The effects of nitrate: ammonium ratios and dicyandiamide on the nitrogen response of Zea mays L. in a high rainfall area on an acid soil

Abstract

Hydroponic studies under controlled environmental conditions indicated that maize plants respond better to combinations of nitrate and ammonium nutrition than to either form supplied separately but that this response depended upon the total N concentration. An attempt was made to maintain different nitrate: ammonium ratios and concentrations in the soil by the addition of a nitrification inhibitor. Five nitrate: ammonium ratios at three N application rates were tested with and without dicyandiamide (H2NC(NH)NHCN) on a low-pH, sandy soil for two years. Treatments were applied to field-grown maize in two applications, one at planting and the other at 21 to 30 days after planting. Under favourable climatic conditions for crop growth the optimum nitrate: ammonium ratio for grain yield was between 3 : 1 and 1 : 1 over all N rates. Under unfavourable climatic conditions, ratios of 3 : 1 and 1 : 1 showed in contrast to all other ratios no grain yield depressions at high N rates. Dicyandiamide did not interact with N rates or ratios, but did increase grain yield over all N treatments under favourable conditions. N ratio interactions with N rates and dicyandiamide were also shown for N concentrations of the leaves at anthesis, for the grain at harvest and for mineral N in different soil layers at anthesis. These results imply that nitrate: ammonium ratios between 3 : 1 and 1 : 1 should be recommended at the optimum N rate on a low-pH sandy soil in a high rainfall area for maize production.

Enzyme activity and C and N pools in soil following application of mulches

“…Results comparing lower concentrations of both forms of available N, N-NH 4 + and N-NO 3 -at T 2 , suggested a greater potential reduction of N-NH 4 + (average value 70%) than N-NO 3 -(average value 50%) from the root zone, confirming the findings of Adriaanse and Human (1991). Water soluble carbon WSC reveals the appearance of C compounds during the mineralization process, increasing for all treatments with respect to the control and from T 1 to T 2 .…”

Application of nitrogen in NO 3 − form increases rhizosphere alkalisation in the subsurface soil layers in an acid soil

“…This high recovery efficiency of nitrate indicates that plants were better able to take up added NO 3 − -N than NH 4 + -N in this low-pH soil. Preferential uptake of NO 3 − by plants at low soil pH has been reported by Blair et al (1970), Vessey et al (1990) and Adriaanse and Human (1991). Uptake of NO 3 − occurs via an H + -cotransport system in the plasma membrane (Mistrick and Ullrich 1996).…”

Root Nitrogen Acquisition and Assimilation

“…However, conversion of NH + 4 to NO − 3 by nitrification compromises some of these benefits. Nitrification inhibitors have been used in agriculture to enhance soil NH + 4 contents (Adriaanse and Human, 1991;Bock, 1987). The availability of NH + 4 within the soil may, however, also be severely limited, because it is tightly held by the micaceous clay minerals of the soil, and readily utilized by micro-organisms effectively removing it from the soil solution until mineralisation occurs (Lewis, 1986).…”