Nutrient concentrations in leachate and runoff from dairy cattle lots with different surface materials

Abstract

Nitrogen (N) and phosphorus (P) loss from agriculture persists as a water quality issue, and outdoor cattle lots can have a high loss potential. We monitored nutrient concentrations in leachate and runoff from constructed dairy heifer lots. Lots were about 42 m², excavated to 1.1 m deep, and backfilled with three surface materials (soil, sand, and bark mulch). Plots were lined with a rubber membrane to allow total collection of all leachate. Runoff was collected from soil plots only. Runoff and leachate were monitored for dissolved and total N and P from 2010 to 2015. Dairy heifers were placed on plots several times per year for a week at a time. The percentage of annual precipitation collected as leachate averaged 34% for soil, 75% for sand, and 64% for mulch plots. For soil plots, the percentage of annual precipitation collected as runoff ranged from 13% to 44%, with the percentage increasing with precipitation. Leachate N concentrations in soil and sand plots behaved similarly, with ammonium-N (NH₄-N) much less than nitrate-N (NO₃-N) and total N. Leachate N concentrations were generally least from mulch, suggesting more microbial N retention. Soil had the least and sand plots had the greatest P concentrations in leachate, with mulch in between. In runoff from soil plots, particulate N and P were well related to sediment concentrations. Compared with leachate from soil and sand, soil runoff NO₃-N concentrations were typically less, but were similar to leachate in mulch plots. Conversely, soil runoff NH₄-N concentrations were typically greater than in leachate for any treatment. Soil runoff dissolved P concentrations increased slightly over time, with most runoff P when cows were recently on plots, and were typically greater than in leachate for any treatment. Overall, nutrient dynamics in sand and soil plots appeared controlled by mineral processes, while biological processes dominated more in mulch plots. Sand plots had the most nutrients in leachate, implying more potential to recover nutrients in liquid form. Soil and mulch retained more nutrients within plot materials, which means on-farm nutrient use would require transfer of materials to cropland or growing crops directly in previous cattle lots.