I investigated how land use and human activities affect the sources and cycling of carbon (C) in subtropical rivers. Annually rivers receive a large amount of terrestrial C, process a portion of this C and return it to the atmosphere as CO2. The rest is transported to the ocean. Land use and human activities can affect the sources and fate of terrestrial C in rivers. However, studies on these effects are limited, especially in the humid subtropics. I combined measurements of the partial pressure of dissolved CO2 (pCO2), C isotopes (13C and 14C) and solid-state 13C nuclear magnetic resonance (NMR) to study C cycling in three subtropical rivers in Texas, two small rivers (Buffalo Bayou and Spring Creek) and a midsized river (the Brazos). My pCO2 data show that small humid subtropical rivers are likely a large source of atmospheric CO2 in the global C cycle. My measurements on pCO2, C isotopic and chemical composition of dissolved inorganic C (DIC) and particulate organic C (POC) revealed four types of effects of land use and human activities on river C cycling. First, oyster shells and crushed carbonate minerals used in road construction are being dissolved and slowly drained into Buffalo Bayou and the lower Brazos and may be a source of river CO2 released to the atmosphere. Second, river damming and nutrient input from urban treated wastewater stimulate algal growth and reduce CO2 evasion of the middle Brazos. Third, urban treated wastewater discharge is adding old POC to the middle Brazos and decomposition of the old POC adds to the old riverine DIC pool. Fourth, agricultural activities coupled with high precipitation enhance loss of old organic C (OC) from deep soils to the lower Brazos, and decomposition of the old soil OC contributes to the old CO2 evaded. I document for the first time the river C cycling effects of the use of carbonate minerals in construction and the riverine discharge of urban wastewater. Results presented here indicate the need to study disturbed river systems to better constrain the global C budget.