EFFECTS OF WATER-TABLE DEPTH ON NITROGEN ACCUMULATION AND POD YIELD OF PEANUT

Peanut is an important leguminous row crop in the Coastal Plain of the southeastern U.S. Its growth and yield are dependent upon dinitrogen fixation, a biological process that is adversely affected by water def icits or excesses. Drought and excessive rainfall in the same growing season present special problems for crop production in this region. A two-year field experiment was conducted to assess the effects of contr olled drainage/subirrigation on peanut nitrogen accumulation and pod y ield. The experiment was located on a lateral of the main channel in a PL-566 drainage district that was equipped with a water table managem ent structure. Water table depths (WTD) were obtained from natural var iations in the surface elevation; four, nonrandomized blocks contained WTD of 0. 61, 0.76, 0.91, 1.07, and 1.22 m (2, 2.5, 3, 3.5, and 4 ft) . Plots consisted of eight 15 m (49 ft) rows on 0.91 m (3 ft) spacings . Peanuts (Arachis hypogaea L., CV Florigiant) were planted during the second week in May. Shoots and roots were sampled from 0.30 m (1 ft) of row; shoots uere analyzed for total nitrogen, and roots were assaye d for dinitrogen fixation by acetylene reduction. Yields were taken fr om 30 m (98 ft) of row. Rainfall accumulations were 406 and 634 mm (16 and 25 in) in the first and second year, respectively. Pod yields ran ged from 3.5 to 5.5 Mg ba-1 (1.6 and 2.5 t ac-1) and uere > 4.0 Mg ba- 1 (1.8 t ac-1), if WTD were < 0.91 m. Dry matter accumulation acetylen e reduction, and peanut pod yield were negatively correlated with WTD in the first year (R2 values > 0.37), but there uere no meaningful cor relations between plant characteristics and WTD in the second season b ecause sufficient rainfall bad eliminated peanut water stress. However , differences in the partitioning of dry matter between the peanut veg etation and fruit produced differences in estimated net soil nitrogen accumulation. When dinitrogen fixation was assumed to provide 45 perce nt of the total N, estimated net N accumulations were negative and poo rly correlated to depth (R2 of 0.02) in the drier year. In the wetter year, estimates of net soil nitrogen accumulation ranged from 64 to 4 kg ha-1 (57 to 3.6 lb ac-1), and the R2 and slope of the linear regres sion of net N and WTD were 0.40 and -1.05, respectively. Thus, water t able management and controlled drainage/subirrigation may offer a meth od that will reduce excess nitrogen and the associated nitrate polluti on potential as well as stabilize peanut yield.