Nitrate assimilation of raspberry and pin cherry in a recent clearcut

Abstract

An experimental study conducted in a recently clear-cut hardwood forest was used to determine the reduction of nitrate in the leaves of Rubus idaeus L. and Prunus pensylvanica L.F., early successional species and shrub competitors of forest tree regeneration. A factorial experimental design was chosen and the main factors were the shrub competitors and the replicates (blocks). The nitrate reductase enzyme assay was chosen as a physiological marker since it is substrate induced and it is the first key enzyme implicated in the assimilation of nitrate. Results show that for 2 consecutive years, pin cherry had extremely low enzymatic activity in their leaves (0.80 and 0.70 μmol NO2/(g dry wt.∙h)), whereas raspberry always had high values (2.52 and 2.32 μmol NO2/(g dry wt.∙h)). The induction of the enzyme with 800 kg/ha N–P–K fertilizer showed that only raspberry responded to the treatment. These findings could explain the differing peaks of dominance of these early succession species, especially in relation with the changing nitrification rate found in disturbed hardwood forest ecosystems. Pin cherry, a successional shrub reaching maximum abundance following raspberry, clearly does not acquire the large quantities of nitrogen that it does by the uptake of the nitrate form. Nitrogen assimilation of the NH4+ form predominantly would allow pin cherry to short-circuit nitrification by reducing the substrate for nitrification, thus rendering soil conditions less favourable for raspberry and leading to its successional replacement by pin cherry. Key words: disturbed forest ecosystem, nitrate reductase activity, Prunus pensylvanica L.F., Rubus idaeus L.