Recycling composted organic residues in agriculture can reduce the need of mineral fertilizers and improve the physicochemical and biological properties of cultivated soils. However, some trace elements may accumulate in soils following repeated applications and impact other compartments of the agrosystems. This study aims at evaluating the long-term impact of such practices on the composition of soil leaching water, especially on trace metal concentrations.

Main results

Despite the benefits of compost application on arable soils, attention must be paid to possible accumulation of persistent contaminants in soils and their eventual transfer to crops, plant consumers, waters and nearby ecosystems. Several studies demonstrated the increase of inorganic contaminants like copper, zinc, lead or cadmium in horizons receiving organic amendments issued from urban wastes, even under conditions that conform to relevant regulations [1].

The objective of the study is to evaluate the long-term effects of organic residue amendments on the composition of soil draining waters, especially on their concentrations of trace metal contaminants. The present work is based on a large field experiment that has received regular amendments over 15 years to study the long-term impacts of urban waste recycling on arable soils.

The field experiment QualiAgro started in 1998 on typical loess Luvisol of the Paris Basin, with a maize–wheat crop succession and five modalities: spreading of three different urban waste composts, farmyard manure (FYM), and no organic amendment (CTR). Inputs of trace metals have been close to regulatory limits, but supplies of organic matter and nitrogen overpassed common practices. Soil solutions were collected from wick lysimeters at 45 (figure 1) and 100 cm in one plot for each modality, during two drainage periods after the last spreading.

Figure 1 : Installation of lysimeters and hydrodynamic sensors in Feucherolles.

Despite wide temporal variations, a significant effect of treatments on major solutes appears at 45 cm: dissolved organic carbon (DOC), Ca, K, Mg, Na, nitrate, sulphate and chloride concentrations were higher in most amended plots compared to CTR. Cu concentrations were also significantly higher in leachates of amended plots compared to CTR, whereas no clear effect emerged for Zn. The influence of amendments on solute concentrations appeared weaker at 1m than at 45 cm, but still significant and positive for major anions and DOC. Average concentrations of Cu and Zn at 1m depth lied in the ranges [2.5; 3.8] and [2.5; 10.5 μg/L], respectively, with values slightly higher for plots amended with sewage sludge compost or FYM than for CTR. However, leaching of both metals was less than 1% of their respective inputs through organic amendments. For Cd, most values were <0.05 μg/L (figure 2).

Figure 2 : Variations of trace metal concentrations with time, in waters collected by lysimeters at 45 cm depth (●SGW— square MSW—✳FYM—○BIO—▲CTR). The lowest dots in the Zn, Ni and Cd graphs correspond to unquantified values, set at half the quantification limit (QL); QL(Zn) changed between 2012 and 2013.

So, metals added through spreading of compost or manure during 14 years may have increased metal concentrations in leachates of amended plots, in spite of increased soil organic matter, factor of metal retention. Indeed, DOC, also increased by amendments, favours the mobility of Cu; whereas pH variations, depending on treatments, influence negatively the solubility of Zn. Generic adsorption functions of these variables partly explain the variations of trace metal concentrations and helped to unravel the numerous processes induced by regular amendments with organic waste products.

References

1. Houot S, Cambier P, Benoit P, Bodineau G, Deschamps M, Jaulin A, et al. Effet d'apports de composts sur la disponibilité de micropolluants métalliques et organiques dans un sol cultivé. Etude Gest Sols 2009;16:255–74.

Affiliations

a INRA, UMR, 1402 Ecosys Ecologie fonctionnelle et Ecotoxicologie des agroécosystèmes, Thiverval-Grignon, France
b Veolia Environnement Recherche & Innovation, 291 Avenue Dreyfous Ducas, 78520 Limay, France