Distribution And Degradation Of Terrestrial Organic Matter In Tropical River Sediments Among Different Climate Types

Land and ocean are the two major carbon pools and rivers are considered as an important link between them.A series of transformations take place during the transportation of terrestrial organic matter along the river.The burial process of terrestrial organic matter within river sediments plays an important role in the global carbon cycle.The sources and transformation processes are the key issues of the study on terrestrial organic matter in river sediments.Global climate change has changed the rainfall patterns and intensity,and the frequency of climate events,such as El Ni?o,La Nina,and extreme floods/drought have shown an increase in recent years.The anthropogenic activities such as deforestation,damming,and expansion of agricultural land has also altered the natural conditions.These factors have a great impact on the source,degradation and burial process of terrestrial organic matter from source to sink?transport from river to ocean?.Tropical rivers have a faster carbon cycle than boreal and temperate rivers,the transportation of terrestrial organic matter from tropical rivers is of great importance in understanding the complexities of the global carbon cycle.Previous studies have focused on large tropical rivers,such as Amazon River and Congo River,and the biogeochemical cycle of other small tropical rivers has been neglected,while the contribution of these small rivers to the global transportation of terrestrial organic matter should not be ignored.Tropical zones experienced more dramatic land use change than the boreal and temperate regions in last half century due to the urgent demand of economic development.The changes in monsoon circulation patterns due to climate change also greatly affect the rainfall pattern and intensity in tropical zones.Smaller rivers with low discharge are more sensitive and respond to the changes in these environmental conditions much faster than other large tropical rivers.Here we carried out studies in tropical rivers among different climatic types—Malaysian peat-draining rivers?Rajang River,Maludam River,Sebuyau River and Simunjan River,tropical rainy climate?,Indus Delta?Subtropical arid climate,closer to the tropical zone?and Pangani River Basin?tropical savanna climate?.Biomarker?lignin?,organic carbon?OC%?,total nitrogen?TN%?,carbon to nitrogen molar ratio?C/N?,stable carbon isotope(?¹³C)and grain size were used in our research.Plant and soil samples were also collected to trace the sources,degradation and burial processes of organic matter in the sediments of the above rivers.The effects of human activities and changes of natural processes on terrestrial organic matter in river sediments were also discussed.In the sediments of peat-draining rivers of Malaysia,the dominant source of organc matter was C3 plants.Cluster analysis and principle component analysis showed that,sedimentary organic matter in Rajang River had different characteristics compared with the other three small rivers.Grain size of Rajang River had a stronger influence on OC%and lignin content,while the effects were lesser on the other three small rivers.The occurrence of La Ni?a phenomenon led to similar rainfall between flood season and dry season,which might be the main reason for the few seasonal changes in the organic matter characteristics of river sediments.In sediments of the lower Rajang River and the three small rivers,the?Ad/Al?v was between 0.4 and 0.6,indicating moderate levels of lignin degradation,while the?Ad/Al?v in the upper reaches of Rajang River was higher than 0.8,showing higher levels of lignin degradation.The presence of low pH and high nitrogen content in these sediments may be the main reason for the low degradation of lignin in the three small rivers.The sources of organic matter in sediments of Indus Delta are greatly affected by vegetation cover present in different regions.The results of?8 and?¹³C indicated that the source of angiosperm tissues of C4 plant is more obvious in the sediment collected from the mainstream,possibly related to the dense cultivation of C4 crops?sugarcane,sunflowers?in this region.The positive correlation between?8 and mean grain size suggested that the contributions from plant detritus cannot be ignored in the mangrove dominated tidal channel?Waddi Khuddi Creek?.The sediments of Indus Delta are generally characterized by low carbon and low nitrogen,and the bulk properties(OC%,TN%,C/N and?¹³C)from the tidal channel displayed almost equal values for the wet and dry season.The lignin contents??8 and?8?along with the degradation parameters??Ad/Al?v and P/?S+V??showed variation between the wet and dry season,suggesting the lignin degradation is more sensitive to the source changes rather than the bulk properties,and can be used to trace changes from smaller sources.Due to the construction of Kotri dam and the impact of agricultural irrigation,the flow of water from the Indus River has reduced,and there is no discharge to Arabian Sea during certain periods.Owing to the high temperature and drought climate,the degradation of lignin in most sediments of Indus Delta was high??Ad/Al?v>0.6 and P/?S+V?>0.4?.The lignin contents in the sediments of Waddi Khuddi Creek were more"fresh"??Ad/Al?v?0.4?during the rainy season,which may be resulted from the input of plant detritus,and is consistent with the indication of?8 and grain size.Vegetation cover and soil characteristics of the Pangani River Basin displayed altitudinal variation,leading to differences in sources of organic matter within the river sediments.The vegetation from the mountainous areas?>1100 m above sea level?near to the tributaries was mainly composed of C3 plants,and soils erosion was obviously due to deforestation.The result of?8 and?¹³C also showed that,the organic matter of sediments from the mountainous areas was mainly from C3 plants and soils.There has large scale cultivation of C4 crops?sugarcane,corn?along the main stream?>700 m above sea level?.Therefore,in addition to C3 plants and soil sources,there are obvious contributions from C4 plant sources to the sediments from individual sites.Similar to Indus Delta,the seasonal differences in lignin characteristics of the Pangani River sediments are more pronounced than the overall properties,and the grain size has a certain controlling effect on OC%and lignin content.The?Ad/Al?v of sediments from the tributaries of the mountainous regions was lower than 0.5 for the dry season,and higher than 0.6 for the wet season.Combined with the analysis of organic sources and the high values of DHBA/V and?Ad/Al?v in soils,we can state that,the increase of rainfall and surface runoff after rainy seasons leads to increased input of soil-derived organic matter,resulting in higher degradation of lignin in sediments.In summary,the differences in natural conditions and human activities lead to different sources and degradation of terrestrial organic matter in small tropical river sediments under different climatic conditions.The input of fresh C3 plant detritus and the inhibition of low pH and the high-content nitrogen element inhibits the degradation process,leading to the presence of terrestrial organic matter are“fresh”in Malaysian peat-draining river sediments under the tropical rainforest climate.On the contrary,the coupling of arid climatic conditions and dam construction lead to a higher level of organic matter degradation in the sediments of Indus Delta.The important sources of organic matter to the sediments of Pangani River were C3 plants and soils.The contribution of C4 crops to the sedimentary organic matter in lower Pangani River and the lower reaches of the Indus River can not be ignored.With the intensifying of climate change and human activities,the way terrestrial organic matter continues to change during its transportation from small and medium tropical rivers will have important implications in terms of storage and transport.These processes will affect the global biogeochemical cycles,and the impacts of this process on the regional and global carbon cycle are worth further study.