Comparison of Nutrient Loss Pathways: Run‐off and Seepage Flow in Vertisols

Recent studies have highlighted the agronomic and environmental importance of phosphorus (P) movement through the soil profile. Thus, faced with challenges such as high-profile cases of P enrichment of surface water, better understanding of nutrient movement through soil is needed to better manage agricultural fertilizers and manures and their contribution to water quality degradation. In particular, field-scale research is especially needed in soils with preferential flow transport pathways. Thus, we collected nitrogen (N) and P transport data in run-off and seepage (lateral subsurface return flow) from 13 field- and farm-scale watersheds on Vertisols in Central Texas for a 14-year period. For 2004-2017, seepage accounted for similar to 20% of the total surface flow, and nutrient concentrations were generally similar in run-off and seepage. As surface run-off contributed similar to 80% of the flow, it follows that median annual N and P loads in run-off were significantly greater than in seepage for every watershed. N loads in both run-off and seepage flow from cultivated land were an order of magnitude greater than in native prairie and improved pasture, and the highest run-off and seepage P loads both occurred on cultivated land with organic fertilizer sources. Increasing watershed scale (size) did not to produce consistent patterns in N or P loss in run-off or seepage. Land use and watershed scale produced significant differences in seepage volume but did not affect run-off volumes or total surface flow/rainfall. Although less significant in terms of total offsite flux, nutrient movement in vadose zones has important agronomic and environmental implications as considerable N and P are transported through and within the root zone and eventually offsite. And in terms of P, this contradicts the traditionally held scientific viewpoint that P movement through the vadose zone is unimportant agronomically and environmentally.