Wetting Pattern of Cow Urine Patch in an Andisol Assessed through Bromide Concentration Distribution: A Pilot Study

Abstract

Cow urine is a rich source of mobile nutrients such as nitrate (NO3−) and potassium (K+). The aim of this experiment was to evaluate the wetting pattern distribution through soil profile of cow urine patch in an andisol. Two field experiments across two consecutive years were carried out to compare cow urine patches in relation to initial wetting pattern and volume of soil affected. Bromide (Br−) has successfully been used as an inert hydrologic tracer to indicate the movement of NO3− and K+ in soil–water systems. The distribution of Br− (used as a urine tracer) on the soil surface and down the profile was irregular in all the patches. Cow urine patches covered a surface area of 0.27 and 0.35 m2, respectively, and penetrated to a depth of 70 cm. The rapid downward movement of urine occurred through macropore flow but even so, between 27% and 40% of the applied Br− was detected in the 0–5 cm soil layer. Br− showed concentrations greater than 1500 mg kg−1 and up to 3000 mg kg−1, and as the concentration of Br− decreases, the frequency and depth of affected layers increases. Despite the differences in moisture and in the distribution of the Br− concentration in both years, the concentration frequency of 500 to 1500 mg kg−1 represented around 37% of the affected volume of soil (bulb of urine) in both years. Up to 40% of the bulb represented N equivalent rates between 187 and 975 kg N ha−1. These values can potentially be emitted in gases such as NH3, N2O, and N2. It is suggested that the presence of N in the volume of affected soil could vary due to the moisture content of the soil, and that in andisols of southern Chile under permanent grasslands there are a large number of macropores that would induce preferential flows.