Zinc–amino acid complexes are more stable than free amino acids in saline and washed soils

• Soil washing, AA type and Zn complexation significantly affected degradation of AA.
• Complexation with Zn reduced mineralization of amino acids in soil.
• The t1/2 of ZnAAC was higher than free amino acids.
• Washing of a natural saline soil decreased degradation of free amino acids.
• Washing of a natural saline soil had inhibitory effect on microbial activity.

Biodegradability of metal–amino acids (AA) complexes has remained undocumented. Such knowledge is necessary to assess role of AA in metal mobility and bioavailability in the soil environments. In this study, biodegradation of free glycine (Gly), glutamine (Gln), arginine (Arg) and histidine (His) was compared with their Zn complexes in a natural saline soil before and after washing. The soil samples were collected from 0 to 15 cm depth of a saline soil before and 7 days after washing. The heavy washing of the saline soils using low salinity irrigation water (EC < 2 dS m−1) is a common practice applied by the farmers in the region before plant seeding. Mineralization of AA, revealed by CO2 release (35–66% in saline soil and 20–42% in washed soil) after 48 h incubation indicating rapid degradation of AA in soil. Complexation with Zn significantly reduced mineralization of AA. In saline soil treated with AA, the cumulative release of CO2 after 48 h incubation varied from 12.4 to 47.5 mg C kg−1, while it was 5.8–37.9 mg C kg−1 in the same soil treated with Zn–AA complexes (ZnAAC). The effect of complexation with Zn on CO2 release was dependent on AA type. Lower C mineralization and thus, higher stability of ZnAAC compared with free AA supports a conclusion that complexation of metal greatly decreases degradability of AA. According to the results, the effectiveness of complexation with proteinaceous AA in metal mobilization in soil has to be given greater consideration than that reported previously.