Spatial decoupling of legacy phosphorus in cropland: Soil erosion and deposition as a mechanism for storage

Agricultural production continues to be implicated as one cause of eutrophication of water courses and water bodies in many agro-ecosystems. The generally-held consensus is that soil erosion by water is a major mechanism for loss of phosphorus (P) to adjacent water bodies and down-stream reservoirs from crop and grazing lands. Nonetheless, it does not explain the whole story of the fate of P losses applied to soils. This study assessed the spatial distribution of P as a consequence of historical erosion and redistribution of soil in an agricultural field in the Lake Erie basin in southern Ontario. Combining stratified random sampling with geospatial analysis of landscape position, plant-available (Olsen) P concentrations and stocks were measured and compared to other soil properties and related to soil erosion and redistribution. Toe-slope and foot-slope positions had two to three times the Olsen P stocks compared to summit landscape positions (mean of 52 +1- 9 kg P ha(-1) and 47 +1- 9 kg P ha(-1) versus 20 +/- 2 kg P ha(-1), respectively) which was attributed to both water and tillage erosion. Burial of P beneath the plowing depth showed that the toe-slopes had over two to three times more Olsen P stocks below 25 cm depth compared to all other landscape positions. Olsen P stocks were significantly related (R-2 = 0.50; p < 0.001) to soil organic carbon (SOC) stocks which was a consequence of historical soil erosion and redistribution in the field. The fate of legacy P in soils may be dependent upon the history of erosion and redistribution resulting in deposition of P in toe-slope and foot-slope positions in an agricultural landscape.

Automated summary

Agricultural production is a major contributor to water eutrophication. This study found that historical soil erosion and redistribution can result in the deposition of phosphorus in toe-slope and foot-slope positions in agricultural landscapes. The storage of phosphorus is closely related to soil organic carbon stocks.