Partitioning carbon sources between wetland and well-drained ecosystems to a tropical first-order stream – implications for carbon cycling at the watershed scale (Nyong, Cameroon)

[摘要] Tropical rivers emit large amounts of carbon dioxide (CO 2 ) tothe atmosphere, in particular due to large wetland-to-river carbon (C)inputs. Yet, tropical African rivers remain largely understudied, and littleis known about the partitioning of C sources between wetland andwell-drained ecosystems to rivers. In a first-order sub-catchment (0.6 km 2 ) of the Nyong watershed (Cameroon 27 800 km 2 ), we fortnightly measured C in all forms and ancillaryparameters in groundwater in a well-drained forest (hereafter referred to asnon-flooded forest groundwater) and in the stream. In the first-order catchment, thesimple land use shared between wetland and well-drained forest, togetherwith drainage data, allowed the partitioning of C sources between wetlandand well-drained ecosystems to the stream. Also, we fortnightly measureddissolved and particulate C downstream of the first-order stream to the mainstem of order 6, and we supplemented C measurements with measures ofheterotrophic respiration in stream orders 1 and 5. In the first-orderstream, dissolved organic and inorganic C and particulate organic C (POC)concentrations increased during rainy seasons when the hydrologicalconnectivity with the riparian wetland increased, whereas the concentrationsof the same parameters decreased during dry seasons when the wetland wasshrinking. In larger streams (order  >  1), the same seasonalitywas observed, showing that wetlands in headwaters were significant sources oforganic and inorganic C for downstream rivers, even though higher POCconcentration evidenced an additional source of POC in larger streams duringrainy seasons that was most likely POC originating from floatingmacrophytes. During rainy seasons, the seasonal flush of organic matter fromthe wetland in the first-order catchment and from the macrophytes inhigher-order rivers significantly affected downstream metabolism, asevidenced by higher respiration rates in stream order 5 (756  ±  333 gC-CO 2  m −2  yr −1 ) compared to stream 1 (286  ±  228 gC-CO 2  m −2  yr −1 ) . In the first-order catchment, the sum of the Chydrologically exported from non-flooded forest groundwater (6.2  ±  3.0 MgC yr −1 ) and wetland (4.0  ±  1.5 MgC yr −1 ) to the streamrepresented 3 %–5 % of the local catchment net C sink. In the first-ordercatchment, non-flooded forest groundwater exported 1.6 times more C thanwetland; however, when weighed by surface area, C inputs from non-floodedforest groundwater and wetland to the stream contributed to 27 %(13.0  ±  6.2 MgC yr −1 ) and 73 % (33.0  ±  12.4 MgC yr −1 ) of the total hydrological C inputs, respectively. At the Nyong watershedscale, the yearly integrated CO 2 degassing from the entire rivernetwork was 652  ±  161 GgC-CO 2  yr −1 (23.4  ±  5.8 MgC CO 2  km −2  yr −1 when weighed by the Nyong watershed surfacearea), whereas average heterotrophic respiration in the river and CO 2 degassing rates was 521  ±  403 and 5085  ±  2544 gC-CO 2  m −2  yr −1 , which implied that only ∼  10 % of theCO 2 degassing at the water–air interface was supported by heterotrophicrespiration in the river. In addition, the total fluvial C export to theocean of 191  ±  108 GgC yr −1 (10.3  ±  5.8 MgC km −2  yr −1 when weighed by the Nyong watershed surface area) plus the yearlyintegrated CO 2 degassing from the entire river network represented ∼  11 % of the net C sink estimated for the whole Nyongwatershed. In tropical watersheds, we show that wetlands largely influenceriverine C variations and budget. Thus, ignoring the river–wetlandconnectivity might lead to the misrepresentation of C dynamics in tropicalwatersheds.

[学科分类] 大气科学