Effect of organic and mineral fertilizers applications in pasture and no-tillage system on crop yield, fractions and contaminant potential of Cu and Zn

Organic wastes, fertilizers, organominerals, and minerals contain nutrients essential to crops and contribute to increase productivity. However their use can also lead to the accumulation of copper and zinc, which changes the distribution of their fractions and enhances the risk of toxicity to plants and the environment, thereby working against the sustainable development goals. However, the effect of manure applications on clayey soils and the effects of increasing Cu and Zn contents over the years on crop productivity remain unknown. This study aimed to evaluate the distribution and the potential for contamination of heavy metals in clayey soil with a history of the application of different types and amounts of fertilizers under Cynodon spp. grazing and no-tillage system for soybeans. In Experiment 1, Cynodon spp. were cultivated for hay production and submitted to applications of the following treatments: 0, 200, and 400 kg ha⁻¹ of total-N in the form of pig slurry and urea. In experiment 2, the soybean was grown and subjected to the following treatments: control, pig slurry, commercial granulated solid organic fertilizer, organomineral fertilizer blended with granules, and mineral fertilizer blended with granules. The dose of each nutrient source was determined based on the soil analysis and the recommendations of the liming and fertilization manual. The experiments were conducted for four agricultural crops where soil samples were collected in the layer of 0.00–0.10 m layer and submitted to chemical fractionation of Cu and Zn. The use of pig manure increased the availability of Cu and Zn in the soil, causing an increase in the distribution of copper organic (40.73% and 57.64% for PS200 and PS400 treatments, respectively, in Experiment 1; 67.33% for PS treatment, in Experiment 2) and residual fractions (10.01% and 7.60% for PS200 and PS400 treatments, respectively, in Experiment 1; 9.80% for PS treatment, in Experiment 2), while zinc was predominant in clay-mineral (61.04% and 92.01% for PS200 and PS400 treatments, respectively, in experiment 1; 278.90% for PS treatment, in Experiment 2) and residual fractions (2.36% for PS200 treatment in experiment 1; 9.90% for PS treatment, in Experiment 2). Additionally, the use of swine manure increased the Cu and Zn contents in the exchangeable fraction, which may potentiate the toxicity to plants and increase the potential for contamination of subsoil water. However, the increased bioavailability of Cu and Zn did not caused a loss of crop productivity but instead increased dry mass production and crop yield.