Effect of Decomposed Rice Straw Solution on AgNP Uptake by Rice Roots

Decomposed rice straw solution was used as amendment of organic matter to investigate effects of natural organic matter (NOM) on AgNP phyto-availability. AgNP uptake rate decreased with increasing levels of total organic carbon (TOC, 0-20 mg·L^-1) and then tended to level off when TOC rose beyond 20 mg·L^-1 till 80 mg·L^-1 in concentration. Analysis of the dynamic process of AgNP in the solution demonstrates that dissolution of AgNP was inhibited by NOM and decreased by 27.5%-95.5% in a dose-dependent manner as against the CK. Obviously the amendment of NOM leads to a reduction of the concentration of silver ion dissolved in the exposure medium and consequently a lower AgNP uptake rate of the crop. Meanwhile, the NOM tended to promote aggregation of AgNP, thus expanding the size of AgNP particles, and making them hard to be absorbed by rice. In addition, composition of NOM in molecular weight would also affect AgNP uptake. NOM low in molecular weight (<3 kDa) was found to have an effect on AgNP uptake comparable to that the pristine NOM did, indicating that the fraction of low molecular weight NOM plays an important role in AgNP uptake by rice roots.