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

Different theories have been brought forward to explain the commonly observed delta N-15 enrichment with depth in soil profiles, including the discrimination against N-15 during N decomposition and the buildup of N-15-enriched microbial residues. A combination of soil organic matter (SOM) size and density fractionations, N-15 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 N-15-enrichment pattern and to link N-15 SOM enrichment with microbial community composition. Nitrogen-15 enrichments were greater for the microaggregate (< 150 mu m) than for the macroaggregate (> 150 mu m) size fraction, with Rayleigh isotope enrichment factors averaging -8.5% and -3.7%, respectively. The macro-organic matter density fractions (> 150 mu m) showed intermediate enrichment factors of -5.1% and -7.3% for the light (< 1.37 g cm-(3)) and heavy (> 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 N-15 enrichment of SOM fractions to total microbial abundance and, as such, corroborates existing theories of microbial-induced N-15 enrichment. Isotopic fractionation during microbial N decomposition processes alone could not explain the large N-15 enrichment in the microaggregate size fraction (-8.5%) and the heavy density fraction (-7.3%). We therefore suggest that microbial turnover and accretion of N-15-enriched microbial (especially fungal) compounds was an additional driver for N-15 enrichment of this soil profile.