TRACING NITROGEN MOVEMENT IN CORN PRODUCTION SYSTEMS IN THE NORTH-CAROLINA PIEDMONT - ANALYSIS OF NITROGEN POOL SIZE

The fate of N in North Carolina Piedmont corn (Zea mays L.) was traced using N pool size analysis. In 1989 and 1990, selected N pools [corn; weeds; surface residues; and soil NH4+, NO3-, and potentially mineral izable N (N(o))] were sequentially sampled in four reduced chemical in put systems: (i) crimson clover (Trifolium incarnatum L.) with convent ional tillage (clover-till), (ii) no cover crop with conventional till age and 70 kg N ha-1 as NH4NO3 (fertilizer-till), (iii) crimson clover with strip-tillage (clover-strip), and (iv) no cover crop with no til lage and 70 kg N ha-1 as NH4NO3 (fertilizer-no-till). The largest of t he repeatedly measured N pools was soil N(o), which accounted for <10% of the total Kjeldahl soil N. Soil N(o) was highest in the clover-til l treatment, and had large seasonal fluctuations: 170 to 255 kg N ha-1 in clover-till, and 117 to 210 kg N ha-1 in other treatments. Fertili zer treatments had higher inorganic N levels than did clover treatment s only immediately after fertilizer application. Although clover shoot s contained 97 to 134 kg N ha-1 yr-1, more than the N applied to ferti lizer treatments, no significant differences among treatments in plant (corn + weed) aboveground N accumulation (38 to 132 kg N ha-1) were o bserved. Of these totals, weed N accounted for 5 to 30 kg N ha-1. Surf ace residue N content was highest in reduced tillage treatments. In re duced chemical input systems such as these, N transfers among soil N(o ), weed, and residue pools may be as large or larger than N transfers among soil inorganic and crop pools.