A 15N-Labeling Study of the Capture of Deep Soil Nitrate from Different Plant Systems

The objective of this study was to determine the efficiency of different plant systems in capturing deep soil nitrate(NO3-) to reduce NO3- leaching in a field plot experiment using15N labelling.The study was conducted on a calcareous alluvial soil on the North China Plains and the plant systems evaluated included alfalfa(Medicago sativa),American black poplar(Populus nigra) and cocksfoot(Dactylis).15N-labelled N fertilizer was injected to 90 cm depth to determine the recovery of 15N by the plants.With conventional water and nutrient management,the total recovery of 15N-labeled NO3- -N was 23.4% by alfalfa after two consecutive growth years.The recovery was significantly higher than those by American black poplar(12.3%) and cocksfoot(11.4%).The highest proportion of soil residual 15N from the labeled fertilizer N(%Ndff) was detected around 90 cm soil depth at the time of the 1st year harvest and at 110-130 cm soil depth at time of the 2nd year harvest.Soil %Ndff in 0-80 cm depth was significantly higher in the alfalfa treatment than those in all the other treatments.The soil %Ndff below 100 cm depth was much lower in the alfalfa than those in all the other treatments.These results indicated that 15N leaching losses in the alfalfa treatment were significantly lower than by those in the black poplar and cocksfoot treatments,due to the higher root density located in nitrate labeling zone of soil profile.In conclusion,alfalfa may be used as a plant to capture deep soil NO3- left from previous crops to reduce NO3- leaching in high intensity crop cultivation systems of North China Plain.