The Colombian Andean Mountains host the headwaters of the main basins of the country. However, the interactions between high-mountain ecosystems and the isotopic composition of water in this region has been poorly studied. Here we present and analyze the first set of stable isotopes data collected in the Central Andes of Colombia. Stable isotopic composition of stream water and precipitation was determined for a period between 2017 and 2018 in the Upper Claro River Basin. The driving factors influencing the spatial and temporal variability of δ H, δ O and d-excess were identified and compared to daily air temperature and precipitation at seven meteorological stations. A Local Meteoric Water Line was defined as δ²H = 8.13 δ O + 12.5, R =0.98. δ H, δ O and d-excess values of precipitation were more negative during the rainy season and changes were more related to precipitation events and amounts than to temperature. An altitude effect of -0.11 ‰ / 100 m and -0.18 ‰ / 100 m was estimated for stream water and precipitation, respectively, where the latter showed a non-linear behavior. The data set was compared to stations of the Global Network of Isotopes in Precipitation (GNIP) database in Colombia and a back-trajectory analysis of air masses was conducted to compare with d-excess. δ O weighted means changed with respect to the position of the Central Andes and the altitudinal range 2,100 to 3,100 m a.s.l.. High d-excess can be attributed to moisture recycling enhanced by the local ecosystems and the travel of precipitable water from the Amazon basin across the northern Andes. The results showed a high range of variation due to the differences in elevation, seasonality and atmospheric circulation patterns across the year. The present study contributes to fill the gap of spatial and temporal isotopic composition data in the northern Andes as well as to the implementation of the first “National Network for Isotopes” in Colombia.

Abstract. Microbial release of CO2 from soils to the atmosphere reflects how environmental conditions affect the stability of soil organic matter (SOM), especially in massive organic-rich ecosystems like the peatlands and grasslands of the Qinghai-Tibetan Plateau (QTP). Radiocarbon (14C) is an important tracer of the global carbon cycle and can be used to understand SOM dynamics through the estimation of time lags between C fixation and respiration, often assessed with metrics such as age and transit time. In this study, we incubated peatland and grassland soils at four temperature (5, 10, 15 and 20 °C) and two water-filled pore space (WFPS) levels (60 and 95 %), and measured the 14C signature of bulk soil and respired CO2. We compare the relation between the Δ14C of the bulk soil and the Δ14CO2 of respired carbon as a function of temperature and WFPS for the two soils. To better interpret our results, we used a mathematical model to analyse how the calculated number of pools, decomposition rates of carbon (k), transfer (α) and partitioning (γ) coefficients affect the Δ14C -bulk and Δ14CO2 relation, with their respective mean age and mean transit time. From our incubations, we found that 14C from peatland was significantly more depleted (old) than from grassland soil. Our results showed that changes in temperature did not affect the Δ14C values of respired CO2 in either soil. However, changes in WFPS had a small effect on the 14C CO2 in grassland soils and a strong influence in peatland soils, where higher WFPS levels led to more depleted Δ14CO2. In our models, we observed large differences between slow and fast cycling systems, where low values of k modified Δ14C patterns due to the incorporation of 14C-bomb in the soil. Hence, the correspondence between Δ14C and age and transit time strongly depended on the internal dynamics of the soil (k, α, γ and number of pools) as well as on model structure. We conclude that the stability of carbon in these systems depends strongly on the direction of change in temperature and moisture and how it affects the rates of SOM decomposition. Finally, Δ14C modelling along with empirical data from SOM dynamics is a useful tool to improve predictions on interactions between terrestrial and atmospheric carbon.

The Colombian Andean Mountains include the headwaters of the main basins of the country. However, the isotope composition of water in these high mountain ecosystems has been poorly studied. In this study, we analysed the first set of stable isotope data collected along a wide elevation range (2600-4950 m a.s.l.) in the Central Andes of Colombia. The stable isotope composition of stream water and precipitation was determined for a period between 2017 and 2018 in the Upper Claro River basin. The driving factors influencing the spatial and temporal variability of δ 2 H, δ 18 O, and d-excess were identified, and compared with daily air temperature and precipitation data from seven meteorological stations. The local regression line was described by δ 2 H = 8.2 δ 18 O + 12.3, R 2 = 0.98. The δ 2 H and δ 18 O values showed more depletion in heavy isotopes, and the d-excess values were more negative during the rainy season. An altitude effect of À0.11‰/100 m and À0.18‰/100 m was estimated for stream water and precipitation δ 18 O values, respectively, with the latter showing non-linear behaviour. The dataset was compared with Colombian stations of the Global Network of Isotopes in Precipitation database, and a back-trajectory analysis of air masses was conducted and compared with the d-excess values. The δ 18 O weighted mean values changed with respect to the position in the Central Andes, indicating contrasting altitude effects depending on the moisture sources. The most positive d-excess values were attributed to moisture recycling enhanced by local ecosystem conditions and the origin of precipitation from the Amazon basin, which change during the year and across the northern Andes. The results showed a high level of variation because of differences in elevation, seasonality, and atmospheric circulation patterns during the year. This study contributes to knowledge of spatial and temporal isotope composition data in the northern Andes, delineation of water supply basins, and to the definition of ecosystem boundaries in the high mountains of Colombia.