Using An Exchange Frequency Index to Identify Sites At Risk for Over-Winter Leaching.

Loss of nitrate-N (NO₃-N) present in the fall through over-winter leaching in humid climates can affect starter-N decisions for winter cereals or the deployment of cover crops to improve water quality. A multi-year series of studies were conducted on Maryland’s Atlantic Coastal Plain and Piedmont that followed the loss of fall residual NO₃-N or fall applied bromide (Br) from the 0-60 cm depth of a group of well-drained soils with a wide range of textures.  A simple soil solution exchange frequency index (EFI) was calculated for each sample where:  EFI over time interval Dt = (Precipitation, mm)_{Dt  *} [soil available water holding capacity (AWHC), mm]^-1. This index assumes: i) surface runoff is negligible, ii) the AWHC approximates the active soil pore-volume participating in leaching, and iii) the initial soil moisture is near field capacity. The quadratic relationship between EFI and the percent loss of Br or NO₃-N was highly significant (P<0.001), while the linear and quadratic coefficients for Br loss vs. NO₃-N loss were not different (P > 0.05).  An EFI >2.5 (Dt = 15 Oct. – 30 Nov.) successfully identified conditions with very high (> 60%) NO₃-N loss potential. Based on a 141-year precipitation dataset from Maryland a loamy sand, sandy loam, and silt loam soil will fall into this very-high category in 21, 1, and 0% of years, respectively. These findings show that the EFI can categorize soil and weather conditions that put NO₃-N at risk for leaching. This index could be used to identify sites for alternative winter wheat N management or for deployment of cover crops.