Kinetics of organic carbon mineralization and its changes in three soil types treated with different plant residues (case study: soils around the Lake of Urmia)

Soil C dynamics will be influenced differently following incorporation of various plant residues with different qualitative characteristics. One of the most important ways to improve soil organic matter is to manage the correct use of plant residues in agricultural products, so that by bringing these residues back to the soil, the average annual carbon input to soil is increased and part of the carbon output from decomposition microbial is compensated. Hence, the purpose of this study was to investigate the effect of various plant residues on the mineralization kinetics of carbon and organic carbon content in different soils around the Urmia lake.

This study was conducted as a laboratory incubation experiment in a completely randomized factorial design, including three soil types (calcareous, saline-sodic and sodic) and five plant residues (corn stalks, sunflower stalks, wheat straw, clover and vetch residues) with control treatment (soil without plant residues) in three replications. Plant residues (20 g kg-1) were added to the soils and incubated for 10 weeks (70 days) at 25±1°C. CO2 emissions were measured once a week for 10 weeks, and then the parameters of the kinetic equation of mineralization of carbon were calculated. Finally, the organic carbon content of soils was measured immediately after the completion of the experiment on all treatments.

The results of analysis of variance showed that the soil type and plant residue significantly (p < 0.01) affected C mineralization. In all treatments, the highest CO2 emissions occurred during the first week of incubation, and then there was a decreasing trend until the end of the incubation time. The highest amount of cumulative C mineralized obtained from calcareous soils treated with vetch and clover residues. There was a positive and significant correlation between plant N concentration and C mineralized, while there was a negative and significant correlation between ratios of C/N, lignin/N, cellulose/N, hemicellulose/N, cellulose+lignin/N, N/P ratio and C mineralized. Carbon mineralization in all three studied soils is better estimated with C0K than C0. Also, with the return of plant residues to the soils, the amount of organic carbon increased in all three studied soils. The order of increasing organic carbon in different treatments of plant residues in all three studied soils was wheat> corn> sunflower> vetch> clover.

The results of this study showed that the highest cumulative mineral carbon in calcareous soils among the three studied soils, indicates the further decomposition of residues in these soils. Therefore, it should be compensated annually for the return of more residues to these soils, part of the carbon output resulting from microbial decomposition. Considering that the main purpose of using organic matter in soils was to increase soil organic matter, the results of this study showed that the greatest effect was on organic carbon increase in all three soils (calcareous, sodic and saline-sodic) of wheat residues and then corn with high C/N ratios and the smallest role obtained from clover and vetch with the lowest C/N. This suggests that increasing the amount of soil organic carbon content by adding plant residues depends on the quality of plant remains (C/N, lignin/N, cellulose/N, hemicellulose/N, cellulose + lignin/N and N/P). Therefore, the use of plant residues with high C/N ratio is recommended for carbon sequestration in these soils. Keywords: Plant residues, carbon mineralization, organic carbon, C/N ratio, soil type