The Use of Stable Zinc Isotope Soil Labeling to Assess the Contribution of Complex Organic Fertilizers to the Zinc Nutrition of Ryegrass

Abstract

Manure and sewage sludge are known to add significant amounts of zinc (Zn) and other metals to soils. However, there is a paucity of information on the fate of Zn that derives from complex organic fertilizers in soil-plant systems and the contribution of these fertilizers to the Zn nutrition of crops. To answer these questions, we grew Italian ryegrass in the presence of ZnSO₄, sewage sludge, and cattle and poultry manure in an acidic soil from Heitenried, Switzerland, and an alkaline soil from Strickhof, Switzerland, where the isotopically exchangeable Zn had been labeled with ⁶⁷Zn. This allowed us to calculate the fraction of Zn in the shoots that was derived from fertilizer, soil, and seed over 4 successive cuts. In addition, we measured the ⁶⁷Zn:⁶⁶Zn isotope ratio with the diffusive gradients in thin films technique (DGT) on soils labeled with ⁶⁷Zn and incubated with the same fertilizers. After 48 days of growth, the largest fraction of Zn in the ryegrass shoots was derived from the soil (79-88%), followed by the Zn-containing fertilizer (11-20%); the least (<2.3%) came from the seed. Only a minor fraction of the Zn applied with the fertilizer was transferred to the shoots (4.7-12%), which indicates that most of the freshly added Zn remained in the soil after one crop cycle and may thereby contribute to a residual Zn pool in the soil. The ⁶⁷Zn:⁶⁶Zn isotope ratios in the DGT extracts and the shoots measured at cut 4 were identical, suggesting that the DGT and plant took up Zn from the same pool. The proportion of Zn derived from the fertilizers in the DGT extracts was also identical to that measured in ryegrass shoots at cut 4. In conclusion, this work shows that stable Zn isotope labeling of the soil available Zn can be used to precisely quantify the impact of complex organic fertilizers on the Zn nutrition of crops. It also demonstrates that DGT extractions on labeled soils could be used to estimate the contribution of Zn fertilizers to plant nutrition.

Keywords: DGT (diffusive gradients in thin films); manure; plant nutrition; sewage sludge; source tracing.

Figure 1

Uptake of Zn derived from the Zn-containing fertilizers (QZndffert_sum; A) and soils (QZndfsoil_sum; B) in the Italian ryegrass shoots (sum of 4 cuts) cultivated in a growth chamber and grown in soil from Heitenried (pH = 4.9) and Strickhof (pH = 7.7) without (no Zn) and with the addition of distinct Zn-containing fertilizers. Data are average values ± standard deviation (bars in the figures) calculated from n = 4 replicates. QZndffert_sum values are corrected for the uptake of Zn derived from the seed. For each soil, different letters indicate significant differences (p < 0.05) among treatments. The effects of the soils and fertilizers tested by two-way ANOVA are shown at the bottom of each plot.

Figure 2

Bland–Altman plot comparing the ⁶⁷Zn:⁶⁶Zn ratios measured in the DGT extracts in Heitenried soil and Strickhof soil incubated for 48 days after the addition of distinct Zn-containing fertilizers and in the Italian ryegrass shoots at cut 4 subjected to the same treatment cultivated in the same soil in the same growth chamber. For each treatment, the results measured in the DGT extracts and the shoots were averaged (x-axis) and plotted against their difference (y-axis). The dotted line shows the bias and the dashed lines represent the 95% confidence interval (CI) of the Bland–Altman analysis.

Figure 3

Bland–Altman plot comparing the proportion of Zn derived from the Zn-containing fertilizers in the DGT extracts (Zndffert_DGT) and in Italian ryegrass shoots at cut 4 (Zndffert₄) subjected to the same treatment in the same soil. For each treatment, the values of Zndffert_DGT and Zndffert₄ were averaged (x-axis) and plotted against their difference (y-axis). The dotted line shows the bias and the dashed lines represent the 95% confidence interval (CI) of the Bland–Altman analysis.