Fusarium Mycotoxins in Maize Field Soils: Method Validation and Implications for Sampling Strategy.

While mycotoxins are generally regarded as food contamination issues, there is growing interest in mycotoxins as environmental pollutants. The main sources of trichothecene and zearalenone mycotoxins in the environment are mainly attributed to Fusarium infested fields, where mycotoxins can wash off in infested plants or harvest residues. Subsequently, mycotoxins inevitably enter the soil. In this context, investigations into the effects, fate, and transport are still needed. However, there is a lack of analytical methods used to determine Fusarium toxins in soil matrices. We aimed to validate an analytical method capable of determining the toxins nivalenol (NIV), deoxynivalenol (DON), 15-acetyl-deoxynivalenol (15-AcDON), and zearalenone (ZEN), at environmentally relevant concentrations, in five contrasting agricultural soils. Soils were spiked at three levels (3, 9 and 15 ng g-1), extracted by solid-liquid extraction assisted with ultrasonication, using a generic solvent composition of acetonitrile:water 84:16 (v:v) and measured by LC-HRMS. Method validation was successful for NIV, DON, and 15-AcDON with mean recoveries > 93% and RSDr < 10%. ZEN failed the validation criteria. The validated method was applied to eight conventionally managed maize field soils during harvest season, to provide a first insight into DON, NIV, and 15-AcDON levels. Mycotoxins were present in two out of eight sampled maize fields. Soil mycotoxin concentrations ranged from 0.53 to 19.4 ng g-1 and 0.8 to 2.2 ng g-1 for DON and NIV, respectively. Additionally, we found indication that "hot-spot" concentrations were restricted to small scales (<5 cm) with implications for field scale soil monitoring strategies.

Effects of temperature and soil fauna on the reduction and leaching of deoxynivalenol and zearalenone from Fusarium graminearum-infected maize stubbles.

A microcosm study was conducted at two different temperatures under laboratory conditions to investigate the regulatory capacity and the interactive performance of two soil fauna species (Aporrectodea caliginosa, earthworms, and Proisotoma minuta, collembolans) on the reduction of Fusarium toxins in contaminated maize stubbles. Single and mixed species treatments were exposed to artificially infected maize stubbles highly contaminated with the mycotoxins deoxynivalenol (DON) (10,462 µg kg-1) and zearalenone (ZEN) (2,780 µg kg-1) at 17 °C and 25 °C for time periods of 3 and 6 weeks. Immediately after the respective end of incubation, the microcosms were heavily watered to determine the leaching potential of DON and ZEN from contaminated maize stubbles. Maize residues, soil, and eluted water (percolate) samples were analysed for mycotoxin content using liquid chromatography coupled to mass spectrometry. The biomass of introduced earthworms and number of collembolans were monitored to get information about their adaptability to the experimental conditions. While the decline of ZEN was temperature-dependent, but not influenced by faunal activities, a reduction of DON due to faunal impact was observed by trend. In the leaching experiment, 67-82% of the DON content in the residual maize stubbles leached from the plant material by irrigation and was detected in the soil (1.9-3.4 µg kg-1) and in the percolate (12-295 µg L-1). In the case of ZEN, 27-50% of the mycotoxin leached from the residual maize stubbles due to watering but was only occasionally detected in traces in the soil and not found in the percolate. The results clearly reveal a leaching potential of both DON and ZEN, respectively, but a mobilisation with water was only observed for DON. Temperature confirmed to be a key factor, affecting the fate of the mycotoxins in the soil by driving the interaction between different soil fauna members as well as functional and trophic levels within the soil food web.

Biocontrol of the toxigenic plant pathogen Fusarium culmorum by soil fauna in an agroecosystem.

In 2011 and 2013, a field experiment was conducted in a winter wheat field at Adenstedt (northern Germany) to investigate biocontrol and interaction effects of important members of the soil food web (Lumbricus terrestris, Annelida; Folsomia candida, Collembola and Aphelenchoides saprophilus, Nematoda) on the phytopathogenic fungus Fusarium culmorum in wheat straw. Therefore, soil fauna was introduced in mesocosms in defined numbers and combinations and exposed to either Fusarium-infected or non-infected wheat straw. L. terrestris was introduced in all faunal treatments and combined either with F. candida or A. saprophilus or both. Mesocosms filled with a Luvisol soil, a cover of different types of wheat straw and respective combinations of faunal species were established outdoors in the topsoil of a winter wheat field after harvest of the crop. After a time span of 4 and 8 weeks, the degree of wheat straw coverage of mesocosms was quantified to assess its attractiveness for the soil fauna. The content of Fusarium biomass in residual wheat straw and soil was determined using a double-antibody sandwich (DAS)-ELISA method. In both experimental years, the infected wheat straw was incorporated more efficiently into the soil than the non-infected control straw due to the presence of L. terrestris in all faunal treatments than the non-infected control straw. In addition, Fusarium biomass was reduced significantly in all treatments after 4 weeks (2011: 95-99%; 2013:15-54%), whereupon the decline of fungal biomass was higher in faunal treatments than in non-faunal treatments and differed significantly from them. In 2011, Fusarium biomass of the faunal treatments was below the quantification limit after 8 weeks. In 2013, a decline of Fusarium biomass was observed, but the highest content of Fusarium biomass was still found in the non-faunal treatments after 8 weeks. In the soil of all treatments, Fusarium biomass was below the quantification limit. The earthworm species L. terrestris revealed a considerable potential as an effective biocontrol agent contributing to a sustainable control of a Fusarium plant pathogen in wheat straw, thus reducing the infection risk for specific plant diseases in arable fields.