Relative contribution of mass flow and diffusion to nitrate transport towards roots

Experimental data from a field experiment with oats and faba beans were used to quantify the contribution of mass flow to nitrate transport towards roots. Two different approaches were used. The first was the calculation of apparent mass flow from average nitrate concentration and water uptake rate. The second was the computation of the increase of nitrate transport rate caused by mass flow. This was done using a mathematical model for nutrient transport. A comparison was made for a reference situation of transport by diffusion only. Calculation of apparent mass flow resulted in a nitrate transport to faba bean roots of 88 and 42 kg N . ha(-1) in 1982 and 1983, respectively. This accounted for 27 and 16% of total N accumulation of the faba bean shoots. For oats a transport of 38 and 23 kg N . ha(-1) by mass flow was calculated. These values correspond to 30 and 16% of total N uptake of oats in both years. The model calculations, however,showed that apparent mass flow overestimated the contribution of mass flow drastically when the nitrate concentration at the root surface is much lower than the average nitrate concentration of the soil solution as was found for faba beans in the soil layer 40-60 cm depth. In that case apparent mass flow overestimated the real increase of nitrate transport to roots by mass flow by a factor of 2.7. Under the given conditions diffusion was the more important process involved in the nitrate transport towards roots. The increase in maximum nitrate transport rate towards roots is much lower than apparent mass flow.