Abstract
Free amino acids (FAAs) in soil play an important role in the soil nitrogen cycle and plant nutrition. However, the attributing factors and migration characteristics of free amino acid pools in paddy soils after green manure application during the entire growth period of rice have not been elucidated. In this study, a single application of chemical fertilizer (CK) was used as a control under equal nitrogen, phosphorus and potassium conditions, and different application rates of milk vetch (15 000 kghm−2(CL), 30 000 kghm−2(CM) and 45 000 kg hm−2(CH)) were selected to investigate the dynamic of FAAs concentration and composition in paddy soil. Soil FAAs concentration at different growth stages under the same fertilization treatments was highest at the seedling stage and lowest at the tillering stage. The concentration of threonine, alanine, valine, isoleucine, leucine, phenylalanine was most abundant under different fertilization treatments during the growth period, accounting for 59.42 %–76.46 % of the respective FAAs pool. The application of milk vetch was shown to increase the soil FAAs concentration, especially glutamic acid, which increased by 368.17 %–680.78 %, but excessive application had an inhibitory effect. Soil pH, organic matter, protease, bacterial biomass and community were critical factors affecting the concentration of soil FAAs. Bacteroidetes, Firmicutes and Nitrospirae significantly affected the dynamics of FAAs in bacterial communities, and their total contribution rate was 56.89 %. FAAs displayed significant vertical profile characteristics, and the mobility of serine, glycine and proline was high. Conclusively, the application of milk vetch was able to significantly change the concentration and composition of soil FAAs, which may affect the capture of N by plants.
Highlights
More than 90% of total soil nitrogen (N) is organic and, within the organic forms of N, amino acids are the most abundant (Rovira et al, 2008)
The temporal variation in concentration of Free amino acids (FAAs) displayed a similar pattern under different fertilization treatments (Fig. 1), with the concentration increasing from background to the peak in seedling stage and rapidly decreased to the lowest level in tillering stage, increasing gradually to a second peak in tillering stage and decreased gradually
The results of structural equation in this study showed that the pH could indirectly affect FAAs concentrations through protease, bacterial biomass and bacterial community structure, which might be due to pH affecting the decomposition of soil soluble organic matter to produce FAAs by affecting soil bacterial enzyme activity (Min et al, 2014)
Summary
More than 90% of total soil nitrogen (N) is organic and, within the organic forms of N, amino acids are the most abundant (Rovira et al, 2008). Free amino acids (FAAs) are usually present in soil solution and pores, which are less abundant but more readily absorbed by plant roots and microorganisms (Gao et al, 2020). FAAs are mainly generated by hydrolysis of soil protein and peptides using extracellular enzymes (Jan et al, 2009), exudation and death from plant roots (Feng et al, 2018), and microbial turnover and excretion (Hill et al, 2019). These amino acids cycle very rapidly in soil, can be selectively absorbed by plants and microorganisms (Harrison et al, 2007), and in addition, migrate and are subsequently lost (Nie et al, 2018). It was reported that FAAs concentration varied greatly when comparing different ecosystems, and the concentrations of amino acids in boreal forest, agricultural soil and alpine meadow soils were 438.00-4867.00 ng N g-1 dry soil (Werdin-Pfisterer et al, 2009), 12.87-48.93 ng N g-1 dry soil
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