Effects of organic micropollutants on the stability of soil organic matter: the hidden effect of the irrigation

The global increase in food demand and the effects of climate change require ever higher area irrigation of arable land. On the other hand, the concentrations of organic micropollutants (OMPs) in irrigation water sources are also steadily increasing. However, the effects of OMPs on the microbiota and, thus, on soil organic matter (SOM) stability are still poorly understood.

Consequently, our primary objective is to investigate the effects of pharmaceutically active compounds (PhACs) and their metabolites (Carbamazepine -CBZ, Trans-carbamazepine - TCBZ, 17α-ethynylestradiol - EE2, 17β-estradiol - βE2, 17α-estradiol - αE2, Estrone - E1, Estriol - E3, Diclofenac-sodium  - DFC, 5-Hydroxydiclofenac  - 5-HDFC), an antifungal compound (difenoconazole - DCL) and an antibiotic (Ciprofloxacin - CPX) on the stability of soil organic matter. Soils were incubated after seven treatments: control, PhACs, CPX, DCL, CPX+PhACS, DCL+PhACs, CPX+DCL+PhACs.

We have selected a soil type (Phaeozem) for our research, which is typical of the Earth's arable lands. The soil studied has not been documented to have been tilled for several decades, so past human impacts are now negligible. A series of 90-day incubation experiments were performed, in which soils were pretreated with defined concentrations of OMPs.

Quantitative and qualitative parameters of soil organic matter were determined using a CHNS elemental analyser, an NDIR CN analyser, a UV-Vis spectrophotometer (SUVA254 and phosphatase and hydrolase activities), and a spectrofluorometer (Coble peaks and PARAFAC analysis). Microbial communities were also monitored (16S and 18S RNA sequencing). Since the data were not normally distributed, the Kruskal-Wallis test was used for statistical data analysis.

The ratio of labile fractions of SOM was primarily estimated based on the dissolved fraction ratio (DOC/SOC), aliphatic components, and protein-like components. The microbial activity was estimated based on enzyme activities.

The soil chemistry and microbiological parameters studied were significantly different for all treatments. The proportion of labile fractions decreased over time in all treatments. Contrary to our initial assumption, the proportion of labile fractions decreased most significantly in the combined treatment. The microbial communities changed in different patterns during the incubation.

Our results show that the presence of OMPs in irrigation water affects the ratios of labile and stable fractions SOM and also influences the rate of decomposition.

Hungarian Scientific Found funded the study, project no. K142865 and DKOP-23-03.