How does land use link terrestrial and aquatic carbon in western North America?: Implications from an agricultural case study in central Montana

Abstract:

The fate of soil organic matter with expanding human land use is of increasing concern for planetary health and ecological sustainability. In North American grasslands, cultivation has commonly resulted in loss of stored soil organic carbon to dissolved phases in groundwater and surface water, as well as to atmospheric CO₂ via decomposition. In addition, cultivation has released nutrients stored in organic matter and facilitated water movement through soils to benefit crops, increasing groundwater recharge rates. This has altered groundwater chemistry both by changing biogeochemistry of the terrestrial-aquatic interface and by increasing addition of nutrients, herbicides, and pesticides to these systems. In this presentation, we consider the effects of food production practices on terrestrial-aquatic carbon linkages in former grassland ecosystems of western North America. Our data from an agricultural area in central Montana begin to reveal how elevated nitrate and pesticide levels in groundwater on an isolated landform reflect transformation over the last century of a temperate grassland ecosystem for wheat and cattle production. Rates and pathways of carbon and nitrogen loss are inferred from the concentration and isotopic character of both water and carbon and nitrogen over three years in soils, shallow groundwater, emergent springs and surface waters. In this semi-arid, non-irrigated context, the fate of soil organic matter is linked with redistribution of pedogenic carbonate as well as other soil and rock derived solutes. We consider implications for future trends in dissolved carbon and nitrogen in surface waters in the region.