Contrasting responses of gross N transformation to oxalic acid and glucose supplement in paddy soil

Abstract

Labile organic carbon (C) substrates could accelerate microbial transformation of soil N pool by stimulating the decomposition of large molecule organic N. However, it remains unclear how gross N transformation processes (protein depolymerization, amino acid uptake, microbial N mineralization and NH₄⁺-N uptake rates) in response to individual C substrates. Typical paddy soil was incubated with the supplement of oxalic acid or glucose under simulated field water conditions for 16 days to assess the gross N transformation rates by ¹⁵N pool dilution assays. A mixture of ¹⁵N labeled amino acid was applied to gross protein depolymerization and amino acid uptake rates measurement, and ¹⁵N-(NH₄)₂SO₄ was used to gross microbial N mineralization and NH₄⁺-N uptake rates analyses. Oxalic acid supplement promoted the gross protein depolymerization, gross microbial uptake of amino acid, and gross N mineralization rates at the early stage. It was attributed that oxalic acid supplement urged microbes to decompose large molecular organic N to acquire amino acid derived C and excluded the superfluous N via mineralization as evidenced by the increase of NH₄⁺-N. By contrast, glucose supplement diminished the gross N transformation processes, since microbes prefer to utilize the native NH₄⁺-N to meet their N demand supported by the decreasing NH₄⁺-N concentration in soil, and consequently inhibited the decomposition for the large molecule organic N. With the increase of microbial growth, especially for bacteria, glucose amendment stimulated the large molecular organic N depolymerization to acquire amino acid to maintain the microbial C/N stoichiometric balance. Compared to glucose treatment, oxalic acid supplement stimulated more N allocation into microbial growth but not for mineralization, and thus led to higher microbial N use efficiency, which was adverse for available inorganic N supply for rice growth in paddy ecosystem. Overall, this study emphasizes that low molecular organic C substrates of organic acid and glucose exerted contrasting influences on gross N transformation, and help to improve our understanding of the mechanism of the coupling biotransformation of C and N in paddy soil.

Keywords: (15)N pool dilution; Coupling C and N biotransformation; Gross N transformation; Labile C substrate; Microbial N use efficiency; Paddy soil.