Nutrient elements in micro- to nanosized fractions: occurrence, transport, and plant uptake in agroecosystems

Abstract

As phosphorus (P) is a non-substitutable nutrient for all organisms and thus pivotal in supporting crop productivity, limited and unevenly distributed phosphate rocks drive research concerns on substituted P fertilizers and management of soil legacy P. My thesis mainly focused on 1) the availability and transformation of P from alternative P fertilizers within soil size fractions; 2) a detailed understanding of colloidal P transport and potential associations with other elements in different arable soil layers; 3) a general exploration of plant uptake preferences of elements from nanoparticles and dissolved salts. As data on nanoparticulate P uptake were rare, I focused on different metallic elements in this third part to achieve a broader understanding of the main factors controlling metal accumulation in crops from metallic nanoparticles (MNPs) applications.<br /> To achieve the above aims, firstly, bone char (BC) and sulfur modified bone char (BCplus), as potential substitutes for mineral P fertilizers, were assessed after 5-year field fertilizations in the P fractions and pools within soil size fractions by wet-sieving, centrifugation, and tangential flow filtration followed by UV-Vis and ICP-OES determinations. Moreover, the colloidal P in different arable soil layers and artificial drainage systems were determined with FFF-OCD-ICP-MS; potential associations between P and other elements were estimated via cluster analysis. To elucidate the uptake of MNPs by crops, data were collected and studied by a meta-analysis, with effect sizes (standardized mean difference) of various factors on metal accumulations in crop tissues calculated for the three most studied non-essential and essential metals, respectively.<br /> Field trial results showed that BC and BCplus fertilization mostly had no detrimental effects on soil macro- and micro-aggregations, nor on different soil P fractions and pools compared to triple superphosphate (TSP) fertilization. Similar to TSP, BC and BCplus increased the mass proportions of large micro-aggregates compared to the no P control.<br /> Detected arable soil colloids consisted of three size fractions including: nanocolloids (0.66-20 nm) enriched in organic carbon (Corg) and calcium (Ca); fine and medium-sized colloids (20-170 and 170-450 nm, respectively) rich in Corg, iron (Fe), aluminum (Al), and silicon (Si). Colloidal P stocks among three size fractions were 0.6-6.6, 0.2-2.9 and 0.9-7.1 kg ha-1 at Ap, Bw, and C horizons, respectively, highlighting that colloidal P contributes only partly to plant fertilizer needs. A cluster analysis revealed high similarity of soil nanocolloids in the C horizon to medium-sized colloids in water, suggesting their potential translocation to water bodies via ditch and tile drainage systems.<br /> The meta-analysis revealed that current studies on crops’ uptake of MNPs have focused on worldwide main cereal and vegetable crops (wheat, tomato, bean, maize, rice, and cucumber), and mainly on six main metals: Zn, Ag, Cu, Fe, Ce and Ti. The uptake preference of the elements from nanoparticles or salts were element-specific and varied among tissues. Plants generally accumulated higher concentrations of the three essential metals (Zn, Fe, and Cu) than of the non-essential ones (Ag, Ce, and Ti), and uptake rates were more efficient upon foliar exposure to MNPs than upon supplements to roots. Shoot metal concentrations increased with decreasing particle size diameters and increasing negative zeta potential.<br /> I conclude that alternative sources of P fertilization have potential. The BCs reveal similar effects on P status than TSP, but also nanocolloidal P could contribute to both plant nutrition and ecosystem loss. The mechanisms of nanoparticulate P uptake renders further attention, but studies using MNPs revealed that there are plant uptake preferences depending on both the element and nanoparticle properties, particularly particle size or surface charge.