The role of N-depleted fertilizers as tracers in N cycling studies in agroecosystems

A search of the literature (principally Google Scholar, using the keywords given below) revealed that 120 research papers have been published where N-depleted (N-enriched) fertilizers, including ammonium sulfate, ammonium nitrate, urea and ammonia forms were used as tracers. The studies included annual cereals, vegetables, a grain legume and a fibre crop, and perennial crops including grasses, shrubs (berries), trees (fruits and nuts) and N-fixing forage, grain and woody legumes and an actinorhizal species. Other minor examples include perennial ornamentals and trees harvested for wood. Applications included estimation of fertilizer recovery in crops, in post-harvest soil and estimation of N losses in the soil–plant system by mass balance. The residual value of N-depleted fertilizers and the recovery of N-depleted legume residues have also been reported. N depleted fertilizers have played an important role in differentiating the relative uptake of fertilizer N to N mobilized in storage tissues with respect to the N nutrition of deciduous horticultural trees, shrubs and vines in spring. The symbiotic dependence of N-fixing species and transfer to companion non-legumes has been indirectly determined using N-depleted fertilizers. Thus N-depleted fertilizers have shared the same applications as their N-enriched counterparts. Because of their relative cheapness compared with N-enriched fertilizers, larger and more representative unconfined plots can be employed in field studies.