched from positive for drier (South Africa, Tibet and USA) vs. negative for wetter (India) biomes, though were not significant for the remaining biomes. In contrast, only the India biome showed a significant relationship between water availability and evenness, which was negative. Globally, the richness-water availability relationship was hump-shaped, however, not significant for evenness. At the regional scale, a positive richness-evenness relationship was found for grassy biomes in India and Inner Mongolia, China. In contrast, this relationship was weakly concave-up globally. These results suggest that different, independent factors are determining patterns of species richness and evenness in grassy biomes, resulting in differing richness-evenness relationships at regional and global scales. As a consequence, richness and evenness may respond very differently across spatial gradients to anthropogenic changes, such as climate change.

Higher than expected N2O emissions from soybean crops in the Pampas Region of Argentina: Estimates from DayCent simulations and field measurements.

In developing countries, agriculture generally represents a large fraction of GHG emissions reported in National Inventories, and emissions are typically estimated using Tier 1 IPCC guidelines. However, field data and locally adapted simulation models can improve the accuracy of IPCC estimations. In this study we aimed to quantify anthropogenic N2O emissions from croplands of Argentina through field measurements, model simulations and IPCC guidelines. We measured N2O emissions and their controlling factors in 62 plots of the Pampas Region with corn, soybean and wheat/soybean crops and in unmanaged grasslands. We accounted for gross emissions from crops and background emissions from unmanaged grasslands to calculate net anthropogenic emissions from crops as the difference between them. We calibrated and evaluated the DayCent model and then simulated different weather and management scenarios. Finally, we applied IPCC guidelines to estimate anthropogenic N2O emissions at the same plots. The DayCent model accurately simulated annual N2O emission for all crops as compared to measured data (RMSE = 1.4 g N ha-1 day-1). Measured and simulated emissions in soybean crops were higher than in corn and wheat/soybean crops. Gross N2O emissions ranged from 1.4 to 5.1 kg N ha-1 yr-1 for current environmental (soil and weather) and management (crops and fertilizer doses) conditions. Background emissions ranged between 1.1 and 1.3 kg N ha-1 yr-1, and therefore net anthropogenic emissions ranged from 0.3 to 4.0 kg N ha-1 yr-1. IPCC Tier 1 emission factors underestimated N2O releases from soybean, that were on average 4.87 times greater when estimated with DayCent and observations (0.53 vs 2.47 and 2.69 kg N ha-1 yr-1, respectively). On the contrary, IPCC estimates for corn and wheat/soybean crops were similar to modeled and measured values. Our results suggest that N2O emissions from the vast 15 million ha of soybean croplands in the Pampas Region may be substantially underestimated.

Livestock exclusion reduces the temporal stability of grassland productivity regardless of eutrophication.

Changes in livestock loads and eutrophication associated with human activities can modify the stability of grassland's aboveground net primary productivity (ANPP), by modifying the mean (µ) and/or standard deviation (σ) of ANPP. The changes in attributes of the plant community (i.e., species richness, species asynchrony, dominance) might in turn explain the ecosystem temporal (inter-annual) stability of grassland production. Here, we evaluated the interactive effects of changes in livestock loads and chronic nutrient addition on the temporal stability of ANPP (estimated as µ/σ) in temperate grasslands. We also assessed the role of different attributes of the plant community on ecosystem stability. We carried out a factorial experiment of domestic livestock exclusion and nutrient addition (10 g.m-2.year-1 of nitrogen, phosphorus, and potassium; n = 6 blocks) during five consecutive years in a natural grassland devoted to cattle production (Flooding Pampa, Argentina). Domestic livestock exclusion reduced ANPP stability by 65%, regardless of nutrient load, mainly by the increase of ANPP standard deviation. This reduction in ANPP stability after livestock exclusion was associated mostly with higher plant species dominance and also with reductions in plant effective richness and in the asynchrony of grassland's species. Despite not finding direct negative effects of eutrophication on ANPP stability, chronic nutrient addition decreased effective species richness and asynchrony, which may translate into reductions in ANPP stability in the future. Our findings highlight that the presence of livestock maintains the temporal stability of ANPP mainly by lowering the dominance of the plant community. However, increases in nutrient loads in grasslands devoted to livestock production may threaten grassland's stability.

Nutrient identity modifies the destabilising effects of eutrophication in grasslands.

Nutrient enrichment can simultaneously increase and destabilise plant biomass production, with co-limitation by multiple nutrients potentially intensifying these effects. Here, we test how factorial additions of nitrogen (N), phosphorus (P) and potassium with essential nutrients (K+) affect the stability (mean/standard deviation) of aboveground biomass in 34 grasslands over 7 years. Destabilisation with fertilisation was prevalent but was driven by single nutrients, not synergistic nutrient interactions. On average, N-based treatments increased mean biomass production by 21-51% but increased its standard deviation by 40-68% and so consistently reduced stability. Adding P increased interannual variability and reduced stability without altering mean biomass, while K+ had no general effects. Declines in stability were largest in the most nutrient-limited grasslands, or where nutrients reduced species richness or intensified species synchrony. We show that nutrients can differentially impact the stability of biomass production, with N and P in particular disproportionately increasing its interannual variability.

Negative effects of nitrogen override positive effects of phosphorus on grassland legumes worldwide.

Anthropogenic nutrient enrichment is driving global biodiversity decline and modifying ecosystem functions. Theory suggests that plant functional types that fix atmospheric nitrogen have a competitive advantage in nitrogen-poor soils, but lose this advantage with increasing nitrogen supply. By contrast, the addition of phosphorus, potassium, and other nutrients may benefit such species in low-nutrient environments by enhancing their nitrogen-fixing capacity. We present a global-scale experiment confirming these predictions for nitrogen-fixing legumes (Fabaceae) across 45 grasslands on six continents. Nitrogen addition reduced legume cover, richness, and biomass, particularly in nitrogen-poor soils, while cover of non-nitrogen-fixing plants increased. The addition of phosphorous, potassium, and other nutrients enhanced legume abundance, but did not mitigate the negative effects of nitrogen addition. Increasing nitrogen supply thus has the potential to decrease the diversity and abundance of grassland legumes worldwide regardless of the availability of other nutrients, with consequences for biodiversity, food webs, ecosystem resilience, and genetic improvement of protein-rich agricultural plant species.

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

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 ), that is, 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 an important context-dependent knowledge for grassland management worldwide.

Soil bacterial communities remain altered after 30 years of agriculture abandonment in Pampa grasslands.

Old fields are spreading in the world because of agriculture abandonment, and they show a persistence of exotic plant species with little recovery towards the original vegetation composition. Soil biota may also differ between old fields and native grasslands, but were comparatively less studied than plant communities, despite their importance in biogeochemical processes. Here we compared soil bacterial communities of exotic-dominated old fields with those of remnants of native grasslands in the Inland Pampa, Argentina, using the 16S rRNA gene amplicon sequencing approach. We also characterized plant communities, soil physico-chemical properties, and soil respiration. We expected more diverse soil bacterial communities, with higher heterogeneity, in remnant grasslands than in old fields because of a more diverse and more heterogeneous plant community. However, our results showed that soil bacterial communities had higher Shannon diversity in old fields than in remnant grasslands, but richness was not significantly different. Also we found different bacterial community compositions between grasslands even at a low taxonomic level. On the other hand, old fields harbored less heterogeneous bacterial communities than remnants, and bacteria and plant beta diversity were correlated. Despite contrasting plant and bacterial composition between old fields and remnant grasslands, soil physico-chemical properties were quite similar between grasslands. Overall, our results showed that bacterial communities in grassland soils were associated with changes in plant communities after agricultural abandonment. Plant-microbial feedbacks might regulate plant and soil bacterial community assemblage in old fields, yet further research is needed to demonstrate this potential feedback mechanism.

Microbial processing of plant remains is co-limited by multiple nutrients in global grasslands.

Microbial processing of aggregate-unprotected organic matter inputs is key for soil fertility, long-term ecosystem carbon and nutrient sequestration and sustainable agriculture. We investigated the effects of adding multiple nutrients (nitrogen, phosphorus and potassium plus nine essential macro- and micro-nutrients) on decomposition and biochemical transformation of standard plant materials buried in 21 grasslands from four continents. Addition of multiple nutrients weakly but consistently increased decomposition and biochemical transformation of plant remains during the peak-season, concurrent with changes in microbial exoenzymatic activity. Higher mean annual precipitation and lower mean annual temperature were the main climatic drivers of higher decomposition rates, while biochemical transformation of plant remains was negatively related to temperature of the wettest quarter. Nutrients enhanced decomposition most at cool, high rainfall sites, indicating that in a warmer and drier future fertilized grassland soils will have an even more limited potential for microbial processing of plant remains.

Nutrient addition increases grassland sensitivity to droughts.

Grasslands worldwide are expected to experience an increase in extreme events such as drought, along with simultaneous increases in mineral nutrient inputs as a result of human industrial activities. These changes are likely to interact because elevated nutrient inputs may alter plant diversity and increase the sensitivity to droughts. Dividing a system's sensitivity to drought into resistance to change during the drought and rate of recovery after the drought generates insights into different dimensions of the system's resilience in the face of drought. Here, we examine the effects of experimental nutrient fertilization and the resulting diversity loss on the resistance to and recovery from severe regional droughts. We do this at 13 North American sites spanning gradients of aridity, five annual grasslands in California, and eight perennial grasslands in the Great Plains. We measured rate of resistance as the change in annual aboveground biomass (ANPP) per unit change in growing season precipitation as conditions declined from normal to drought. We measured recovery as the change in ANPP during the postdrought period and the return to normal precipitation. Resistance and recovery did not vary across the 400-mm range of mean growing season precipitation spanned by our sites in the Great Plains. However, chronic nutrient fertilization in the Great Plains reduced drought resistance and increased drought recovery. In the California annual grasslands, arid sites had a greater recovery postdrought than mesic sites, and nutrient addition had no consistent effects on resistance or recovery. Across all study sites, we found that predrought species richness in natural grasslands was not consistently associated with rates of resistance to or recovery from the drought, in contrast to earlier findings from experimentally assembled grassland communities. Taken together, these results suggest that human-induced eutrophication may destabilize grassland primary production, but the effects of this may vary across regions and flora, especially between perennial and annual-dominated grasslands.

Nitrous oxide emissions decrease with plant diversity but increase with grassland primary productivity.

Nitrous oxide (N2O), a main greenhouse gas that contributes to ozone layer depletion, is released from soils. Even when it has been argued that agriculture is the main cause of its increase in the atmosphere, natural ecosystems are also an important source of N2O. However, the impacts of human activities on N2O emissions through biodiversity loss or primary productivity changes in natural ecosystems have rarely been assessed. Here, we analyzed the effects of vegetation attributes such as plant diversity and production, as drivers of N2O emission rates, in addition to environmental factors. We measured N2O emissions monthly during 1 year in 12 sites covering a large portion of the Rio de la Plata grasslands, Argentina, and related these emissions with climate, soil and vegetation attributes. We performed spatial and temporal models of N2O emissions separately, to evaluate which drivers control N2O in space and over time independently. Our results showed that in the spatial model, N2O emissions decreased with increments in plant species richness, with concomitant reductions in soil [Formula: see text] whereas N2O emissions increased with primary productivity. By contrast, in the temporal model, monthly precipitation and monthly temperature were the main drivers of N2O emissions, with positive correlations, showing important differences with the spatial model. Overall, our results show that biological drivers may exert substantial control of N2O emissions at large spatial scales, together with climate and soil variables. Our results suggest that biodiversity conservation of natural grasslands may reduce regional greenhouse gas emissions, besides maintaining other important ecosystem services.

Gross, Background, and Net Anthropogenic Soil Nitrous Oxide Emissions from Soybean, Corn, and Wheat Croplands.

Agricultural soils are the largest single source of NO emissions globally. However, soils left uncultivated would still release NO. Distinguishing anthropogenic from natural emissions (i.e., background emissions) in crops is important if we want to assess the net effect of human activity. This study aimed to characterize NO emissions from croplands and unmanaged grasslands to estimate the net anthropogenic emissions and to gain a better insight into their main drivers. We established a replicated manipulative field experiment in the Pampas Region of Argentina to quantify soil NO emissions from corn ( L.), wheat ( L.), and soybean [ (L.) Merr.] crops, and from adjacent unmanaged grassland plots for 1 yr. We also analyzed the main controls of NO emissions and the correlation between the normalized difference vegetation index (NDVI) and NO fluxes. Background emissions represented between 21 and 32% of total emissions from croplands, depending on crop type. No differences were detected in NO emissions between total and background during winter and peak crop growing season. NDVI showed a significant correlation with NO fluxes which was positive in grasslands and negative in growing season of soybean crops. Our results showed that NO emissions from croplands were higher than background emissions, but also that background represented an important fraction of cropland emissions. Higher emissions in croplands occurred during pre-seeding, after harvest, and after N fertilization in fertilized crops. In addition, our study informs about NO emissions from crops and unmanaged systems in South America where field data are very scarce.

Local loss and spatial homogenization of plant diversity reduce ecosystem multifunctionality.

Biodiversity is declining in many local communities while also becoming increasingly homogenized across space. Experiments show that local plant species loss reduces ecosystem functioning and services, but the role of spatial homogenization of community composition and the potential interaction between diversity at different scales in maintaining ecosystem functioning remains unclear, especially when many functions are considered (ecosystem multifunctionality). We present an analysis of eight ecosystem functions measured in 65 grasslands worldwide. We find that more diverse grasslands-those with both species-rich local communities (α-diversity) and large compositional differences among localities (ß-diversity)-had higher levels of multifunctionality. Moreover, α- and ß-diversity synergistically affected multifunctionality, with higher levels of diversity at one scale amplifying the contribution to ecological functions at the other scale. The identity of species influencing ecosystem functioning differed among functions and across local communities, explaining why more diverse grasslands maintained greater functionality when more functions and localities were considered. These results were robust to variation in environmental drivers. Our findings reveal that plant diversity, at both local and landscape scales, contributes to the maintenance of multiple ecosystem services provided by grasslands. Preserving ecosystem functioning therefore requires conservation of biodiversity both within and among ecological communities.

Asynchrony among local communities stabilises ecosystem function of metacommunities.

Temporal stability of ecosystem functioning increases the predictability and reliability of ecosystem services, and understanding the drivers of stability across spatial scales is important for land management and policy decisions. We used species-level abundance data from 62 plant communities across five continents to assess mechanisms of temporal stability across spatial scales. We assessed how asynchrony (i.e. different units responding dissimilarly through time) of species and local communities stabilised metacommunity ecosystem function. Asynchrony of species increased stability of local communities, and asynchrony among local communities enhanced metacommunity stability by a wide range of magnitudes (1-315%); this range was positively correlated with the size of the metacommunity. Additionally, asynchronous responses among local communities were linked with species' populations fluctuating asynchronously across space, perhaps stemming from physical and/or competitive differences among local communities. Accordingly, we suggest spatial heterogeneity should be a major focus for maintaining the stability of ecosystem services at larger spatial scales.

Asymmetric responses of primary productivity to precipitation extremes: A synthesis of grassland precipitation manipulation experiments.

Climatic changes are altering Earth's hydrological cycle, resulting in altered precipitation amounts, increased interannual variability of precipitation, and more frequent extreme precipitation events. These trends will likely continue into the future, having substantial impacts on net primary productivity (NPP) and associated ecosystem services such as food production and carbon sequestration. Frequently, experimental manipulations of precipitation have linked altered precipitation regimes to changes in NPP. Yet, findings have been diverse and substantial uncertainty still surrounds generalities describing patterns of ecosystem sensitivity to altered precipitation. Additionally, we do not know whether previously observed correlations between NPP and precipitation remain accurate when precipitation changes become extreme. We synthesized results from 83 case studies of experimental precipitation manipulations in grasslands worldwide. We used meta-analytical techniques to search for generalities and asymmetries of aboveground NPP (ANPP) and belowground NPP (BNPP) responses to both the direction and magnitude of precipitation change. Sensitivity (i.e., productivity response standardized by the amount of precipitation change) of BNPP was similar under precipitation additions and reductions, but ANPP was more sensitive to precipitation additions than reductions; this was especially evident in drier ecosystems. Additionally, overall relationships between the magnitude of productivity responses and the magnitude of precipitation change were saturating in form. The saturating form of this relationship was likely driven by ANPP responses to very extreme precipitation increases, although there were limited studies imposing extreme precipitation change, and there was considerable variation among experiments. This highlights the importance of incorporating gradients of manipulations, ranging from extreme drought to extreme precipitation increases into future climate change experiments. Additionally, policy and land management decisions related to global change scenarios should consider how ANPP and BNPP responses may differ, and that ecosystem responses to extreme events might not be predicted from relationships found under moderate environmental changes.

Pushing precipitation to the extremes in distributed experiments: recommendations for simulating wet and dry years.

Intensification of the global hydrological cycle, ranging from larger individual precipitation events to more extreme multiyear droughts, has the potential to cause widespread alterations in ecosystem structure and function. With evidence that the incidence of extreme precipitation years (defined statistically from historical precipitation records) is increasing, there is a clear need to identify ecosystems that are most vulnerable to these changes and understand why some ecosystems are more sensitive to extremes than others. To date, opportunistic studies of naturally occurring extreme precipitation years, combined with results from a relatively small number of experiments, have provided limited mechanistic understanding of differences in ecosystem sensitivity, suggesting that new approaches are needed. Coordinated distributed experiments (CDEs) arrayed across multiple ecosystem types and focused on water can enhance our understanding of differential ecosystem sensitivity to precipitation extremes, but there are many design challenges to overcome (e.g., cost, comparability, standardization). Here, we evaluate contemporary experimental approaches for manipulating precipitation under field conditions to inform the design of 'Drought-Net', a relatively low-cost CDE that simulates extreme precipitation years. A common method for imposing both dry and wet years is to alter each ambient precipitation event. We endorse this approach for imposing extreme precipitation years because it simultaneously alters other precipitation characteristics (i.e., event size) consistent with natural precipitation patterns. However, we do not advocate applying identical treatment levels at all sites - a common approach to standardization in CDEs. This is because precipitation variability varies >fivefold globally resulting in a wide range of ecosystem-specific thresholds for defining extreme precipitation years. For CDEs focused on precipitation extremes, treatments should be based on each site's past climatic characteristics. This approach, though not often used by ecologists, allows ecological responses to be directly compared across disparate ecosystems and climates, facilitating process-level understanding of ecosystem sensitivity to precipitation extremes.