Nitrogen use efficiency is a key factor for the economic and environmental sustainability of farms. It comprises the ability for crops to recover applied fertilizer N in their aboveground biomass, called fertilizerN uptake efficiency (NupE). Sweet pearl millet [Pennisetum glaucum (L.) R.BR.] and sweet sorghum [Sorghum bicolor (L.) Moench] are C4 annual crops known for their capacity to produce high yield under N-limiting conditions, suggesting high NupE. A field study was conducted for 2 years on sandy loam soils in eastern Canada. The NupE was determined using a 15N-tracing approach. Comparisons were made for both species in regards to their response to (1) increasing mineral N rate (0–160 kg N ha−1), (2) mineral versus organic N (liquid swine and dairy cattle manures), and (3) single versus split N application. For mineral N treatments, NupE ranged from 54 to 82 %, which is greater than values generally reported for conventional crops such as corn. Moreover, NupE increased with N rates. These findings suggest than both species expressed N luxury consumption. The NupE was lower with organic than with mineral N, and a larger proportion of N remained in the soil, suggesting that N immobilization occurred. The presumed losses (unrecovered 15N) were <24 kg N ha−1 in all mineral and most organic N treatments. Splitting N application had little effect on NupE. Our results confirm that sweet pearl millet and sweet sorghum are highly efficient at recovering fertilizer N and, with adequate residue management, may represent a low environmental risk.
