Copper-based fungicide application shifts the soil bacterial community structure and the soil nitrogen cycle

Abstract

Purpose

Through high sorption affinities of Copper (Cu) to soil organic matter, excessively applied Cu generates complex responses by numerous soil functions, e.g. microbial nutrient cycling, that are relying on organic matter decomposition.

Methods

Effects of Cu on soil nitrogen (N) transformation were investigated on a bacterial community level with complying N-functional genes, enzymatic activities and N-pools. Up to 5000 mg Cu kg⁻¹ were applied to an acidic sandy loam (AS) and a calcareous silt loam (CL). An eco-toxicological greenhouse pot experiment was carried out and sampled after 28 and 106 days during the growth of Medicago sativa.

Results

In both soils, the urease activity and ammonium increased with Cu, whereas nitrate decreased. This reduced the mineral N, much stronger in the AS than in the CL. The microbial N rather declined, but the ammonia oxidation via amoA increased, especially in the AS. The NO₂⁻-reductase via nirS declined continuously, while nirK increased up to 200 mg Cu kg⁻¹. The dominating denitrifying Pseudomonas decreased, the community shifted towards saprotrophs at 500 and 1500 mg Cu kg⁻¹ in the AS and CL, respectively.

Conclusions

Shifts in the N-cycle and pool sizes resulting from excessive Cu became apparent in this study. The soil texture- and pH-governed bioavailability affected the Cu-derived responses and indicated a higher sensitivity of the AS. The increasing abundance of Pseudoxanthomonas, i.e. solely capable of dissimilatory nitrite reduction to ammonium suggested Cu-related shifts to this N-pathway. This explains the elevated ammonium concentrations, but not the reduced mineral N-pool.

Graphical Abstract