Carbon and nitrogen dynamics in preferential flow paths and matrix of a forest soil

Natural abundance (&) of the stable isotopes C-13 and N-15 has gained acceptance for studying C and N cycling in forests. In most studies, bulk soil samples are collected to determine isotope abundance. Such sampling overlooks the potential impact of preferential flow on isotope distribution. The objective of this study was to investigate the effects of preferential flow on the distribution of soil organic carbon (SOC), total N, delta C-13, and delta N-15 in a forest soil in Central Switzerland. Preferential flow paths in the soil were identified with a dye tracer, Brilliant Blue (Pluss-Staufer AG, Oftringen, Switzerland), that was homogeneously applied to the soil surface. In the stained preferential flow paths, concentrations of SOC and total N were 15 to 75% higher than in the soil matrix. The total increase of SOC in preferential flow paths ranged from 740 to 960 g C m(-2) in four individual soil plots. Values of delta C-13 and WIN were lowest in tree leaves and in the forest floor, and increased with soil depth, thus with the degree of decomposition of SOC. In the mineral soil, preferential flow paths were significantly depleted in C-13 by 0.15 to 0.4 parts per thousand as compared with the soil matrix. The delta N-15 values increased with soil depth from 0.9 to 4.7 parts per thousand in the preferential flow paths and from 0.5 to 6 parts per thousand in the soil matrix. Adding a highly enriched N-15-tracer homogeneously to the soil surface showed a higher recovery of N-15 in the soil and in the fine roots sampled from preferential flow paths than in those sampled from the soil matrix. Our results suggest that in preferential flow paths, SOC is younger and N cycling is more rapid than in the soil matrix.