Abstract
The cycling of especially large size organic nitrogen (N) from plants into stable microbial derived soil organic carbon (C) and N pools is understudied, in spite of organic N composing 90% of soil N and the intimate link between organic N and soil C stabilization. We investigated the fate of peptide-size and protein-size organic N fractions in soils from two long-term field experiments markedly differing in conditions for microorganisms. We combined amino acid stable isotope probing (AA-SIP) fingerprinting with PLFA-SIP to trace organic N into the soil microbial biomass. Contrary to the present paradigm, we found for both soils that greater molecular size did not protect against decomposition of these compounds neither did protection via strong sorption to the soil mineral phase. Instead, we found strong evidence that gram-positive bacteria are the key actors in the decomposition of protein-sized nitrogen compounds and that amino acids bound in large organic nitrogen compounds directly contribute to the build-up of bacterial tissue. We conclude that when large organic nitrogen compounds are dissolved, turnover occurs rapidly, irrespective of molecular size, and the bacterial incorporation of these rapid cycling compounds makes an important contribution to soil organic matter formation.
Highlights
The cycling of especially large size organic nitrogen (N) from plants into stable microbial derived soil organic carbon (C) and N pools is understudied, in spite of organic N composing 90% of soil N and the intimate link between organic N and soil C stabilization
For the 1–10 kDa and > 100 kDa fractions, we found a negative correlation between respiration and sorption of total organic N showing that the larger the organic N fraction the stronger it is sorped to the soil (Fig. 1), supporting that microbial decomposition is controlled by the accessibility of organic N 23,24
We determined the degradation of the added organic N compounds by analyzing the isotopic values of soil-bound amino acids from the two size fractions (Fig. 2a–f)
Summary
The cycling of especially large size organic nitrogen (N) from plants into stable microbial derived soil organic carbon (C) and N pools is understudied, in spite of organic N composing 90% of soil N and the intimate link between organic N and soil C stabilization. The objective of this study was to determine the mechanisms controlling large molecular weight (Mw) organic N cycling in soil, we added triple-labeled (14C, 13C, 15N) white clover sap to study the short-term fate of non-structural organic N compounds in two molecular size classes above the 0.6 kDa threshold (peptide size class, 1–10 kDa, and protein size class, > 100 kDa) for direct microbial assimilation in topsoils from two renowned long-term field experiments (LTE) in D enmark[19]. The two LTE sites have contrasting management strategies; the Jyndevad LTE has manipulated liming and phosphorus fertilization since 1 94220 and the Askov LTE have treatments with animal manure and mineral fertilizer since 1 89421 The utilization of these unique LTE aims to exemplify the legacy effect of these management practices on the fate of organic N, but to further facilitate a broadened contextual understanding of the mechanisms controlling the fate of organic N under different environmental conditions. We determined organic N sorption and degradation versus retention of two size-fractions of organic N compounds with AA-SIP fingerprinting[19], and identified the active microbial groups degrading the organic N compounds with PLFA-SIP22
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