Longevity of soil water repellency in a former wastewater disposal tree stand and potential amelioration

Previous studies have shown that long-term irrigation with wastewater can lead to the development of soil water repellency. Little is known about the longevity of this effect. Here we address this research gap by examining the effect of long-term (~ 20 years) use of low-quality wastewater for disposal purposes, followed by 6 years of ‘recovery’ with no irrigation, on the wettability of calcareous sandy soil (Xerofluvent) under a Populus alba tree stand used as a “green filter” in SE Spain. Water repellency (WR) and soil organic matter content (SOM) were determined for 120 air-dry samples from the plot and 80 control samples from adjacent and otherwise similar non-irrigated areas. To account for plot micro-topography 40 samples each were taken from ridges (R; 0-5 cm depth), furrows (F; 0-5 cm), and furrows at depth (FD; 5-10 cm). The controls included 40 samples each (0-5 cm depth) from unvegetated and unploughed soil, and from soil under the P. alba plantation.All control samples were non-repellent whereas at the irrigated plot, water repellency was present for 48, 95 and 93% of ridge, furrow and furrow-depth samples respectively. WR and SOM was strongly correlated within the whole sample population (R² = 0.623**) and within two sample groups (R: R² = 0.783**; FD: R² = 0.424**), but weakly within F samples (R² = 0.072 n.s.). The latter showed the highest frequency and persistence (WDPT) of WR, indicating that not only quantity of SOM is controlling WR. Exploratory kaolinite clay additions (0.5-8%) to samples substantially reduced WR even at the lowest concentration, indicating that this could be a promising amelioration treatment for the WR in the soils investigated here.We conclude that for the conditions studied here (i) long-term use with poor-quality wastewater for disposal had led to the development of soil WR, (ii) a 6-year period of ‘recovery’ (i.e. non-irrigation) was insufficient to eliminate the induced WR, and (iii) kaolinite addition could be a promising amelioration treatment for these sandy soils.