Solute transport in a hummocky landscape: II. Vertical and seasonal redistribution of bromide and 15N-labelled nitrate

Abstract

Bromide (Br-1) tracer is often used to estimate potential nitrate-nitrogen (NO3-N) leaching in soil. An alternative is to use 15N-labelled fertilizer to provide a direct measure of NO3-N leaching losses. We employed a dual application of Br-1 and 15N to estimate NO3-N leaching in a hummocky landscape. The study was carried out near Brandon, Manitoba, in 2007 and 2008, using two separate plots (Site-2007 and Site-2008). The plots were delineated into three landscape positions as upper (UPP), middle (MID) and lower (LOW) slope. A microplot at each landscape received KBr at the rate of 200 kg Br-1 ha-1, and 15N-labelled KNO3 fertilizer at the rates of 0, 90 and 135 kg N ha-1. Site-2007 and Site-2008 were seeded to canola and winter wheat, respectively. Soil samples were taken in the fall and spring. The soil samples for the 90 and 135 kg N ha-1 fertilization treatments were analyzed for vertical distribution of Br-, 15N, and NO3-N. The smallest amounts of Br-1, nitrogen derived from 15N-labelled fertilizer (NDFF), and NO3-N were measured in the soil profile (0 to 120 cm) at the LOW slope position for both N fertilizer rates. The greatest amounts were at the MID slope, indicating that the downward movement of solute followed the order of: LOW>UPP>MID (PB<0.05). Crop uptake of fertilizer N was 35 and 63% in canola and winter wheat, respectively, while Br-1 uptake was negligible. In the absence of crop uptake, Br-1 transport was similar to that of NDFF as 38% of each solute was lost between fall and spring in Site-2007 and 33% in Site-2008. Unlike Br-1 and NDFF, NO3-N distribution remained unchanged between fall and spring measurements, possibly due to its replenishment from native soil N sources. The dual tracer technique, as used in this study, showed the limitations of using soil profile NO-3N alone as an indicator of nitrate leaching and for making treatment comparisons.