Relationships between total-N, biomass-N and active-N in soil under different tillage and N fertilizer treatments

Studies assessing the effects of tillage and N fertilizer treatments on relationships between different N pools in soil can provide information concerning the influence of such treatments on the distribution of N in agricultural soils. To assess the effects of treatments on relationships between total-N, biomass-N and active-N in soil, these N pools were measured in samples of soil collected at different depths (0–2.5, 2.5–7.5 and 7.5–15 cm) from long-term field experiments located in two geographic regions (piedmont and coastal plain) and containing plots under annual treatments of plow-or no-tillage and 0 or 135 kg ha−1 of fertilizer N. Results showed that whereas geographic location, N fertilization and depth generally had little influence on the slopes of regression lines describing the linear relationships between total-N, biomass-N and active-N, tillage had a marked influence on these slopes. Such results indicated that tillage may substantially influence the distribution of N among different pools within agricultural soils. The overall results for linear regressions indicated that the active-N pool, as measured by an isotope-dilution method, was very closely related (r = 0.96) to the biomass-N pool, as measured by a fumigation-incubation method, but that measurements for the active-N pool were approximately twice those for the biomass-N pool. These observations suggested that such methods may measure a common pool of soil N, but place substantially different bounds on the size of this N pool.