Can Incorporating Brassica Tissues into Soil Reduce Nitrification Rates and Nitrous Oxide Emissions?

New Zealand agriculture is composed predominantly of pastoral grazing systems; however, forage crops have been increasingly used to supplement the diet of grazing animals. Excreta from grazing animals has been identified as a main contributor of N2O emissions. Some forage crops, such as brassicas (Brassica spp.), contain secondary metabolites that have been identified to inhibit soil N cycling processes, and nitrification in particular. Our objective was to determine if secondary metabolites released from brassica tissues inhibited nitrification and reduced N2O emissions when incorporated into soil, which was amended with a large amount of urea N (such as derived from urine patches deposited during grazing). Three brassica tissues (kale [Brassica oleracea L.], turnip [Brassica rapa L.] bulb, and turnip leaf and stem) and ryegrass (Lolium perenne L.) tissue were incorporated into soil with and without urea solution, and N2O, NO3-, and NH4+ were measured during a 52-d incubation. All brassica tissues reduced urea-derived N2O emissions relative to ryegrass tissues when incorporated into soil. According to the mineral N and microbial community data, this reduction, however, could not be attributed to inhibition of nitrification. Although there was less N2O from urea in the brassica treatments, total N2O emissions increased after incorporation of all tissue residues into soil, so this tradeoff must be explored if brassica tissues are to be considered as a tool for N2O reduction.