###### Abstract  
Agricultural soils play a key role in the achievement of a circular nutrient economy. The use of sewage sludges as fertilizers is important for such an achievement, assisting in the maintenance of soil health and nutritional crop value. This study was established, after 23 years of a fertilization experiment, in calcareous soil under a maize monoculture. The treatments included mineral fertilization as a control (MIN, 225 kg N ha\({}^{-1}\)) and two sludge treatments, where doses followed a threshold sludge nitrogen criterion (SNC, 170 kg org-N ha\({}^{-1}\)) or a threshold soil phosphorus criterion (SPC; when the soil Olsen-P value exceeded 40-60 kg P ha\({}^{-1}\), the sludge application was stopped). A detailed study was performed on Cd, Cu, F, Mn, Pb, and Zn soil extractable with diethylenetrimamepentaacetic acid (DTPA), as well as the nutrient and heavy metal concentration of different fractions of the maize plant (grain, cob, and the rest of the plant). Extractions were also quantified. No biomass-yield differences were observed in the different parts of the maize plant in the year of sampling. Sludges increased the soil DTPA extraction of Cd, Cu, Fe, and Zn and diminished Mn extraction, without differences in extractable Pb. The SNC, when compared with MIN, showed increased P cob concentrations, and in grain, it showed increased Fe, Cr, and Co concentrations. The SPC figures of the studied parameters were, in general, between both treatments (MIN and SNC), although Cr extractions in grain diminished vs. SNC. Based on the results, the SPC can be recommended as it also avoids excessive available-P build up. cadmium; chrome; cobalt; DTPA; iron; lead; manganese; nitrogen; phosphorus; zinc +
Footnote †: journal: Medical  
in plants is mainly affected by soil typeto a greater extent than the SS dose [8]. Many field experiments are established on acid [9] or neutral [10; 11] soils, or using acid soils under controlled conditions, usually with short crop cycles [12; 13]. Moreover, some experiments in calcareous soils are short-term experiments [14] focused on crop quality [15]. There is therefore a lack of knowledge about crop composition related to the use of SS on soils of high pH. Furthermore, under field conditions, when SSs are applied to calcareous soils, an increase in the soil bioavailability of some trace metals, and also in barley concentrations, has been observed [16]. In a previous work, eight years before the current study, changes in soil heavy metals were also found, but they were far below the thresholds established by regulations and without changes in maize grain concentrations [17]. Maize is an interesting crop for SS studies as it produces high grain-biomass yields. Additionally, the rest of the plant and cob kernel can be reused for animal feeding [18], which means that the composition of maize parts, linked to SS use, is of interest in the context of a circular nutrient economy. The limits of heavy metals in soil and sewage sludges (Cd, Cu, Ni, Pb, Zn, Hg, Cr (VI), and inorganic As) regarding their agricultural use are stipulated in the Spanish laws [19; 20]. Legislation has been developed in order to avoid their excessive accumulation in soil. Furthermore, the EU [21] established a limit of As, Cd, Hg and Pb in animal feed and human food [22]. The official analytical methods used to quantify heavy metals in soil [20] are based on extraction with aqua regia [23]. However, the most available forms of different heavy metals in neutral and near-alkaline soils are considered to be those obtained through the diethylenetriaminemetaacetic acid (DTPA)-extractable soil fraction, mainly for Zn and Cu bioavailability [24]. The application of SSs, as organic fertilizers, is also based on N criteria according to plant demand and/or general restrictions on water quality protection across Europe. In the area of this study, restrictions on the available-P soil level also apply.