Urea is carried by blood to the kidneys and bladder where it is processed out and stored until excreted. We have not been able to find studies specific to horse urine on trails. Because of the immense variability in soil composition and soil microbial contents, such studies would be difficult to generalize from and are probably unwarranted given the risk factors discussed below. At this point in time, allegations regarding the environmental risk of horses urinating on the trails and providing nutrient enrichment to weeds are only speculative and not supported by science or experience. While horses can readily defecate on trails, they do not as readily urinate on trails.
Most horses urinate in their paddocks or pastures.
They are allowed to stop walking. Horses then stretch their bodies out in a rather awkward position to urinate, often standing on the front edges of all of their hooves simultaneously in a splayed posture. This places them in a vulnerable position if attacked by a predator. 60 million years of evolution and survival means this is an activity not undertaken lightly by the horse. Because of this unique behavior, it is easy for the rider to spur the horse out of a stream to avoid urination in a water body.