Agricultural layering explains variation in sediment P dynamics in streams draining two distinct agricultural biomes

Abstract

Phosphorus (P) enrichment of headwater agricultural streams due to the runoff of fertilizers can lead to the eutrophication of downstream aquatic ecosystems. Agriculture is intensive but heterogeneous in the Mississippi River Basin, with a mixture of pasture, row crops, and patches of confined animal feedlot operations (CAFOs). Many studies have evaluated how a single form of agricultural land use can affect sediment response to P loading, but few have considered the role of agricultural layering (i.e., one vs. two forms of agriculture in the same watershed). We used a synoptic sampling approach to determine the effect of agricultural layering on sediment P cycling in two watersheds in Arkansas (AR) and Michigan (MI). We sampled stream sediments from 10 sites in each watershed across two seasons; sampling from sites draining row crop vs. row crop plus a dairy in MI, and from sites draining chicken CAFOs vs. chicken CAFOs with pasture in AR. The proportion of bioavailable P was highest in MI and lowest in AR sediments, likely due to enhanced runoff of P rich sediment from row crops in MI, compared to sites in AR that were still predominantly forested, despite isolated CAFOs. Sediment P retention capacity was lowest downstream of sites with row crop + dairy in MI and highest at sites draining CAFOs alone in AR. Finally, substrate size differences between watersheds likely influences the reach-scale potential P retention capacity. Overall, we show that agricultural layering along with underlying geology can explain drivers influencing sediment P dynamics in agricultural streams.