Do nitrogen isotope patterns reflect microbial colonization of soil organic matter fractions?

Abstract

Different theories have been brought forward to explain the commonly observed δ15N enrichment with depth in soil profiles, including the discrimination against 15N during N decomposition and the buildup of 15N-enriched microbial residues. A combination of soil organic matter (SOM) size and density fractionations, 15N determinations, and phospholipid fatty acid (PLFA) analyses was conducted on soils from a pristine N-limited Nothofagus forest in southern Chile. The purpose of this study was to investigate which SOM fractions mostly reflect the 15N-enrichment pattern and to link 15N SOM enrichment with microbial community composition. Nitrogen-15 enrichments were greater for the microaggregate ( 150 μm) size fraction, with Rayleigh isotope enrichment factors averaging −8.5‰ and −3.7‰, respectively. The macro-organic matter density fractions (>150 μm) showed intermediate enrichment factors of −5.1‰ and −7.3‰ for the light ( 1.37 g cm−3) fraction, respectively. The abundance of fungal and bacterial PLFAs was significantly higher in the microaggregate compared to the macroaggregate size fraction, but their relative abundance did not change between aggregate size fractions. Our data link differential 15N enrichment of SOM fractions to “total” microbial abundance and, as such, corroborates existing theories of microbial-induced 15N enrichment. Isotopic fractionation during microbial N decomposition processes alone could not explain the large 15N enrichment in the microaggregate size fraction (−8.5‰) and the heavy density fraction (−7.3‰). We therefore suggest that microbial turnover and accretion of 15N-enriched microbial (especially fungal) compounds was an additional driver for 15N enrichment of this soil profile.