Modification of copper and cadmium biovailability to faba bean plant (Vicia Faba L.) as affected by soil organic matter and salinity

Određivanje mobilnosti i biopristupačnosti metala u tlu predstavlja osnovu za procjenu rizika od njihovog potencijalnog uključivanja u hranidbeni lanac. Stoga je cilj rada bio definirati mehanizme kojima organska tvar (OT) i salinitet tla rezultiraju promjenama u mobilnosti i biopristupačnosti bakra (Cu) i kadmija (Cd) u tlu. Pretpostavljeno je da će povećanje OT i zaslanjenosti tla utjecati na mobilnost/biopristupačnost Cu i Cd te mikrobiološku dehidrogenaznu aktivnost (DHA) u tlu. Istraživanje je obuhvatilo 2 pokusa koji su provedeni u kontroliranim uvjetima plastenika na Agronomskom fakultetu u Zagrebu, s bobom (Vicia faba L. cv. Aguadulce) kao test kulturom. Eksperimentalni dizajn pokusa bio je split-split-plot u 3 ponavljanja. Glavni faktor bio je sadržaj OT u 2 razine: tlo uzeto iz područja Donje Neretve (OT1) te isto tlo miješano s tresetom u omjeru 4:1 (OT2). Podfaktor bio je NaCl salinitet u 3 razine (0, 50 i 100 mM), a podpodfaktor kontaminacija tla s 250 i 500 mg kg-1 Cu (pokus 1), odnosno 5 i 10 mg kg-1 Cd (pokus 2) i kontrolna (nekontaminirana) varijanta. Rezultati istraživanja ukazuju da je OT jedna od najznačajnijih pedovarijabli koje utječu na mobilnost/biopristupačnost istraživanih metala u tlu. Dodatak OT u varijanti OT2 smanjio je biopristupačnost Cu u tlu procesima sorpcije i kompleksacije s OT (ukupna koncentracija Cu u tlu povećana je za 5 %, dok je koncentracija Cu u mahuni boba smanjena za 14 %). Rezultati upućuju i na smanjenu biopristupačnost Cu u uvjetima NaCl saliniteta i/ili smanjeno primanje Cu u test kulturu iz zaslanjene rizosfere. Dodatak OT u varijanti OT2 i posljedično povećanje koncentracije DOC-a u otopini tla, smanjuje aktivnost slobodnog Cd2+, a time i njegovo primanje u korijen (koncentracija Cd u listu boba smanjena je za 28 %, u mahuni za 40 % te u sjemenu za 20 %). Primjena NaCl nije rezultirala povećanom biopristupačnošću Cd, nego samo njegovom redistribucijom unutar krute faze tla. Kontaminacija tla Cu i Cd nije utjecala na fotosintetske pokazatelje lista boba, upućujući na zaključak da nije utjecala na produktivnost biljaka boba, iako je indeks sadržaja klorofila (CCI) smanjen za 4 % s Cd10 tretmanom. Dodatak OT u varijanti OT2 povećao je za 30-65 %, a NaCl salinitet smanjio za 28-29 % pri NaCl100 tretmanu dehidrogenaznu aktivnost (DHA) u tlu. Kontaminacija tla Cu smanjila je DHA za 50 % pri Cu500 tretmanu. Kontaminacija tla Cd nije utjecala na DHA, ali modeliranje udjela različitih kemijskih oblika Cd u otopini tla primjenom programa Visual MINTEQ i modela NICA-Donnan upućuje na mogućnost da kompleksi Cd i klorida (CdCln2-n) imaju izraženiji inhibitorni učinak na DHA u tlu.The aim of this research was to study the mechanisms by which soil organic matter (SOM) and increased soil salinity affect copper (Cu) and cadmium (Cd) mobility in soil, as well as the importance of afore mentioned mechanisms for their bioavailability in soil. Therefore, to study the Cu and Cd bioavailability in soil, two pot experiments were conducted in a greenhouse at the experimental station Maksimir of the Faculty of Agriculture University of Zagreb (Croatia). In a half of both experiments alluvial soil from cultivated land in a Neretva valley (Croatian Mediterranean region) (OT1) was used, and in the other half the same alluvial soil with added peat to increase SOM content (OT2). The soil (OT1 and OT2) was spiked with Cu and Cd at two levels: 250 and 500 mg Cu kg-1, and 5 and 10 mg Cd kg-1. The same treatment with NaCl salinity was applied in both experiments: control, 50 and 100 mM NaCl. Faba bean (Vicia faba L. cv. Aguadulce) was used as a test plant. Copper solubility and therefore bioavailability in soil is lowered by sorption and complexation processes with SOM. Visual MINTEQ speciation revealed that the Cu in the soil solution is mostly present as Cu-dissolved organic carbon (DOC) complex. The proportion of other Cu species in the soil solution rise with increasing Cu concentration, but only if SOM level is not increased. Faba bean uptake of Cu increased with raising soil Cu concentration, although plants may have restricted the translocation of Cu, behaving like metal excluder plants. However, application of SOM decreased plant tissue Cu concentration, yet significantly only for faba bean pod. Nevertheless, results suggest that Cu phytoavailability may be affected by the increase in SOM, and that faba bean pod tissue provides a best (shoots) reflection of bioavailable Cu in soil. The possibility of reduced Cu mobility in salinized soils and/or reduced plant uptake of Cu from a saline rhizosphere is also suggested by obtained results. Application of salts either lowered Cu solubility in soil or reduced plant uptake by excessive salts in the rhizosphere. Then again, plant adaptive mechanisms may modify Cu phytoavailability. Results also point to a possible malfunction of faba bean root plasma membrane under Cu toxicity conditions. This study confirms that Cd is highly mobile between different soil fractions, depending on its total concentration and the presence of inorganic (chlorides) and organic (humics) ligands in surrounding media. Cd speciation was greatly affected by all investigated trial factors, i.e. soil Cd contamination enhanced the proportion of the free Cd2+, raised SOM content induced Cdorgano-complexation with DOC, and increased soil salinity promoted the formation of CdCln2n complexes in soil solution, all of which consequently modified the mobility of Cd in soil. Soil Cd contamination led to an increased plant Cd uptake. The addition of SOM caused a decreased Cd availability to plants, possibly because of the redistribution of Cd to a less available organically-bound Cd fraction. However, bean plants have uptaken the highest amount of Cd in a non-saline environment, indicating that, if soil adsorption capacity for Cd has not been exceeded, increased Cd mobility in soil does not have to result in its increased phytoavailability, than only in Cd transfer between soil solid phases. Additionally, leaf photosynthetic parameters were not affected by soil Cu and Cd contamination, except for chlorophyll content index (CCI) which decreased with soil Cd contamination. Results show that raised SOM can increase soil dehydrogenase acitivity (DHA), whilst increased soil salinity decreases DHA. Although soil Cd contamination did not significantly affect DHA at studied metal concentrations, the results suggested that CdCln2-n has a higher inhibitory effect on DHA than other Cd chemical species in soil solution. DHA decreased with soil Cu contamination, and was even reduced in half at the total soil concentration of 500 mg Cu kg-1. Results also suggest similar inhibitory effect of all Cu species in soil solution on DHA, but also the possibility that DHA may affect Cu soil availability by reducing the formation of metal-DOM complexes in the soil solution