Nitrogen distribution and 15N natural abundances in particle size fractions of a long‐term agricultural field experiment

The present study combined a physical fractionation procedure with the determination of the natural abundance of ¹⁵N to investigate the impact of organic manure and mineral fertilizer application, and fallow on changes of N associated with different soil particle size fractions. The long‐term field experiment was conducted since 1956 in Ultuna, Sweden, on an Eutric Cambisol. Nitrogen in bulk soil and in particle size fractions changed significantly since 1956. The N_t concentrations in bulk soil decreased in all treatments not receiving organic materials. Comparing the N contribution of particle‐size fractions to the total N amount revealed the following ranking: silt > clay > fine clay > fine sand > coarse sand. The relative contribution of N in silt sized particles significantly increased from low to high bulk soil N contents, whereas N in clay and fine clay fractions decreased. The C : N ratios of particle size fractions differed considerably more between treatments than C : N ratios in bulk soils. Generally, the C : N ratios decreased from coarse to fine fractions emphasizing the tendency of smaller fractions being more significant as N sink than as C_org sink. ¹⁵N abundances varied more between particle size fractions of single treatments than between bulk soil from differently treated plots. Within treatments we observed differences of up to 7.1 ‰ between particle size fractions. In most cases δ ¹⁵N values increased with decreasing particle sizes. This pattern on average was similar to changes in δ ¹³ C. Our results suggest that silt sized particles acted as medium‐term sink of introduced N and that ¹⁵N abundances in particle size fractions sensitively reflect changes in N status in response to soil management.