Ammonia transport in a temperate grassland .1. Seasonal transport in relation to soil fertility and crop management

An understanding of N cycling in agricultural systems is necessary to optimize N-use efficiency and reduce N losses to the environment. The objectives of this research were to evaluate N cycling in a highly fertilized grassland in a humid temperate climate and to observe the effects of N surplus and deficit on NH3 absorption-desorption by the grass. Soil, plant, and weather measurements were taken concurrently and soil-plant-atmosphere N transport were determined. After Ei application, soil inorganic N decreased rapidly to background levels, due to possible microbial immobilization and plant N demand. Much of the immobilized N was remobilized during the growth period, but at insufficient rates to avoid Iri stress by the crop, as shown by absorption of NH3. During spring, 452 of the plant N was derived from applied fertilizer, with the balance obtained from mineralized organic N (49%) and absorption of NH3 (6%). During summer, fertilizer N accounted for 60% of accumulated grass N and NH3 absorption accounted for 11% of N not derived from fertilizer (4% of the total N), with the balance coming from mineralized organic N. Soil water content and wind speed were the dominant factors influencing the crop Mt compensation point (CP). The daily NH3 CP was variable, but the average seasonal NH3 CP was about 14 mu g m(-3).