No abstract

There has been considerable recent interest in the potential application of nitrogen isotope analysis in discriminating between organically and conventionally grown crops. A prerequisite of this approach is that there is a difference in the nitrogen isotope compositions of the fertilizers used in organic and conventional agriculture. We report new measurements of delta15N values for synthetic nitrogen fertilizers and present a compilation of the new data with existing literature nitrogen isotope data. Nitrogen isotope values for fertilizers that may be permitted in organic cultivation systems are also reported (manures, composts, bloodmeal, bonemeal, hoof and horn, fishmeal and seaweed based fertilizers). The delta15N values of the synthetic fertilizers in the compiled dataset fall within a narrow range close to 0 per thousand with 80% of samples lying between-2 and 2 per thousand and 98.5% of the data having delta15N values of less than 4 per thousand (mean=0.2 per thousand n=153). The fertilizers that may be permitted in organic systems have a higher mean delta15N value of 8.5 per thousand and exhibit a broader range in delta15N values from 0.6 to 36.7 per thousand (n=83). The possible application of the nitrogen isotope approach in discriminating between organically and conventionally grown crops is discussed in light of the fertilizer data presented here and with regard to other factors that are also important in determining crop nitrogen isotope values.

The role of atmospheric deposition of iron, nitrogen and phosphorus in supplying nutrients to marine systems has been described, individually, in previous works. Here we examine atmospheric dry deposition of all these nutrients simultaneously, using samples collected during two meridional transects of the Atlantic Ocean. We find that, in line with previous work, desert dust supplies excess iron to the water column. However, primary production promoted by aerosol nitrogen can be sufficient to consume all of the soluble aerosol iron input in some situations. Aerosol N:P is universally very high, so that aerosol is always deficient in P relative to phytoplankton requirements. Nitrogen fixation stimulated by any excess atmospheric iron supply and phytoplankton utilisation of atmospheric nutrient inputs will therefore tend to drive the ecosystem towards P limitation. This emphasises the need to study the biogeochemical impact of atmospheric nutrient deposition in an integrated manner.