PREDICTION OF PATHWAY FOR IMIDACLOPRID DEGRADATION IN ISTRIA AND KVARNER OLIVE ORCHARD SOILS

U suvremeno doba poljoprivreda se suočava s velikim problemom primjene prekomjernih količina kemijskih sredstava (insekticida) u suzbijanju raznih štetnika i njihovih razvojnih oblika, često puta bez ikakve kontrole i provođenja redovitih monitoringa njihovih ostataka u okolišu. U skupinu najčešće primjenjivanih insekticida prvenstveno zbog visoke učinkovitosti, selektivnosti i dugotrajne zaštite nasada ubraja se neonikotinoidni insekticid imidakloprid. U Hrvatskoj imidakloprid je učinkovito sredstvo u zaštiti šećerne repe, ali svoju primjenu pronašao je i u maslinicima Kvarnera i Istre u zaštiti od bolesti i štetnika na kulturama maslina, pogotovo u suzbijanju maslinove muhe (Bactrocera oleae). Iako je aktivan pri niskim dozama, njegova višekratna primjena tijekom sezone rasta i nekontrolirana uporaba predstavljaju javno-zdravstveni problem zbog ulaska u ciklus kruženja tvari u okolišu s toksičnim potencijalom prema vodnim resursima, tlu, zraku i živim bićima. Proučavana je postojanost imidakloprida u tlima maslinika s područja Istre i otoka Krka u ovisnosti o koncentraciji insekticida i fizikalno-kemijskim karakteristikama tla te kinetika razgradnje uz primjenu matematičkih modela. Rezultati ukazuju da je brža razgradnja imidakloprida u tlu Istra nego u tlu Krk, uzrokovana višim kationskim izmjenjivačkim kapacitetom i višem udjelu organskog ugljika i gline. Pri višoj dozi imidakloprida (5 mg/kg) Model kinetike prvoga reda najbolje opisuje razgradnju u tlu Krk dok se u tlu Istra uočava bifazna razgradnja. Bifazno ponašanje razgradnje je uočeno kod oba tla pri nižoj dozi imidakloprida (0,5 mg/kg). Vrijeme poluživota od 91 dan ubraja imidakloprid u umjereno postojane pesticide. Pretpostavka je da se u tlu Istra razgradnja imidakloprida odvijala uz abiotičku kiselu hidrolizu te da u ovom blago kiselom tlu, veliki udio gline uzrokuje vezu između brzina sorpcije i razgradnje i na taj način olakšava i ubrzava reakciju hidrolize. U tlu Krk prisutnost mikrobne populacije vjerojatno je utjecala na postojanost imidakloprida, a istodobno nije postignuta poveznica s procesima sorpcije. Rezultati ukazuju da se matematičko modeliranje može koristiti kao vrlo prikladan alat za predviđanje postojanosti imidakloprida kojim se omogućuje praćenje ostataka imidakloprida u onečišćenom tlu. Nadalje, primjena imidakloprida treba biti strogo kontrolirana, posebno u tlima s niskim sadržajem organske tvari gdje je rizik od onečišćenja tla i podzemnih voda visok zbog dulje postojanosti i mogućnosti površinskog ispiranja i procjeđivanja kroz tlo.
Nowadays, agriculture industry faces with the great problem of applying excessive chemicals (insecticides) amounts in the controling of various pests and their developmental forms, often without any control and regular monitoring of their residues in the environment. One the most commonly used insecticides, primarily due to its high efficiency, selectivity and long-term plantation protection is the neonicotinoid insecticide imidacloprid. In Croatia, imidacloprid is most used as an an effective mean in the protection of sugar beet, but its application has been recorded in Kvarner and Istria olive orchards for the protection of olive tree against diseases and pests such as olive fruit fly (Bactrocera oleae). Although imidacloprid is active at low doses, its repeated application during the growing season and uncontrolled use, poses it as a public health problem since it was entered to the matter cycle in the environment with toxic effect for aquatic resources, soil, air and living beings. The imidacloprid persistence in the soils from Istria and the Krk island olive orchards as a function of the insecticide concentration and physicochemical soil characteristics was studied and analyzed by mathematical modelling. The results of the study indicate that the degradation of imidacloprid was more rapid in the Istria soil than in the Krk soil what is caused by higher cation exchange capacity and higher organic carbon and clay content in Istria soil. At a higher concentration used (5 mg/kg), the first-order kinetics model gives the best description of imidacloprid degradation in the Krk soil, while the biphasic degradation is observed in the Istria soil. Biphasic degradation was observed in both soils at a lower imidacloprid concentration (0.5 mg/ kg). Whit half-life of 91 days in analyzed soils, imidacloprid belongs to the group of moderately persistent pesticides. It can assumed that in the Istria soil, imidacloprid degradation took place by abiotic acidic hydrolysis where high clay amount stimulated imidacloprid sorption and accelerated the hydrolysis reaction. In the Krk soil, the presence of microbial populations probably affected the imidacloprid persistence, and no connection between degradation and sorption processes was achieved. The results of the study indicate that mathematical modeling can be used as a suitable tool for prediction of imidacloprid persistence, which allows monitoring of imidacloprid residues in contaminated soils. Furthermore, the use of imidacloprid should be carried out under strictly controlled conditions, especially in soils with low organic matter content where the potential of soil and groundwater contamination by imidacloprid is high due to its longer persistence and surface leaching and moving through the soil profile.