Soil mineral nitrogen availability to young maize plants as related to root length density distribution and fertilizer application method

Abstract

Minirhizotron observations from 4 experiments in 1992 and 1993 with maize on a sandy soil in the Wageningen Rhizolab, Netherlands, showed strong vertical and lateral root density gradients during the first 9 weeks after emergence. Root length density (Lrv), as determined in core samples 9 weeks after emergence, was positively related (P <0.01) to the number of roots counted concurrently on minirhizotron walls (n). Lrv:n ratios were 1.13, 1.76, 0.99 and 1.21 cm/cm in the successive experiments. Subsequently, root numbers counted on previous dates in each experiment, were converted into root length density values and related to thermal time. According to this relation, the average vertical root extension rates were 0.7 and 1.1 cm/d at temperatures of 13 and 16 degrees C, respectively. Corresponding values for the lateral extension rate were 1.0 and 1.6 cm/d. Calculations showed that the nitrogen (N) content of a 9 weeks old maize crop could not be explained by mass flow only. Transport distances between roots and mineral N in the soil, may have restricted the availability of N as suggested by preferential uptake of mineral N from soil compartments with a high root length density. The recovery of N was only slightly improved by fertilizer N positioning close to the plant as compared to broadcast N or placement of N halfway between the rows. Recoveries based on the difference method and the isotopic dilution method, yielded similar values. Dry matter yields were not significantly affected by the application method of N. Apparently, the root extension rate and the initial availability of N in the soil prior to the application of fertilizer-N, were sufficient to cover shoot demand under the prevailing circumstances.