Nitrogen-rich compounds constitute an increasing proportion of organic matter with depth in O_i-O_e-O_a-A horizons of temperate forests

Soil organic matter (SOM) in O horizons of temperate forests forms a natural gradient of increasing decomposition, as operationally separated into the Oi (leaf litter), Oe (partially decomposed), and Oa (highly decomposed) sub-horizons. This study determines the contribution of extractable nitrogen (N)-rich biomolecules and characterizes changes in SOM composition within O horizon decomposition gradients and in underlying A horizons of three temperate forest stands. Four biomolecules were chosen for this investigation: chlorophyll, protein, DNA, and chitin; combined, they represent the majority of identifiable N-rich organic compound classes present in soils. Selective chemical extractions and purification methods provide estimates of concentrations of each distinct biomolecule class, while short-term respiration measurements provide insight into possible relationships with microbial activity. Changes in the molecular character of SOM are probed using Fourier Transform Infrared (FTIR) and Carbon K-edge X-ray Absorption Near Edge Structure (C-XANES) spectroscopies. Differences in biomolecule concentrations and respiration by horizon reveal two contrasting trends with increasing decomposition and depth: chlorophyll and respiration decrease, whereas protein, DNA and chitin increase. Increasing contribution of these three extractable N-rich biomolecules with progressive decomposition and depth at all sites correspond to decreasing concentrations of C and N and to increases in C:N ratio. These trends are consistent with FTIR spectral results, which indicate a relative increase of N–H bending and aromatic C[dbnd]N vibrations, and with C-XANES spectral results, which show a relative increase of aromatic C[dbnd]N transitions. These multiple lines of evidence demonstrate that extractable N-rich biomolecules constitute a growing proportion of the organic C and N content within an organic matter decomposition gradient and that this trend continues into the underlying mineral soil. We posit that the persistence of these intact N-containing biomolecules represents a growing contribution from the microbial biomass pool, but also from non-specific interactions among biomolecules in the Oe and Oa horizons and from biomolecule-mineral interactions in mineral soils. The results of this investigation (increases in the proportion of extractable N-rich biomolecules) also provide valuable insight into the potential bioavailability and biochemical structure of SOM in relation to degree of decomposition.