Soil water repellency after slurry fertilization in a dryland agricultural system

In the context of the circular economy, the use of animal excrements as fertilizers is encouraged, but the addition of such organic materials can develop soil water repellency (SWR) or hydrophobicity. There is a lack of consensus about how to assess SWR in agricultural soils. This work evaluated SWR when pig slurries were applied onto soil using the two most common testing methods: the water drop penetration time (WDPT) and the molarity of ethanol droplet (MED) tests. The experiment consisted of five different slurry treatments plus a control (no slurry added). At sowing, slurry from fattening pigs (SF) or no slurry application was combined with a second application at the cereal tillering stage, in which SF, slurry from sows (SS) or no-slurry were the treatments. Soil water repellency was tested at cereal tillering before slurry application and then during the following 47 days. At each sampling date, hydrophobicity was tested in undisturbed samples at field-moist and after 25 °C, 65 °C, and 105 °C oven drying. Disturbed samples were tested after 40 °C oven drying. Soil disturbance removed SWR. Under field conditions, undisturbed samples attained the maximum SWR expression 7d from pig slurry application; moderate and very severe scores using WDPT and MED were respectively attained. From 14d to the end of the experiment, the highest SWR was observed after 105 °C oven drying and when SF applications at sowing (900 kg TOC ha−1) had been combined with SS applications at cereal tillering (1894 kg TOC ha−1). Slurry hydrophobic compounds rather than slurry dry matter influenced SWR expression which is enhanced as the soil dries. The persistence of repellency (WDPT test) was more sensitive to detecting changes in SWR between treatments than the changes in its severity (MED test). The importance of the SWR described hereafter slurry application will depend on plant cover over the soil and the effects in slaking prevention in order to avoid superficial water runoff.Field assistance from J.C. Estruga, J.M. Llop, B. Martínez and N. Mateo and laboratory assistance from M. Antúnez and S. Porras (University of Lleida); D. Pezzolla, G. Gigliotti and “Laboratorio di Chimica Agraria – Chimica delle Biomasse di Uso Agrario” (University of Perugia) is fully acknowledged. Funding: This work was supported by the Spanish Ministry of Economy and Competitiveness and the Spanish National Institute for Agricultural Research and Experimentation (MINECO-INIA) through the projects RTA2013-57-C5-5 and RTA2017-88-C3-3. JADE-Plus scholarship from Bank of Santander-University of Lleida for D.E. Jiménez-De-Santiago PhD studies. Grant for linguistic revision from the Language Institute of the University of Lleida