Aboveground nitrogen in relation to estimated total plant uptake in maize and bean

The main objective of this field study was to estimate the total plant uptake of soil mineral N in maize ( Zea mays L.) and common bean ( Phaseolus vulgaris L.) grown in crop rotations under different N content in Nicaragua. Secondary objectives were to estimate the fraction of the measured soil mineral N content taken up in this way, and to determine how the measured N in plant aboveground parts was related to the total mineral N uptake. A large variation in N content was obtained by using data from fertilisation experiments. Plant total N uptake was estimated as the residual N in a mass balance calculation of soil mineral N. Mineral N content in the top 0-0.3 m soil layer in the field cultivations and in tubes isolated from root uptake, and N content in aboveground plant parts were measured every 30 days. Estimated plant total uptake of soil mineral N varied considerably ( 2.5- 14 g N m(-2) 30 day(-1)) over periods and N treatments. The range of variation was similar for maize and bean. The fraction of the soil mineral N that was taken up by the plant daily varied more in maize ( about 0.03 - 0.12 day(-1)) than in bean ( about 0.05 - 0.08 day(-1)). Our results suggest that monthly changes in N in aboveground plant parts were linearly related to plant total N uptake during the same period. Aboveground plant N constituted between about 55% and 80% of total uptake of soil mineral N in maize depending on period within season, whereas for bean it was more constant and smaller ( about 40%).