Monitoring Nitrates in Ground Water of an Apple Orchard

Nitrate (NO3–) pollution is of high interest in modern agriculture. Although the amount of nitrogen (N) usually supplied to an apple orchard is lower than other crops, it could become a factor of pollution because of the lower tree root density and of the low nitrogen use efficiency. In this study we monitored the concentration of nitrate in soil water solution in a 3-year old apple orchard located near Grand Rapids (Mich.) during 1996 and 1997. The orchard had no history of fertilization, but alfalfa crop was previously planted for several years. Eight lysimeters, 1.8 m tall and 1.8 m of diameter, were installed in 1995 in the orchard site without changing the soil profile. One tree was planted in each lysimeter in spring 1996. Leached soil solution was collected bi-weekly in a connected 143-L storage container. Additionally, three soil suction tubes (SST) were placed inside and outside the lysimeters at a depth of 0.6, 1.05 and 1.55 m, in order to collect water solution to evaluate the level of nitrate at different soil layers. Daily water (L/ha) and nitrate (kg/ha) flow rate were linearly correlated with an R2 of 0.9. Nitrate flow rates were highest and during the wet spring and winter periods. The variation of NO –3 flow rate during the year is a result of root and bacteria activity. All year long, the nitrate concentration in the SST inside the lysimeters was higher than those outside. These results show that the application rate for N could be different from orchard to orchard and it must be necessary to know the soil N background, cropping history and N-NO –3 availability, in order to maximize productivity and avoid environmental risks.