Interactive effects of catchment mean water residence time and agricultural area on water physico-chemical variables and GHG saturations in headwater streams

Greenhouse gas emissions from headwater streams are linked to multiple sources influenced by terrestrial land use and hydrology, yet partitioning these sources at catchment scales remains highly unexplored. To address this gap, we sampled year-long stable water isotopes (δ$^{18}$O and δ$^2$H) from 17 headwater streams differing in catchment agricultural areas. We calculated mean residence times (MRT) and young water fractions (YWF) based on the seasonality of δ$1^{18}$O signals and linked these hydrological measures to catchment characteristics, mean annual water physico-chemical variables, and GHG % saturations. The MRT and the YWF ranged from 0.25 to 4.77 years and 3 to 53%, respectively. The MRT of stream water was significantly negatively correlated with stream slope (r$^2$ = 0.58) but showed no relationship with the catchment area. Streams in agriculture-dominated catchments were annual hotspots of GHG oversaturation, which we attributed to precipitation-driven terrestrial inputs of dissolved GHGs for streams with shorter MRTs and nutrients and GHG inflows from groundwater for streams with longer MRTs. Based on our findings, future research should also consider water mean residence time estimates as indicators of integrated hydrological processes linking discharge and land use effects on annual GHG dynamics in headwater streams

Anthropogenic activities significantly increase annual greenhouse gas (GHG) fluxes from temperate headwater streams in Germany

Anthropogenic activities increase the contributions of inland waters to global greenhouse gas (GHG; CO$_2$, CH$_4$, and N$_2$O) budgets, yet the mechanisms driving these increases are still not well constrained. In this study, we quantified year-long GHG concentrations, fluxes, and water physico-chemical variables from 28 sites contrasted by land use across five headwater catchments in Germany. Based on linear mixed-effects models, we showed that land use was more significant than seasonality in controlling the intra-annual variability of the GHGs. Streams in agriculture-dominated catchments or with wastewater inflows had up to 10 times higher daily CO$_2$, CH$_4$, and N$_2$O emissions and were also more temporally variable (CV > 55 %) than forested streams. Our findings also suggested that nutrient, labile carbon, and dissolved GHG inputs from the agricultural and settlement areas may have supported these hotspots and hot-moments of fluvial GHG emissions. Overall, the annual emission from anthropogenic-influenced streams in CO$_2$ equivalents was up to 20 times higher (∼ 71 kg CO$_2$ m$^{−2}$ yr$^{−1}$) than from natural streams (∼ 3 kg CO$_2$ m$^{−2}$ yr$^{−1}$), with CO$_2$ accounting for up to 81 % of these annual emissions, while N$_2$O and CH$_4$ accounted for up to 18 % and 7 %, respectively. The positive influence of anthropogenic activities on fluvial GHG emissions also resulted in a breakdown of the expected declining trends of fluvial GHG emissions with stream size. Therefore, future studies should focus on anthropogenically perturbed streams, as their GHG emissions are much more variable in space and time and can potentially introduce the largest uncertainties to fluvial GHG estimates

Greenhouse gas emission from livestock water pans and water points along tropical streams in Taita Hills, Kenya

GHG emissions from the livestock sector are expected to increase with the increasing global demand for livestock products. In sub-Saharan Africa, rivers and artificially dug water pans are used by small scale and large scale livestock farmers to provide water for their livestock. The rivers and water pans receive loads of manure and urea during livestock watering activities, making them potential hotspots for GHG (CO2, CH4 and N2O) emissions through in-water biogeochemical processing. However, emissions from such systems remain poorly constrained in the tropical regions, and processes responsible for the emissions quite uncertain. To partly address this, the study was conducted in livestock-influenced aquatic ecosystems in the Taita Hills, Kenya from October 2018 (short rainy season) to December 2019 (dry season). Taita hills region, located within the tropics is no exception, with the lowlands receiving ~ <500 mm of rainfall and most residents practicing livestock farming as a source of livelihood. Sampling was carried out over 4 field campaigns in 10 reference (agricultural) streams, 9 livestock streams and 4 livestock water pans for quantification of water quality parameters, GHG concentration and flux. CH4 and N2O mean fluxes were two orders of magnitude higher in water pans compared to the river systems. Based on livestock densities from low-high during the 4 campaigns, CO2 and CH4 fluxes did not vary significantly with livestock density. However, N2O fluxes were significantly higher in water pans with high livestock densities, and more of sinks in riverine systems with high livestock densities. On possible biogeochemical controls based on the results of a multiple linear regression model, heterotrophy and methanotrophy possibly accounted for CO2 production in riverine systems, supported by a positive relationship between FBOM and pCO2 in livestock streams and a positive relationship between C-CH4 and pCO2 in all river systems. Water pans showed an inverse relationship between FBOM and CO2 implying that CO2 was being produced as FBOM was being degraded by heterotrophs. For methane, hydrogenotrophic methanogenesis (methane production from CO2) and methanotrophy (methane oxidation) were suggested as the controlling processes of methane dynamics in river systems based on the positive relationship between dissolved CO2 and methane and the negative relationship with DO. Water pan methane dynamics seemed to be controlled by photosynthesis and OM input from manure. Lastly, nitrification possibly accounted for N2O production within the river systems and water pans. A negative relationship with DOC and a positive relationship with dissolved CO2 (carbon source for autotrophs) supported this hypothesis. However, based on the results of water quality analysis, livestock sites were net sinks of the gas owing to their high DOC and low nitrate concentrations that favoured complete denitrification of N2O. This meant that N2O dynamics in livestock streams were controlled by nitrification and complete denitrification, with the latter being a more dominant process due to the under saturated concentrations recorded. In addition to nitrification in the water pans, the negative relationship between NO3-N and N2O in water pans suggested that incomplete denitrification might also contribute to the N2O pool. Trends along the longitudinal transect (approximately 14.1 km) in the Bura River suggested that land use controlled variation in gas partial pressures. However, fluxes of all three gases showed a declining trend downwards, a possible indication of the stronger influence of stream hydro geomorphological parameters of velocity and slope. Based on daytime sampling at livestock influenced streams, livestock watering activities were positively correlated to CO2 and CH4 partial pressures, and negatively correlated to N2O. Livestock watering activities potentially lead to hot moments in GHG emissions, where they accounted for an increase instream pCO2 and pCH4, and a decrease in pN2O. Compared to soil emissions from grazed and agricultural lands in Kenya, East and West Africa, mean water pan areal FCH4 and FN2O were mostly 10 - 1000 times greater than reported soil emissions. The emissions from water pans could therefore prove significant in the total emissions from the livestock sector in sub-Saharan Africa that were previously not accounted for, and thus the need for extensive research focussed entirely on these systems

Data_Sheet_1_Interactive effects of catchment mean water residence time and agricultural area on water physico-chemical variables and GHG saturations in headwater streams.pdf

Greenhouse gas emissions from headwater streams are linked to multiple sources influenced by terrestrial land use and hydrology, yet partitioning these sources at catchment scales remains highly unexplored. To address this gap, we sampled year-long stable water isotopes (δ18O and δ2H) from 17 headwater streams differing in catchment agricultural areas. We calculated mean residence times (MRT) and young water fractions (YWF) based on the seasonality of δ18O signals and linked these hydrological measures to catchment characteristics, mean annual water physico-chemical variables, and GHG % saturations. The MRT and the YWF ranged from 0.25 to 4.77 years and 3 to 53%, respectively. The MRT of stream water was significantly negatively correlated with stream slope (r2 = 0.58) but showed no relationship with the catchment area. Streams in agriculture-dominated catchments were annual hotspots of GHG oversaturation, which we attributed to precipitation-driven terrestrial inputs of dissolved GHGs for streams with shorter MRTs and nutrients and GHG inflows from groundwater for streams with longer MRTs. Based on our findings, future research should also consider water mean residence time estimates as indicators of integrated hydrological processes linking discharge and land use effects on annual GHG dynamics in headwater streams.</p