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Effects of agricultural land use on the composition of fluvial dissolved organic matter

Abstract

Nearly 40% of the Earth’s ice-free surface area is cropland or pasture1. Agricultural land use can increase the delivery of nutrients such as nitrogen and phosphorus to fluvial ecosystems2, but the impact of farming on riverine dissolved organic carbon is still largely unknown, despite increasing recognition that rivers act as important modifiers in the global carbon cycle3,4. Here, we examine the character of riverine dissolved organic matter in 34 watersheds along a gradient of agricultural land use. We show that changes in the character of dissolved organic matter are related to agricultural land use, nitrogen loading and wetland loss. Specifically, we find that the structural complexity of dissolved organic matter decreases as the ratio of continuous croplands to wetlands increases. At the same time, the amount of microbially derived dissolved organic matter increases with greater agricultural land use. Furthermore, we find that periods of soil dryness are associated with a decrease in the structural complexity of dissolved organic matter. We suggest that these effects of land use and climate on the character of riverine dissolved organic matter have important implications for global carbon cycling, owing to their potential to control rates of microbial carbon processing (for example, uptake, retention and outgassing) in agricultural systems.

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Figure 1: Relationships between three indices of DOM character and landcover or nutrients.
Figure 2: Changes in fluvial DOM character with proportion of cropland in the watershed.

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Acknowledgements

We thank L. James, K. Warren, J. Norman, J. Plourde, M. Kingsbury, R. Kelly and D. Doherty for providing assistance in the field and laboratory, the Ontario Ministry of Natural Resources for providing us with all geospatial data, Agriculture and Agri-food Canada for providing us with modelled soil moisture data and Environment Canada (P. Chambers and D. McGoldrick) for providing us with samples from the Southern Ontario watersheds. This research was supported by Canada’s Natural Sciences Research and Engineering Research Council (NSERC) Discovery grant to M.A.X., a NSERC University Faculty Award to M.A.X. and NSERC postgraduate scholarships to H.F.W.

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Contributions

H.F.W. and M.A.X. equally contributed to the conceptual and methodological development of the project. H.F.W. collected, processed and analysed the data and wrote the first draft. M.A.X. coordinated the study, contributed to the writing and edited the drafts.

Corresponding authors

Correspondence to Henry F. Wilson or Marguerite A. Xenopoulos.

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Wilson, H., Xenopoulos, M. Effects of agricultural land use on the composition of fluvial dissolved organic matter. Nature Geosci 2, 37–41 (2009). https://doi.org/10.1038/ngeo391

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