Partitioning of nitrate uptake between trees and understory in oak forests

The nitrate reductase activity (NRA) was measured during spring, early summer and late summer in three deciduous forests, where the tree layer was dominated by oak. The nitrate assimilation capacities of the compartments of the above-ground and below-ground biomass of the understory and of the leaves and roots of the tree layer were calculated as NRA per unit biomass and NRA per unit area. The NRA per unit biomass in the above-ground parts of the understory ranged from 0.56 to 13.6 mumol NO2- g(-1) dw h(-1) and was highest in the spring at all sites. The NRA per unit below-ground biomass ranged from 0.12 to 0.94 mumol NO2- g(-1) dw h(-1). For leaves, the NRA per unit biomass varied between 0.27 and 3.51 mumol NO2- g(-1) dw h(-1), and for the roots between 0.14 and 0.91 mumol NO2- g(-1) dw h(-1). Over 90% of the total nitrate uptake (measured as NRA m(-2)) was performed by the understory in the spring, when the nitrification is high, but the unleafed trees showed little activity. Measurements during other periods than spring are scarce in the literature, but results reported here demonstrate the importance of the understory also during the summer, as the NRA per unit biomass of the understory layer was quite high. Fifteen to 50% of the total NRA m(-2) was found in the understory. The capability of the understory and the tree layer to change NRA with nitrate availability and thereby prevent nitrate leaching is further discussed