• Energy Research

Litter decomposition is a key process for carbon and nutrient cycling in terrestrial ecosystems and is mainly controlled by environmental conditions, substrate quantity and quality as well as microbial community abundance and composition. In particular, the effects of climate and atmospheric nitrogen (N) deposition on litter decomposition and its temporal dynamics are of significant importance, since their effects might change over the course of the decomposition process. Within the TeaComposition initiative, we incubated Green and Rooibos teas at 524 sites across nine biomes. We assessed how macroclimate and atmospheric inorganic N deposition under current and predicted scenarios (RCP 2.6, RCP 8.5) might affect litter mass loss measured after 3 and 12 months. Our study shows that the early to mid-term mass loss at the global scale was affected predominantly by litter quality (explaining 73% and 62% of the total variance after 3 and 12 months, respectively) followed by climate and N deposition. The effects of climate were not litter-specific and became increasingly significant as decomposition progressed, with MAP explaining 2% and MAT 4% of the variation after 12 months of incubation. The effect of N deposition was litter-specific, and significant only for 12-month decomposition of Rooibos tea at the global scale. However, in the temperate biome where atmospheric N deposition rates are relatively high, the 12-month mass loss of Green and Rooibos teas decreased significantly with increasing N deposition, explaining 9.5% and 1.1% of the variance, respectively. The expected changes in macroclimate and N deposition at the global scale by the end of this century are estimated to increase the 12-month mass loss of easily decomposable litter by 1.1–3.5% and of the more stable substrates by 3.8–10.6%, relative to current mass loss. In contrast, expected changes in atmospheric N deposition will decrease the mid-term mass loss of high-quality litter by 1.4–2.2% and that of low-quality litter by 0.9–1.5% in the temperate biome. Our results suggest that projected increases in N deposition may have the capacity to dampen the climate-driven increases in litter decomposition depending on the biome and decomposition stage of substrate.

Over the last century, lands used for livestock and agriculture have been progressively abandoned on mountains across Europe. The afforestation of abandoned agricultural lands leads to significant environmental impacts particularly in karst catchments, which are highly responsive to climate change and human disturbances. Due to the lack of long-term historical records and appropriate methods, these impacts are not yet well investigated. The remote Baget forested catchment (Pyrenees Mountains), where a long-term hydrochemical survey has been conducted since 1979, was chosen to develop a method for reconstructing the evolution of landscapes based on heterogeneous and multi-temporal satellite imagery and aerial photographs from 1942 onwards. An indicator of landscape change is proposed, based on three vegetation groups, compatible with the various supporting data. The spatio-temporal landscape evolution evidenced a regular increase of 31% in forest extension between 1942 and 2019, while pioneer (grassland and heathlands) and open forest (early woodland) stages showed an overall but uneven decrease, marked by an accelerated decline of the latter due to an increase in livestock activities. The trends of streamwater discharge and chemistry were related to these environmental changes. A set of processes influencing the changes in the hydrochemical patterns, particularly the increase in dissolved elements in streamwater originating from carbonate dissolution, is proposed and discussed. The forest recolonization (+0.05 Km2.yr−1) enhanced soil organic matter content, generating an increased pCO2 in soils, which in turn enhanced the carbonate dissolution, expressed by a concentration increase of + 5 μeq.L-1.yr−1 for (Ca2++Mg2+) and of + 10 μeq.L-1.yr−1 for HCO3–. The steeper slope of HCO3– could result from decreasing terrestrial Ca2++Mg2+ export together with decreasing SO42- leaching (−4 μeq.L-1.yr−1) due to decreasing acid atmospheric deposition. In the meantime, the observed increase in air temperature (+0.03 °C.yr−1) favoured tree growth, litter decomposition and water demand by trees. This study has demonstrated the powerful indicator of linking the historical evolution of the landscape, quantified using images, to the observed trends in the hydrochemical composition of stream waters, to highlight the internal processes of the critical zone. However, because of nested effects, it still remains a challenge to strictly quantify their respective impact on streamwater chemistry.

El lago de Milluni Grande (LMG) está ubicado en el flanco occidental de los Andes Orientales en el valle de Milluni que posee una clásica forma en U dado que se encuentra en una región glacial. El lago es el más grande de este valle, y dado que se halla situado al cierre de cuenca, resulta estratégico para albergar un registro sedimentario de los últimos 450 años. El registro sedimentario fue estudiado a partir de un testigo del lago, del cual se describieron y analizaron detalladamente parámetros físicos y químicos, sedimentología, mineralogía e indicadores geoquímicos los que permitieron evidenciar una importante variación del clima (VC). Adicionalmente, una datación con 210Pb en los niveles mas superficiales del testigo fue usada para establecer una tasa de sedimentación del lago y estimar una cronología para los niveles sin datación. La cronología definida señala que la variación del clima detectada sucede durante la Pequeña Edad de Hielo (LIA). Además, una correlación detallada fue establecida entre los niveles arenosos del registro sedimentario del LMG y las morrenas glaciarias del Cerro Charquini. Finalmente, algunas evidencias históricas bien documentadas sobre el desarrollo de Pequeña Edad de Hielo en Bolivia son mencionadas.

A dynamic coupled biogeochemical-ecological model was used to simulate the effects of nitrogen deposition and climate change on plant communities at three forest sites in France. The three sites had different forest covers (sessile oak, Norway spruce and silver fir), three nitrogen loads ranging from relatively low to high, different climatic regions and different soil types. Both the availability of vegetation time series and the environmental niches of the understory species allowed to evaluate the model for predicting the composition of the three plant communities. The calibration of the environmental niches was successful, with a model performance consistently reasonably high throughout the three sites. The model simulations of two climatic and two deposition scenarios showed that climate change may entirely compromise the eventual recovery from eutrophication of the simulated plant communities in response to the reductions in nitrogen deposition. The interplay between climate and deposition was strongly governed by site characteristics and histories in the long term, while forest management remained the main driver of change in the short term.