• Energy Research

AbstractQuestionClimate change scenarios predict alterations in interannual variability of precipitation and increases of extreme droughts. Drought effects will reflect ecosystems' sensitivity (i.e. the magnitude of the response relative to the change in amount of precipitation), with different consequences for plant communities and ecosystem functioning. However, sensitivity estimations across ecosystems and among years are contingent upon the metric applied. Here, we asked how different sensitivity indices reflect the responses to drought in contrasting ecosystems.MethodsWe proposed a novel index to estimate sensitivity in drought experiments, which consider the change in the response variable in relation to the reduction in precipitation, standardized by the mean precipitation use efficiency of the site, making it suitable to compare among contrasting ecosystems. We compared the information gained from the new index with two indices commonly used in experimentally induced droughts, through simulated scenarios. Besides, we applied the indices to data from previous field drought experiments to evaluate relative and absolute sensitivity to drought in worldwide grassland ecosystems along a wide precipitation gradient.ResultsWe found that both relative and absolute approaches yielded important information to determine drought sensitivity in different simulated scenarios and under interannual precipitation variability. When the indices were applied to real situations, relative sensitivity to drought showed a significant increase as mean annual precipitation decreases whereas absolute sensitivity did not change significantly with precipitation.ConclusionsOur findings highlighted that absolute and relative indices to estimate drought sensitivity, rather than being redundant, may complement each other to inform how ecosystems respond to drought and to provide a more holistic assessment of vulnerable systems toward increasing interannual precipitation variability. This complementary approach enhances our understanding of ecosystem response under different conditions of precipitation variability and of the mechanisms that determine ecosystem changes during extreme droughts.

Abstract Droughts are projected to increase in magnitude, frequency and duration in the near future. In rangelands, the provision of valuable ecosystem services such as forage supply for livestock productivity is intimately linked to rainfall patterns, which makes it particularly vulnerable to droughts. Nonetheless, rangelands can differ in their sensitivity to droughts as shown by strong differences in the impacts of inter‐annual precipitation changes on vegetation productivity in different sites. The aim of this study was to assess the sensitivity to droughts of nine rangelands located across a broad aridity gradient in Argentina, South America. We experimentally imposed comparable droughts under field conditions by reducing a fixed proportion of each incoming precipitation event within‐year during three consecutive years and tracked changes in total aboveground and forage productivity. We found that arid and semi‐arid rangelands were more severely impaired in their forage provision by drought than mesic rangelands, that is that sensitivity to drought declined as aridity decreased. Forage productivity decreased on average by c. 50%, in arid and semi‐arid rangelands, whereas mesic sites did not exhibit significant changes between drought and control treatments. The negative impact in forage productivity of arid and semi‐arid rangelands was mainly driven by the productivity reduction of few key plant species at each site. In seven of the nine rangelands, we found detrimental effects of drought on forage productivity during the first experimental‐drought year, and in five of them the impact was further accentuated until the end of the experiment, which indicates how serious can these events be. Synthesis and applications. Our main findings indicate that the drought‐induced impacts on forage provision are higher as aridity increases. This pattern highlights the urgent need to implement strategies to mitigate the detrimental consequences of drought, particularly in arid and semiarid rangelands, where forage provision is strongly associated with human well‐being. Management approaches focused on key forage species, such as reducing the grazing pressure during drought periods according to these species' productivity dynamics can attenuate impacts on vulnerable ecosystems, preserving the rangelands' integrity while maintaining high long‐term productivity levels.

Plant biomass tends to increase under nutrient addition and decrease under drought. Biotic and abiotic factors influence responses to both, making the combined impact of nutrient addition and drought difficult to predict. Using a globally distributed network of manipulative field experiments, we assessed grassland aboveground biomass response to both drought and increased nutrient availability at 26 sites across nine countries. Overall, drought reduced biomass by 19% and nutrient addition increased it by 24%, resulting in no net impact under combined drought and nutrient addition. Among the plant functional groups, only graminoids responded positively to nutrients during drought. However, these general responses depended on local conditions, especially aridity. Nutrient effects were stronger in arid grasslands and weaker in humid regions and nitrogen-rich soils, although nutrient addition alleviated drought effects the most in subhumid sites. Biomass responses were weaker with higher precipitation variability. Biomass increased more with increased nutrient availability and declined more with drought at high-diversity sites than at low-diversity sites. Our findings highlight the importance of local abiotic and biotic conditions in predicting grassland responses to anthropogenic nutrient and climate changes.

Overseeding legumes in natural grasslands coupled with phosphorous fertilization are management practices oriented to increase forage production and quality, and to restore nutrient losses generated by livestock. Several studies show increases in forage due to this practice, but less is known about impacts on soil fertility and carbon sequestration. The objective of this study was to evaluate under real farm conditions changes in root C and N stocks and soil organic carbon (SOC) and nitrogen (SON) stocks in two different soil pools, the particulate organic matter (POM) and the mineral associated organic matter (MAOM), after the introduction in natural grasslands of a legume species, Lotus subbiflorus cv. "El Rincón", accompanied with phosphorous fertilization. We also evaluated changes in the natural abundance of 15N and 13C in soils and roots to understand changes in N fixation and species composition. We selected 12 adjacent paddocks of natural grasslands (NG) and natural grasslands overseeded with legumes and fertilized with phosphorous (NGLP) located in commercial farms in Uruguay. We found that overseeding legumes increased root C and N stocks and SOC and SON stocks in some farms but decreased them in others. On average, no significant differences arose between NGLP and NG paddocks in total stocks of 0-30 cm depth. However, higher C stocks were observed in POM of NGLP paddocks in 0-5 cm layer and lower contents in 5-10 cm layer indicating a change in the vertical distribution of C in POM. Changes in δ15N suggest that atmospheric N is being fixed by legumes in NGLP paddocks, but not translated into more N or C stocks in the MAOM fraction, probably due to high N losses promoted by cattle grazing. Our work suggests that carbon sequestration can be achieved after legumes introduction in grazed natural grasslands but will depend on grazing management practices.