A bioassay method for detection mesotrione residues in soils with different physico-chemical properties

Mezotrion je herbicid iz kemijske skupine triketona. Koristi se samo u kukuruzu za suzbijanje jednogodišnjih širokolisnih i nekih uskolisnih korova. Temeljem poluvijeka raspada svrstan je u grupu neperzistentnih herbicida. Budući da je perzistentnost herbicida rezultat interakcije više činitelja, znanstvenim istraživanjima je utvrđeno da perzistentnost mezotriona u određenim uvjetima može značajno varirati (od 4 do 44 dana). Stoga je cilj istraživanja bio metodom biotesta utvrditi utjecaj rezidua mezotriona na kulture koje u plodoredu smjenjuju kukuruz. Tijekom 2016. i 2017. na lokaciji Šašinovečki Lug, na tlima različitih fizikalno-kemijskih značajki, postavljena su dva istovjetna poljska pokusa. U prvoj godini istraživana je osjetljivost suncokreta, soje, uljane repice, zobi, graška i šećerne repe na mezotrion primijenjen u reduciranim (18, 36 i 72 g a. t. ha-1), preporučenoj (144 g a. t. ha-1) i uvećanim (288 i 576 g a. t. ha-1) dozama na hipoglej mineralnom i livadsko fluvijalnom tlu. Osjetljivost kultura utvrđena je vizualnom ocjenom 14, 28 i 35 dana nakon primjene (DNP) mezotriona, te utvrđivanjem svježe i suhe nadzemne mase. Iste kulture sijane su ponovno (2017.) na oba tipa tla 12 mjeseci nakon primjene. Osjetljivost kultura u 2017. na rezidue mezotriona utvrđena je vizualnom ocjenom, na temelju koje je odabrana testna kultura za biotest u kontroliranim uvjetima. Tijekom 2017. u kontroliranim uvjetima postavljeni su biotest sa simuliranim (poznatim) količinama rezidua mezotriona i biotest sa stvarnim (nepoznatim) količinama rezidua preostalim u tlu 14 mjeseci nakon aplikacije mezotriona u preporučenoj i uvećanim dozama. Osjetljivost testne kulture, na simulirane rezidue i nepoznate količine rezidua mezotriona iz polja, utvrđena je vizualnom ocjenom, utvrđivanjem svježe nadzemne mase i utvrđivanjem ukupnog sadržaja karotenoida. Vrijednosti istraživanih parametara osjetljivosti na simulirane rezidue mezotriona, radi utvrđivanja sadržaja rezidua u hipoglej mineralnom i livadsko fluvijalnom tlu, bile su uspoređene s vrijednostima istraživanih parametara osjetljivosti na nepoznate količine rezidua mezotriona. U cilju procjene pouzdanosti biotest metode, uzorci tla s nepoznatim količinama rezidua mezotriona bili su analizirani pomoću tekućinskog kromatografa visoke djelotvornosti. U prvoj godini istraživanja, sve kulture, osim zobi, iskazale su fitotoksični učinak na sve primijenjene doze mezotriona. Značajno veća fitotoksična oštećenja istraživanih kultura utvrđena su na livadsko fluvijalnom tlu. Godinu dana nakon primjene mezotriona fitotoksična oštećenja utvrđena su na grašku i šećernoj repi. Fitotoksična oštećenja na grašku utvrđena su samo na hipoglej mineralnom tlu na tretmanima na kojima je mezotrion bio primijenjen u dvostrukoj (288 g a. t. ha-1) i četverostrukoj (576 g a. t. ha-1) dozi. Oštećenja na šećernoj repi su, osim na dvostrukoj i četverostrukoj dozi, utvrđena i na preporučenoj (144 g a. t. ha-1) dozi na hipoglej mineralnom tlu, zbog čega je odabrana kao testna kultura za biotest u kontroliranim uvjetima. Primjenom simuliranih rezidua mezotriona, veća osjetljivost šećerne repe utvrđena je na livadsko fluvijalnom tlu. Utvrđeno je da simulirane rezidue mezotriona, koje odgovaraju količini 1/32 preporučene doze na livadsko fluvijalnom tlu, odnosno 1/24 preporučene doze na hipoglej mineralnom tlu, mogu uzrokovati 50 % redukcije svježe nadzemne mase šećerne repe. Na livadsko fluvijalnom tlu količina rezidua koja je uzrokovala 50 % redukcije ukupnog sadržaja karotenoida odgovarala je 1/64 preporučene doze, dok na hipoglej mineralnom tlu 1/32 preporučene doze mezotriona. Instrumentalnim metodama rezidue mezotriona nisu utvrđene u nijednom istraživanom tlu, što upućuje na to da su, rezidue mogle biti prisutne samo u količinama manjim od granice detekcije (5 ng g-1). Na temelju rezultata iz simuliranog biotesta, 14 mjeseci nakon primjene mezotriona, veća količina rezidua utvrđena je u hipoglej mineralnom tlu, a iznosila je 1,25 ng g-1. Pretpostavlja se da je u hipoglej mineralnom tlu, zbog većeg sadržaja humusa i organskog ugljika, bila potencijalno veća adsorpcija mezotriona. Istraživanjem je utvrđeno da se jedino biotest metodom, odnosno reakcijom biljke mogu dobiti pouzdani podaci o neznatno malim količinama rezidua koje štete osjetljivim kulturama, a koje instrumentalnim metodama nije moguće utvrditi.A herbicide persistence is an important consideration in the crop production since residues can potentially injure sensitive crops grown in rotation. A herbicide residues, even in a very small amounts, can cause great variability in the plant growth and quality. The length of time a herbicide remains persistent in the soil depends on the physicochemical properties of the herbicide and the environmental conditions (soil type, rainfall, and temperature). Mesotrione is a triketone registered since 2001 as a selective herbicide mainly for maize protection. Its herbicidal property is based on inhibition of the essential plant enzyme 4-HPPD (4-hydroxyphenylpyruvate dioxygenase) in a wide range of a broad-leaved and grass weeds. Mesotrione is classified as a non-persistent herbicide in soil with a typical degradation time (DT50) of 4 to 44 days. A relatively wide range of a half-life suggests that mesotrione may persist longer than expected under certain field conditions. A several studies have shown that mesotrione can cause injury to sensitive crops grown in rotation even in the case of adherence to crop rotation restrictions. That indicates that crop rotation program should be carried out according to the observed field conditions, rather than applied uniformly as it is outlined in the instructions for herbicide use. The purpose of this research was to evaluate the relative susceptibility of sunflower, sugar beet, soybean, rapeseed, pea and oat to mesotrione residues, 12 months after mesotrione application in two agricultural soils differing in physicochemical properties. The specific objectives were to: determine test crop for laboratory bioassay; determine the influence of physicochemical properties of soil on the susceptibility of the test crop; determine the reliability of bioassays in regards to instrumental methods. Two field experiments were conducted during 2016 and 2017 in Šašinovečki Lug (north-western Croatia). The location of the experiments was selected based on the differences in soil physicochemical properties. The soils were hipogley mineral (sand 1.1 %; silt 59.6 %; 39.3 % clay; CEC 33.8 cmol kg-1; pH 7.7; OC 2.5 %; humus 4.2 %) and humofluvisol (sand 11.6 %; silt 66.9 %; 21.5 % clay; CEC 21.8 cmol kg-1; pH 8.2; OC 1.3 %; humus 2.7 %). Sunflower, sugar beet, soybean, rapeseed, pea, and oat were seeded on April 30, 2016, using automatic seed planter. Mesotrione was applied pre-emergence at 0, 18 (1/8), 36 (1/4), 72 (1/2), 144 (1x), 288 (2x) and 576 (4x) g ai ha-1. The labeled pre-emergence dose of mesotrione on the field corn in Croatia is 144 g ai ha-1. The experimental design was a randomized complete block design with a strip-plot arrangement and three replications with herbicide treatment as the main plot and crops as a subplot. Visual crop injury ratings were made on each crop using a 0 (no bleaching effect) to 100 % (crop death) scale. Injury ratings were taken at 14, 28 and 35 days after planting (DAP). Above ground plant samples of each crop were taken from 0.75 m2 of the plot area at 35 DAP for fresh and dry weights. On May 2, 2017 crops were seeded on both soils where 12 months before mesotrione was applied. The susceptibility of crops to mesotrione residues in soils was determined by visual assessment of phytotoxicity assessment. Based on the results from field studies the test crop for laboratory bioassay was selected. Two laboratory bioassay studies were conducted in 2017 at the University of Zagreb, Faculty of Agriculture. One study consisted of spiking untreated soil from the field with known concentrations of mesotrione. The second feild study sampled soils treated with mesotrione in the previous year. Soil samples for both studies and the chemical analysis of herbicide residues in both soils were taken on July 3, 2017. For the spiked-soil laboratory bioassay, soil samples were taken from untreated areas of the field. For the laboratory bioassay, based on the previous-year mesotrione application, soil samples were collected from the untreated and field plots treated with mesotrione at 144, 288 and 576 g ai ha-1. In the spiked-soil bioassay, soil subsamples (200 g) were treated with mesotrione at seven application rates: 1.1 (1/128 R), 2.3 (1/64 R), 4.5 (1/32 R), 9.0 (1/16 R), 18 (1/8 R), 24 (1/6 R) and 36 (1/4 R) g a. i. ha-1, where R is a recommended application rate (144 g a. i. ha-1). The selected application rates reflected the residues of mesotrione which can be expected in soil over time considering its half-life under field conditions. Selected test crop was seeded in the soil samples treated with simulated mesotrione residues and in the soil samples taken from fields treated with mesotrione in the previous year. The experimental design was a randomized complete block design with four (known mesotrione concentrations) and three (unknown mesotrione concentrations) replicates per rate. The pots were kept under controlled conditions (temperatures of 20/15 °C in 16/8 hours of light and dark cycle) for three weeks. All pots were brought to field capacity with water and twice a week during the study soil samples were watered to restore the field capacity weight. The susceptibility of test crop to simulated mesotrione residues and mesotrione residues from the field was determined by visual assessment of phytotoxicity, measuring the fresh weight and the total carotenoids content in test crop. In the first (2016) year of study, all crops except oat responded with phytotoxic effects to all mesotrione rates. Significantly greater phytotoxic effects of the crops were determined on humofluvisol. A 12 months after the mesotrione application, phytotoxic effects were observed on pea and sugar beet. The phytotoxic effects on pea were only determined on hipogley mineral on treatments on which the previous year mesotrione was applied at 288 and 576 g ai ha-1. The phytotoxic effects on sugar beet, except on treatments where mesotrione was applied at 288 and 576 g ai ha-1, was also determined at the recommended (144 g ai ha-1) rate on hipogley. That was the reason for selecting sugar beet as a test crop for laboratory bioassay. High injuries, at all rates of mesotrione simulated residues, were noticed in humofluvisol compared to hipogley mineral. The effective dose (ED50) of mesotrione for 50 % reduction of sugar beet fresh weight was 6.7 g ai ha-1 (1/24 R) in hipogley mineral and 4.9 g ai ha-1 (1/33 R) in humofluvisol. The ED50 of mesotrione for 50 % reduction of total carotenoids content was 5.1 g ai ha-1 (1/30 R) in hipogley mineral and 2.1 g ai ha-1 (1/70 R) in humofluvisol. A greater difference in sugar beet response to the mesotrione residues in soils based on different soil properties was obtained in the determination of total carotenoids content than of fresh weight content. This suggested that the total carotenoids reduction method is a reliable method that should be used for sensitive evaluation of crop susceptibility on change in soil composition. A mass concentrations of mesotrione residues were not detected in any of the studied soils, which suggests that residues could be present in quantities smaller than the limit of detection (5 ng g-1). Based on the results from the simulated bioassay, 14 months after the mesotrione application, a higher amount of residues was determined in the hypogley mineral and the amount was 1.25 ng g-1. It is assumed that due to the higher content of humus and organic carbon, potentially higher adsorption of mesotrione was in hipogley mineral soil. The results of this study showed that only bioassay can obtain reliable data on small amounts of residues that damage susceptible crops, which can't be determined by chemical analysis. This research indicated that soil properties greatly influenced the phytotoxicity of the applied herbicides. Injury on most crops was more severe in humofluvisol than in hipogley mineral. The results confirmed that mesotrione residues can cause injury in susceptible crops grown in rotation. A soil factors had a significant effect on the mesotrione carryover. A one year after mesotrione application in the field, higher injuries were observed on hipogley compared to humofluvisol. It is assumed that due to mesotrione dependence on microbial degradation, it persists longer under high humus and organic carbon content. Therefore, the effective re-cropping intervals for mesotrione may be different for different soil types. The research showed that simulated residue carryover studies are appropriate for determining the potential for injury to sensitive crops grown in the rotation. They also showed that only bioassay can obtain reliable data on small amounts of residues that damage susceptible crops and which can't be determined by chemical analysis. The laboratory bioassay helps to predict potential herbicide residue problems so the grower can make better decisions about crop rotation, planting date and other cultural practices