Bulk electrical conductivity as an indicator of spatial distribution of nitrogen and phosphorous at feedlots

• Conductivity is a good indicator of nitrates for the highest soil water contents
• Conductivity is a good indicator of phosphorous for the highest soil water contents
• The longer is the stocking rate, the higher the nitrogen and phosphorous in soils
• Groundwater quality is strongly influenced by water table depth

The increasing demands of producing beef for domestic and international consumption is leading to the development of feedlots in Argentina. Unfortunately, the great amount of manure produces leachate of nitrates and other ions which may affect groundwater quality. The objective of this paper was to identify the spatial distribution of bulk electrical conductivity (ECb) of non-saturated zone and its association with variability of nitrates and bioavailable phosphorous, water content, topography and electrical conductivity of saturated paste extract (EC).The analysis of the impact was done at two pens with different time of confinement of the animals (Pen1 and Pen 2, with 16 months and 7 years of animal occupation, respectively). The initial exploration phase was done by electrical resistivity tomography (ERT) and electromagnetic induction (EMI) surveys were subsequently performed at a subarea for studying the spatial distribution of ECb. A grid of soil sampling up to 1 m depth was done at the same subarea of the EMI survey. A geostatistical interpolation of data was performed in order to map ECb, water content, nitrogen, bioavailable phosphorous and EC.Anomalies of higher conductivity (between 6 and 10 times the background values) were found near the feedbunk and the water trough, being greater on soils with larger period of manure recharge. Water content, nitrogen of nitrates (N-NO3−), bioavailable phosphorous (Pavail.) and EC showed patterns of spatial distribution similar to ECb. These patterns are mainly associated with the runoff movement of water to the lower zones in the case of a steeper slope and the soil compaction as in the Pen 1. In the Pen 2, the patterns of spatial variability are similar for ECb and water content while anomalies of high ECb were found along an abandoned feedbunk. High values of N-NO3−, Pavail. and EC were found at this site. A longer period of manure recharge resulted in higher values of nitrogen and bioavailable phosphorous in soils.It may be stated that bulk electrical conductivity was a good spatial indicator of water, nitrogen and bioavailable phosphorous in sandy loamy soils in these pens for the investigation depth of 1 m. The bulk conductivity seems to be primarily associated with water content which is the path of transport of ions through the porous medium. In all the cases, the major contents of nitrogen and bioavailable phosphorous in the soil were associated with high bulk electrical conductivity when water content was above an equivalent depth of water of 100 mm. The soil moisture status should be taken into account before an electromagnetic exploration for detecting soil contamination. Regarding the impact on groundwater pollution, the proximity of the water table to the surface seems to be more influential than the animal stocking rate for a longer period.