Phosphorus distributions in alluvial soils of the Lower Mississippi River Basin: A case of dual legacies

Legacies can become intertwined, none more so than the body of work of Dr. Andrew Sharpley examining agricultural nutrient delivery to waterbodies and the phosphorus (P) accumulation in agricultural soils, or "legacy P." Although Sharpley's work focused on the anthropogenic influence on soil P, our study suggests soils of the Lower Mississippi Alluvial Plain (MAP) represent a natural legacy with moderate levels of available P resulting from minimal anthropogenic input. In 2019, we collected surface (0-5 cm) soil samples from four regionally dominant soil series in either cropland or forested land uses, spanning 76 locations within the MAP. Soil chemical and physical properties were measured utilizing a suite of extractions and texture analysis to correlate properties with soil P values. Total soil P did not vary between land uses. Mehlich-3 extractable P was slightly higher in cropland soils due to higher concentrations in Forestdale and Sharkey soils. Dundee, Forestdale, and Sharkey cropland soils showed significant associations between Mehlich-3-extractable iron (Fe) and P. Ratios of total carbon (TC) to total nitrogen (C:N) and TC to P (C:P) were consistent across all sampled soil series but differed between forest and cropland soils. These ratios are critical for establishing baseline soil nutrient values in simulation models and can be used to improve water quality model simulations that help guide P management in the MAP. As Sharpley routinely demonstrated, understanding sources of P is critical for developing an appropriate management strategy. This study provides critical knowledge on soil P dynamics in the MAP region. Patterns in total and Mehlich-3 P were similar across land use and soil series. Forested soils had higher C and N concentrations than cropland, but C:N was consistent across soil series. High native P in Mississippi soils appears to be of depositional origins and not because of fertilizer inputs. Phosphorus is a critical nutrient needed for adequate crop production. In excess, phosphorus can cause environmental problems by increasing algal growth in aquatic ecosystems. Successful phosphorus management is dependent on knowing phosphorus concentrations in soils and understanding potential pathways for phosphorus transport to waterbodies. In the Lower Mississippi Alluvial Plain, soil phosphorus concentrations are higher than expected, considering low phosphorus fertilizer application rates. This study evaluated phosphorus concentrations across dominant soil types in both cropland and forest land uses. We found phosphorus concentrations were relatively similar across soil types and land uses. These results suggest there is a naturally high level of phosphorus in the soils of this region. Understanding phosphorus sources and dynamics will be helpful to guide management decisions and to improve watershed modeling simulations in the region.