• Energy Research

Una amplia gama de investigaciones muestra que la disponibilidad de nutrientes influye fuertemente en el ciclo del carbono (C) terrestre y da forma a las respuestas de los ecosistemas a los cambios ambientales y, por lo tanto, a las retroalimentaciones terrestres al clima. Sin embargo, nuestra comprensión de los controles de nutrientes sigue estando lejos de ser completa y mal cuantificada, al menos en parte debido a la falta de conjuntos de datos informativos, comparables y accesibles a escalas regionales a globales. Una creciente infraestructura de investigación de redes multisitio está proporcionando datos valiosos sobre flujos y existencias de C y está monitoreando sus respuestas al cambio ambiental global y midiendo las respuestas a los tratamientos experimentales. Por lo tanto, estas redes brindan una oportunidad para mejorar nuestra comprensión de las interacciones del ciclo de los nutrientes C y nuestra capacidad para modelarlas. Sin embargo, todavía falta información coherente sobre cómo interactúa el ciclo de nutrientes con los patrones del ciclo C observados. Aquí, argumentamos que complementar las mediciones del ciclo C disponibles de los sitios de monitoreo y experimentación con los datos que caracterizan la disponibilidad de nutrientes mejorará en gran medida su poder y mejorará nuestra capacidad para pronosticar futuras trayectorias del ciclo C terrestre y el clima. Por lo tanto, proponemos un conjunto de mediciones complementarias que son relativamente fáciles de realizar de forma rutinaria en cualquier sitio o experimento y que, en combinación con las observaciones del ciclo C, pueden proporcionar una caracterización sólida de los efectos de la disponibilidad de nutrientes en todos los sitios. Además, discutimos el poder de diferentes variables observables para informar la formulación de modelos y restringir sus predicciones. La mayoría de las mediciones ampliamente disponibles de la disponibilidad de nutrientes a menudo no se alinean bien con las necesidades actuales de modelado. Esto pone de relieve la importancia de fomentar la interacción entre las comunidades empírica y de modelización para establecer futuras prioridades de investigación. Un large éventail de recherches montre que la disponibilité des nutriments influence fortement le cycle du carbone terrestre (C) et façonne les réponses des écosystèmes aux changements environnementaux et donc les rétroactions terrestres sur le climat. Néanmoins, notre compréhension des contrôles des nutriments reste loin d'être complète et mal quantifiée, du moins en partie en raison d'un manque d'ensembles de données informatifs, comparables et accessibles à l'échelle régionale et mondiale. Une infrastructure de recherche croissante de réseaux multi-sites fournit des données précieuses sur les flux et les stocks de C et surveille leurs réponses aux changements environnementaux mondiaux et mesure les réponses aux traitements expérimentaux. Ces réseaux offrent ainsi une opportunité d'améliorer notre compréhension des interactions du cycle C-nutriment et notre capacité à les modéliser. Cependant, il manque encore généralement des informations cohérentes sur la façon dont le cycle des nutriments interagit avec les modèles de cycle C observés. Ici, nous soutenons que le fait de compléter les mesures de cycle C disponibles à partir de sites de surveillance et expérimentaux par des données caractérisant la disponibilité des nutriments améliorera considérablement leur puissance et améliorera notre capacité à prévoir les trajectoires futures du cycle C terrestre et du climat. Par conséquent, nous proposons un ensemble de mesures complémentaires qui sont relativement faciles à effectuer de manière routinière sur n'importe quel site ou expérience et qui, en combinaison avec les observations du cycle C, peuvent fournir une caractérisation robuste des effets de la disponibilité des nutriments sur tous les sites. De plus, nous discutons de la puissance des différentes variables observables pour éclairer la formulation des modèles et contraindre leurs prédictions. La plupart des mesures largement disponibles de la disponibilité des nutriments ne correspondent souvent pas bien aux besoins actuels de modélisation. Cela souligne l'importance de favoriser l'interaction entre les communautés empiriques et de modélisation pour établir les futures priorités de recherche. A wide range of research shows that nutrient availability strongly influences terrestrial carbon (C) cycling and shapes ecosystem responses to environmental changes and hence terrestrial feedbacks to climate. Nonetheless, our understanding of nutrient controls remains far from complete and poorly quantified, at least partly due to a lack of informative, comparable, and accessible datasets at regional-to-global scales. A growing research infrastructure of multi-site networks are providing valuable data on C fluxes and stocks and are monitoring their responses to global environmental change and measuring responses to experimental treatments. These networks thus provide an opportunity for improving our understanding of C-nutrient cycle interactions and our ability to model them. However, coherent information on how nutrient cycling interacts with observed C cycle patterns is still generally lacking. Here, we argue that complementing available C-cycle measurements from monitoring and experimental sites with data characterizing nutrient availability will greatly enhance their power and will improve our capacity to forecast future trajectories of terrestrial C cycling and climate. Therefore, we propose a set of complementary measurements that are relatively easy to conduct routinely at any site or experiment and that, in combination with C cycle observations, can provide a robust characterization of the effects of nutrient availability across sites. In addition, we discuss the power of different observable variables for informing the formulation of models and constraining their predictions. Most widely available measurements of nutrient availability often do not align well with current modelling needs. This highlights the importance to foster the interaction between the empirical and modelling communities for setting future research priorities. تُظهر مجموعة واسعة من الأبحاث أن توافر المغذيات يؤثر بشدة على دورة الكربون الأرضي (C) ويشكل استجابات النظام البيئي للتغيرات البيئية وبالتالي التغذية المرتدة الأرضية للمناخ. ومع ذلك، لا يزال فهمنا لضوابط المغذيات بعيدًا عن الاكتمال وقياسه الكمي ضعيفًا، ويرجع ذلك جزئيًا على الأقل إلى نقص مجموعات البيانات المفيدة والقابلة للمقارنة والتي يمكن الوصول إليها على المستويات الإقليمية والعالمية. توفر البنية التحتية البحثية المتنامية للشبكات متعددة المواقع بيانات قيمة عن تدفقات الكربون والمخزونات وترصد استجاباتها للتغير البيئي العالمي وتقيس استجاباتها للعلاجات التجريبية. وبالتالي توفر هذه الشبكات فرصة لتحسين فهمنا لتفاعلات دورة المغذيات C وقدرتنا على نمذجتها. ومع ذلك، لا تزال المعلومات المتماسكة حول كيفية تفاعل دورة المغذيات مع أنماط الدورة C المرصودة غير متوفرة بشكل عام. هنا، نجادل بأن استكمال قياسات الدورة C المتاحة من مواقع المراقبة والتجربة بالبيانات التي تميز توافر المغذيات سيعزز إلى حد كبير قوتها وسيحسن قدرتنا على التنبؤ بالمسارات المستقبلية للدورة C الأرضية والمناخ. لذلك، نقترح مجموعة من القياسات التكميلية التي يسهل إجراؤها نسبيًا بشكل روتيني في أي موقع أو تجربة والتي، جنبًا إلى جنب مع ملاحظات الدورة ج، يمكن أن توفر توصيفًا قويًا لتأثيرات توفر المغذيات عبر المواقع. بالإضافة إلى ذلك، نناقش قوة المتغيرات المختلفة التي يمكن ملاحظتها للإبلاغ عن صياغة النماذج وتقييد تنبؤاتها. غالبًا ما لا تتوافق معظم القياسات المتاحة على نطاق واسع لتوافر المغذيات بشكل جيد مع احتياجات النمذجة الحالية. وهذا يسلط الضوء على أهمية تعزيز التفاعل بين المجتمعات التجريبية والنمذجة لتحديد أولويات البحث في المستقبل.

Imbalance-P paper. Contact with Bjarni D.Sigurdsson This article describes how natural geothermal soil temperature gradients in Iceland have been used to study terrestrial ecosystem responses to soil warming. The experimental approach was evaluated at three study sites in southern Iceland; one grassland site that has been warm for at least 50 years (GO), and another comparable grassland site (GN) and a Sitka spruce plantation (FN) site that have both been warmed since an earthquake took place in 2008. Within each site type, five ca. 50 m long transects, with six permanent study plots each, were established across the soil warming gradients, spanning from unwarmed control conditions to gradually warmer soils. It was attempted to select the plots so the annual warming levels would be ca. +1, +3, +5, +10 and +20 °C within each transect. Results of continuous measurements of soil temperature (Ts) from 2013-2015 revealed that the soil warming was relatively constant and followed the seasonal Ts cycle of the unwarmed control plots. Volumetric water content in the top 5 cm of soil was repeatedly surveyed during 2013-2016. The grassland soils were wetter than the FN soils, but they had sometimes some significant warming-induced drying in the surface layer of the warmest plots, in contrast to FN. Soil chemistry did not show any indications that geothermal water had reached the root zone, but soil pH did increase somewhat with warming, which was probably linked to vegetation changes. As expected, the potential decomposition rate of organic matter increased significantly with warming. It was concluded that the natural geothermal gradients at the ForHot sites in Iceland offered realistic conditions for studying terrestrial ecosystem responses to warming with minimal artefacts.

Young individuals of a single black cottonwood (Populus trichocarpa Torr. & Gray) clone were raised for three growing seasons in whole-tree chambers and exposed to either ambient or elevated atmospheric carbon dioxide concentration ([CO2]), with either a high or a low mineral nutrient supply, in a factorial experimental design. Nutrient availability had a larger effect on growth and dry matter partitioning than did [CO2]. Total biomass did not differ significantly with CO2 treatment when nutrient availability was low. However, elevated [CO2] increased whole-plant biomass by 47% in the high nutrient availability treatment. Carbon dioxide enrichment reduced leaf area ratio and specific leaf area significantly, but had no significant effect on mean leaf size or leaf mass ratio. Root mass ratio was significantly increased by elevated [CO2] at low, but not at high nutrient availability. A modified "demographic harvesting approach" made possible the retrospective estimation of stem and branch dry masses for different years. The relative growth rates of stem and branch were significantly enhanced by elevated [CO2] with high, but not with low nutrient availability. Canopy productivity index (CPI), i.e., the amount of stem and branch wood produced annually per unit leaf area, was raised 12% by elevated [CO2] when nutrient availability was high, but was reduced when nutrient availability was low, because of increased below ground allocation.

Climate warming is often more detrimental to large body sized organisms than small body sized organisms. Yet, how such differential effects of warming at organismal levels affect aggregate community properties, such as community biomass, remains little understood. Here, using geothermally warmed sub-Arctic grassland soils, we investigate how total biomass (product of density and individual body mass) of two major groups of soil microarthropods (Collembola and mites), which are composed of both large and small body sized species, shift in warmed soils when warmed by ∼3–∼6 °C. Our results show that total biomass of Collembola significantly decreased in warmed soils predominantly due to a decline in the density of large body sized species. In contrast, total mite biomass showed a unimodal response to warming. As a result, there was a shift towards mite biomass dominated microarthropod communities in warmed soils. Within Collembola, the deep soil living eu-edaphic functional group declined the most in total biomass, whereas the unimodal response in mites was most pronounced in oribatid mites. Our study highlights that warming induced shifts in total community biomass of soil microarthropods are likely due to greater detrimental effects of warming on several large body sized Collembola.

Contents Summary 464 I. Introduction 464 II. Net ecosystem exchange and changes in carbon stocks 466 III. Elevated [CO2] 468 IV. Temperature 470 V. Fertilization and nitrogen deposition 471 VI. Disturbances and forest management 472 VII. Feedbacks and interactions 474 VIII. Will we have forest carbon sinks in the future? 475 Acknowledgements 476 References 476

Temperature governs most biotic processes, yet we know little about how warming affects whole ecosystems. Here we examined the responses of 128 components of a subarctic grassland to either 5-8 or >50 years of soil warming. Warming of >50 years drove the ecosystem to a new steady state possessing a distinct biotic composition and reduced species richness, biomass and soil organic matter. However, the warmed state was preceded by an overreaction to warming, which was related to organism physiology and was evident after 5-8 years. Ignoring this overreaction yielded errors of >100% for 83 variables when predicting their responses to a realistic warming scenario of 1 °C over 50 years, although some, including soil carbon content, remained stable after 5-8 years. This study challenges long-term ecosystem predictions made from short-term observations, and provides a framework for characterization of ecosystem responses to sustained climate change.