Agriculture, Ecosystems and Environment 117 (2006) 49–56 www.elsevier.com/locate/agee

Nitrogen losses in runoff waters from a loamy soil treated with sewage sludge G. Ojeda *, D. Tarraso´n, O. Ortiz, J.M. Alcan˜iz CREAF (Centre for Ecological Research and Forestry Applications) and Ecology Unit, Department of Animal and Plant Biology and Ecology, Autonomous University of Barcelona, E-08193 Bellaterra, Spain Received 9 June 2005; received in revised form 15 February 2006; accepted 28 February 2006 Available online 18 April 2006

Abstract Sludge from the same waste-water treatment plant, processed in different ways, was applied superficially to plots of a soil developed on marls (Udic calciustept) at doses equivalent to 10 t ha 1 of dry matter. The concentration of mineral nitrogen (ammonium, nitrate) in runoff waters was measured to assess the effects of composting and thermally drying of sludge on the pollution of surface waters. Significant differences of NH4-N and NO3-N concentration in both runoff waters and soil only appeared during the first five runoff events after sludge application. Thereafter, runoff volume decreased remarkably, possibly due to vegetation growth. The treatment with composted sludge contributes mainly to NO3-N runoff, whereas the NH4-N was predominant in runoff from plots of thermally-dried sludge. The treatment with fresh sludge provided NH4-N and NO3-N to surface runoff in similar amounts. The electrical conductivity of the runoff was not greatly affected by the surface application of the sewage sludges. # 2006 Elsevier B.V. All rights reserved. Keywords: Runoff; Sewage sludge; Nitrate; Ammonium; Composted sludge; Thermally-dried sludge

1. Introduction Runoff generated by rainfall is one of the factors in surface hydrology that can contribute most to the contamination of aquifers through transport of soluble pollutants and suspended particles (Sivakumar et al., 2001). However, the study of runoff is complex as it depends on the intensity and duration of the precipitation and on the properties of the soil surface, which do not necessarily remain constant over time and space (Emmett, 1980). Spatial variability of rainfall and soil physical properties that determines the area contributing to runoff may play an important role in the formation of runoff (Merz and Ba´rdossy, 1998). Water contamination by nitrogen due to agricultural activity is one of the main environmental problems in cattleraising areas of Europe. Although this contamination is * Corresponding author. Tel.: + 34 93 581 14 65;fax: +34 93 581 41 51. E-mail address: [email protected] (G. Ojeda). 0167-8809/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2006.02.017

mainly attributed to leaching processes of nitrate (NO3-N) from the soil, the effects of runoff on surface water are also substantial, especially in Mediterranean climates characterized by irregular and intense rainfall events. The composition of the soil/runoff solution may vary significantly over short-term periods in response to soil amendments (Hesterberg et al., 1994), especially in regions of intensive agricultural activity. Soil erosion is a complex process, which consists of the detachment of particles and aggregates due to the impact of raindrops and their disintegration and transport by surface flow (Huang et al., 1999). Regarding the latter process, Slattery and Burt (1997) state that most sediment transported by runoff is in the form of aggregates. At a microscale level, the aggregated particles are in continuous interaction with their surrounding environment, regulating the quality of the water around them through physical mechanisms as well as, chemical and biological activity (Droppo, 2001). Organic amendments, such as sewage sludge, could form aggregates in the soils to which they are applied (Sort and Alcan˜iz,

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1999a). Addition of sewage sludge to soil will result in an increase of soil organic matter (Loveland and Webb, 2003), and microbial (Kuzyakov et al., 2000) and plant activity (Sollins et al., 1996), and consequently, an improvement of soil aggregate stability. Organic matter can increase the cohesion of aggregates through the binding of mineral particles by organic polymers or physical enmeshment by fine roots or fungi (Chenu et al., 2000), or by decreasing the wettability of soil aggregates (Sullivan, 1990). For this reason in sludge-amended soils, the particles present in the surface runoff events may consist of sludge, soil, sludge-soil aggregates and remains of vegetation. Moreover, runoff contains dissolved substances (minerals and organic matter) from the soil, fertilizers and applied organic residues, among which nitrate (NO3-N) stands out because of its solubility. Organic residues may also contain toxic elements or compounds, which may be transported away from their area of application by the runoff (Rostagno and Sosebee, 2001). Most losses through runoff occur during a small number of very intense rainfall episodes when the runoff water may carry away high concentrations of contaminants (Hubbard et al., 1982; Rostagno and Sosebee, 2001). Moreover, the presence of vegetation cover on the soil surface may affect the size distribution of the eroded sediment, the stability of the aggregates, or the liberation and transport of the sediment itself (Martı´nez-Mena et al., 1999; Cerdan et al., 2002), although each soil response depends on its initial condition. The European Union’s ‘‘Working Document on Sludge’’ (2000) seeks to regulate the use of sewage sludge in agriculture. The high concentrations of organic matter, nitrogen and phosphorous in this waste makes it a good candidate for use as an amendment for soil and as a fertilizer. For this reason, EU policy promotes and regulates the use of sewage sludge in agriculture (Marmo, 2000). The aim of this study was to examine, over a period of a year, the effects of surface application of sewage sludge to a loam soil on the quality of the runoff water, with special attention being paid to soluble nitrogen. As the current trend is to apply sludge of a higher sanitary standard, this study compares nitrogen loss through runoff from a soil treated with fresh (dewatered), composted and thermally-dried sewage sludges.

2. Study site and methods Twelve plots, 20 m long by 5 m wide, were established on a hillside with an average slope of 16% oriented along the line of maximum slope situated in the centre of Catalonia (NE Spain) 590 m above sea level. The climate is sub-Mediterranean with continental features. The mean annual temperature is 12.5 8C. Mean annual rainfall is 750 mm and evapotranspiration is 700 mm y 1. Rainfall is distributed irregularly throughout the year with two

Table 1 Physical and chemical properties of