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Soil moisture profile sensors (SMPSs) have a high potential for climate-smart agriculture due to their easy handling and ability to perform simultaneous measurements at different depths. To date, an accurate and easy-to-use method for the evaluation of long SMPSs is not available. In this study, we developed laboratory and field experiments to evaluate three different SMPSs (SoilVUE10, Drill&Drop, and SMT500) in terms of measurement accuracy, sensor-to-sensor variability, and temperature stability. The laboratory experiment features a temperature-controlled lysimeter to evaluate intra-sensor variability and temperature stability of SMPSs. The field experiment features a water level-controlled sandbox and reference TDR measurements to evaluate the soil water measurement accuracy of the SMPS. In both experiments, a well-characterized fine sand was used as measurement medium to ensure homogeneous dielectric properties in the measurement domain of the sensors. The laboratory experiments with the lysimeter showed that the Drill&Drop sensor has the highest temperature sensitivity with a decrease of 0.014 m3 m−3 per 10 °C, but at the same time showed the lowest intra- and inter-sensor variability. The field experiment with the sandbox showed that all three SMPSs have a similar performance (average RMSE ≈ 0.023 m3 m−3) with higher uncertainties at intermediate soil moisture contents. The presented combination of laboratory and field tests were found to be well suited to evaluate the performance of SMPSs and will be used to test additional SMPSs in the future. [ABSTRACT FROM AUTHOR]

Over the past decades, human‐induced climate change has led to a widespread wetting and warming of the Tibetan Plateau (TP), affecting both ecosystems and the carbon cycling therein. Whether the previously observed climate changes stimulate carbon uptake via enhanced photosynthesis or carbon loss via enhanced soil respiration remains unclear. Here we present 14 years of observations of carbon fluxes, meteorological variables and remotely sensed plant cover estimations from a central Tibetan alpine steppe ecosystem at Nam Co, the third largest lake on the TP. Using modified Mann‐Kendall trend tests, we found a significant increasing daily net carbon uptake of 0.5 g C m−2 decade−1, which can be explained by a widespread greening at the southern shore of lake Nam Co. The Plateau‐wide changes in temperature and precipitation are locally expressed as an increasing diurnal temperature range during winter, higher water availability during spring, higher cloud cover during early summer and less water availability during late summer. While these changes differ over the course of the year, they tend to stimulate plant growth more than microbial respiration, leading to an increased carbon uptake during all seasons. This study indicates that during the 14 years study period, a higher amplitude in winter temperatures and an earlier summer monsoon promote carbon uptake in a central Tibetan alpine steppe ecosystem. Plain Language Summary: Similar to the northern and southern high latitudes, global warming is especially pronounced on the Tibetan Plateau (TP). It remains uncertain, in which way the alpine short grass steppe responds to the observed warming and wetting of the TP. By analyzing nearly 14 years of data describing the carbon dioxide (CO2) exchange between the steppe and the atmosphere, we found that the ecosystem acts as a small carbon sink during recent years. Furthermore, we found that the steppe is becoming greener and stores more CO2 during our study period. The trend in carbon uptake can be attributed to different mechanisms for different seasons. During winter, higher daytime temperatures promote plant growth which leads to higher uptake of CO2 through photosynthesis. An earlier summer monsoon provides more cloud cover during early summer, which relieves drought stress from the plants, thus increasing photosynthetic uptake of CO2. We observed increasing drought conditions during late summer, which hampers soil respiration stronger than plant growth. Together, these changes lead to stronger and earlier greening of the central Tibetan alpine steppe ecosystem and to more carbon storage than release. Key Points: A widespread greening of the alpine steppe at Nam Co was observed, increasing the daily ecosystem carbon uptake by 0.5 g C m−2 decade‐1Larger diurnal temperature range during winter promotes plant growth more than soil respirationEarlier summer monsoon cloud cover and greater aridity in autumn both lead to increased net carbon uptake during these times [ABSTRACT FROM AUTHOR]

The Tibetan alpine steppe ecosystem covers an area of roughly 800,000 km2, containing up to 3.3 % soil organic carbon in the uppermost 30 cm, summing up to 1.93 PgvC for the Tibet Autonomous Region only (472,037 km2). With temperatures rising two to three times faster than the global average, these carbon stocks are at risk of loss due to enhanced soil respiration. The remote location and the harsh environmental conditions on the Tibetan Plateau (TP) make it challenging to derive accurate data on ecosystem-atmosphere exchange of carbon dioxide (CO2) and water vapor (H2O). Here, we provide the first multi-year data set of CO2 and H2O fluxes from the central Tibetan alpine steppe ecosystem, measured in situ using the eddy covariance technique. The calculated fluxes were rigorously quality checked and carefully corrected for a drift in concentration measurements and gas analyzer self heating during cold conditions. A wind field analysis was conducted to identify influences of adjacent buildings on the turbulence regime and to exclude the disturbed fluxes from subsequent computations. The presented CO2 fluxes were additionally gap filled using a standardized approach. The very low net carbon uptake across the 15-year data set highlights the special vulnerability of the Tibetan alpine steppe ecosystem to become a source of CO2 due to global warming. The data is freely available (https://www.doi.org/10.5281/zenodo.3733203, Nieberding et al., 2020b) and may help to better understand the role of the Tibetan alpine steppe in the global carbon-climate feedback. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) is the largest alpine plateau on Earth and plays an important role in global climate dynamics. On the TP, climate change is happening particularly fast, with an increase in air temperature twice the global average. The particular sensitivity of this high mountainous environment allows the observation and tracking of abiotic and biotic feedback mechanisms. Closed lake systems, such as the Nam Co on the central TP represent important natural laboratories for tracking past and recent climatic oscillations, as well as geobiological processes and interactions within their respective catchments. This review gives an interdisciplinary overview of modern and paleoenvironmental changes, focusing on Nam Co as model system. In the catchment area, the steep rise in air temperature forced glaciers to melt, leading to a rise in lake levels and changes in water chemistry. Some studies base their conclusions on inconsistent glacier inventories but an ever-increasing deglaciation and thus higher water availability have persisted over the last decades. The enhanced water availability causes translocation of sediments, nutrients and dissolved organic matter to the lake, as well as higher carbon emissions to the atmosphere. The intensity of grazing has a significant effect on CO2 fluxes, with moderate grazing enhancing belowground allocation of carbon while adversely affecting the C-sink potential through reduction of above- and subsurface biomass at higher grazing intensities. Furthermore, increasing pressure from human activities and livestock grazing are enhancing grassland degradation processes, thus shaping biodiversity patterns in the lake and catchment. The environmental signal provided by taxon-specific analysis (e.g. diatoms and ostracods) in Nam Co have revealed profound climatic fluctuations between warmer/cooler and wetter/drier periods since the late Pleistocene and an increasing input of freshwater and nutrients from the catchment in recent years. Based on the reviewed literature, we outline perspectives to further understand the effects of global warming on geo- and biodiversity and their interplay in the Lake Nam Co, which acts as a case study for potentially TP-wide processes that are currently shaping the earth’s future. [ABSTRACT FROM AUTHOR]

South Patagonian peat bogs are little studied sources of methane (CH4). Since CH4 fluxes can vary greatly on a small scale of meters, high-quality maps are needed to accurately quantify CH4 fluxes from bogs. We used high-resolution color infrared (CIR) images captured by an Unmanned Aerial System (UAS) to investigate potential uncertainties in total ecosystem CH4 fluxes introduced by the classification of the surface area. An object-based approach was used to classify vegetation both on species and microform level. We achieved an overall Kappa Index of Agreement (KIA) of 0.90 for the species- and 0.83 for the microform-level classification, respectively. CH4 fluxes were determined by closed chamber measurements on four predominant microforms of the studied bog. Both classification approaches were employed to up-scale CH4 closed chamber measurements in a total area of around 1.8 hectares. Including proportions of the surface area where no chamber measurements were conducted, we estimated a potential uncertainty in ecosystem CH4 fluxes introduced by the classification of the surface area. This potential uncertainty ranged from 14.2 mg·m-2· day-1 to 26.8 mg·m-2·day-1. Our results show that a simple classification with only few classes potentially leads to pronounced bias in total ecosystem CH4 fluxes when plot-scale fluxes are up-scaled. [ABSTRACT FROM AUTHOR]

The detection of pest infestation is an important aspect of forest management. In the case of the oak splendour beetle (Agrilus biguttatus) infestation, the affected oaks (Quercus sp.) show high levels of defoliation and altered canopy reflection signature. These critical features can be identified in high-resolution colour infrared (CIR) images of the tree crown and branches level captured by Unmanned Aerial Systems (UAS). In this study, we used a small UAS equipped with a compact digital camera which has been calibrated and modified to record not only the visual but also the near infrared reflection (NIR) of possibly infested oaks. The flight campaigns were realized in August 2013, covering two study sites which are located in a rural area in western Germany. Both locations represent small-scale, privately managed commercial forests in which oaks are economically valuable species. Our workflow includes the CIR/NIR image acquisition, mosaicking, georeferencing and pixel-based image enhancement followed by object-based image classification techniques. A modified Normalized Difference Vegetation Index (NDVImod) derived classification was used to distinguish between five vegetation health classes, i.e., infested, healthy or dead branches, other vegetation and canopy gaps. We achieved an overall Kappa Index of Agreement (KIA) of 0.81 and 0.77 for each study site, respectively. This approach offers a low-cost alternative to private forest owners who pursue a sustainable management strategy. [ABSTRACT FROM AUTHOR]

Large areas of the Tibetan Plateau (about 800,000 km²; Miehe et al., 2011) are covered with alpine steppe. During the long, dry and very cold winters, the degradation of organic carbon is largely inhibited. Carbon sequestration mainly takes place in the short vegetation period during the summer monsoon, thus accumulating roughly 2.5 % of terrestrial organic carbon stocks in the grassland soils over time (Wang et al., 2002). The air above these huge carbon stocks heats up about twice as fast as the global average. Together with increased precipitation, there is increased primary production of above- and below-ground biomass, which stores additional carbon in the soil. On the other hand, soil respiration is also stimulated, which in turn leads to higher CO2 emissions. In the long term, this carbon sink can thus become a carbon source, which in turn contributes to global warming. This study investigated which processes drive the ecosystem-atmosphere CO2 exchange, and whether a long-term trend can be identified. For this purpose, a multi-year data set of the CO2 fluxes between alpine steppe and the atmosphere at the second largest central Tibetan lake Nam Co (4730 m a.s.l.) was analyzed. The data was acquired using the Eddy Covariance method and the results were compared with meteorological data from a nearby planetary boundary layer observation tower. The carbon dynamics follow the pronounced seasonal distribution of precipitation, with effective sequestration during the summer monsoon. The onset and strength of monsoonal precipitation differs largely between years, leading to uncertainties in the identification of a long-term trend. Further investigation on possible cause-and-effect relationships are necessary to adequately address the role of alpine steppe in the global carbon cycle.ReferencesMiehe, G., Miehe, S., Bach, K., Nölling, J., Hanspach, J., Reudenbach, C., Kaiser, K., Wesche, K., Mosbrugger, V., Yang, Y.P. & Ma, Y. (2011) Plant communities of central Tibetan pastures in the Alpine Steppe/Kobresia pygmaea ecotone. Journal of Arid Environments, 75, 711–723.Wang, G., Qian, J., Cheng, G. & Lai, Y. (2002) Soil organic carbon pool of grassland soils on the Qinghai-Tibetan Plateau and its global implication. Science of The Total Environment, 291, 207–217. [ABSTRACT FROM AUTHOR]