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Urban ecosystems, the heterogeneous and rapidly changing landscape, showed a considerable impact on the global C cycle. However, studies encompassing the spatial differences in urban land uses on soil C dynamics are limited in tropical ecosystems. In this study, seasonal and temporal variability in soil CO 2 efflux (SCE) and its regulatory physicochemical variables under five urban land use classes viz., Bare (BAR), Agriculture (AGR), Plantation (PLT), Grassland (GRA) and Lawns (LAW) were assessed from 2014 to 2016. Bare land use was considered as the reference for observing the variation for different land uses. Seasonal measurements of SCE, soil temperature, moisture content, pH, ammonium-N, nitrate-N and microbial biomass C (MBC) were performed whereas soil organic C (SOC), soil N, and soil physical properties were measured annually. Our results showed a significant (P < 0.01) increase in SCE by 89%, 117%, 132% and 166% for land use types from BAR to AGR, PLT, GRA and LAW, respectively. The results revealed a two-fold increase in SCE from anthropogenically managed urban lawns as compared to bare soil. PLT and LAW land use classes showed higher SOC and N contents. SCE was found positively correlated with temperature, moisture, SOC, soil N and MBC whereas negatively correlated with ammonium-N and nitrate-N (at P < 0.05) for the overall dataset. Soil moisture, temperature, SOC, porosity and pH were identified as the major determinant of urban SCE by explaining 63% of the variability in overall SCE. Further, temperature for BAR and LAW; moisture for PLT; ammonium-N for GRA; and nitrate-N for AGR were identified as the major regulators of SCE for different land use classes. The findings revealed that the interaction of soil temperature and moisture with nutrient availability regulates overall and seasonal variability in SCE in an urban ecosystem. Since these variables are highly affected by climate change, thus, the soil C source-sink relationships in tropical urban ecosystems may further change and induce a positive global warming potential from urban ecosystems. [Display omitted] • Urban lawns showed 166% higher soil CO 2 efflux (SCE) as compared to bare soil. • Plantation showed higher soil organic C and available nutrients whereas lower SCE. • Strong seasonality observed in SCE with higher value during rainy and lower during winter. • Soil moisture and temperature showed positive whereas nutrient availability showed negative control on the SCE. • Urban lawns showed higher potential for C accumulation with proper management. [ABSTRACT FROM AUTHOR]

Riparian ecosystems are amongst the most vulnerable ecosystems of the world. The natural gradients and increasing human perturbations under these ecosystems can be explored for understanding the soil carbon (C) dynamics, especially soil carbon dioxide (CO 2 ) efflux. However, studies on soil CO 2 efflux and its governing variables under different land uses of dry tropical riparian ecosystems are limited. Therefore, the present study aimed (1) to assess the impact of riparian land use on soil CO 2 efflux, and (2) to identify the key drivers of soil CO 2 efflux along the river Ganga, Varanasi, India. The riparian land uses taken in this study were moist sandy flat (MSF), uncultivated sandy land (USL) and cultivated sandy land (CSL) depending upon their slope and distance from river body to upland, respectively. Soil CO 2 efflux and other soil biophysical properties were measured at 54 locations distributed in six sites having these land uses, in dry season of 2014–15. Soil biophysical properties considered in this study were soil organic C, soil moisture, bulk density, porosity, fine particles, microbial biomass C and soil pH. Riparian land uses were found to have significant impact over soil CO 2 efflux with a respective increase of 222, 424 and 63%, for MSF to USL, MSF to CSL, and USL to CSL land use transitions ( P < 0.01), respectively. Similarly, the regulators of soil CO 2 efflux varied with the land uses. It showed strong positive correlation with soil organic C (r = 0.81), fine particles (r = 0.64) and porosity (r = 0.61), whereas negative correlation with soil moisture (r = 0.61) and bulk density (r = 0.62) for overall dataset. However, soil organic C, fine particles, microbial biomass C and soil pH at MSF; soil organic C and microbial biomass C at USL; and soil moisture, porosity and microbial biomass C at CSL land uses were observed to regulate soil CO 2 efflux. The findings revealed that riparian land uses have significant control over soil CO 2 efflux and its biophysical regulators which have relative control over it. Soil organic C, soil moisture, fine particles, porosity and microbial biomass C were identified as prevalent regulators of soil CO 2 efflux under dry seasons. Overall, the results indicate that the biophysical variables in addition to human interferences (CSL land use) have pronounced regulation over soil CO 2 efflux in dry tropical riparian ecosystems. [ABSTRACT FROM AUTHOR]