Nitrogen cycling in silvopastoral systems in the Pacific Northwest: A review

Clearcuts in interior British Columbia are often aerially seeded with domes tic forages such as orchardgrass and alsike clover to prevent erosion and h elp control understorey vegetation during establishment of lodgepole pine p lantations. These seeded clearcuts provide valuable mid-summer range for ca ttle and with careful management there is little damage to planted trees. T here is growing concern about N losses due to clearcutting and site prepara tion and what effect these might have on long-term forest productivity. The literature indicates that without cattle or forage seeding natural N input s from the atmosphere and N fixation are barely sufficient to provide adequ ate nutrition for a lodgepole pine stand. This literature review addresses what effect introducing domestic forages and cattle grazing can have on N c ycling on clearcuts. Forage seeding alone may reduce N losses due to runoff and leaching and legumes can fix atmospheric N, but grazing by cattle can counteract some of these benefits. Estimates of N losses due to cattle graz ing were calculated using summary data from a long-term grazing trial condu cted in interior British Columbia where experimental pastures were either a erially seeded with clover-orchardgrass mixtures or left unseeded. Using fo rage yields, cattle gains, and a number of assumptions, N losses due to cat tle grazing on the seeded pastures were calculated as 2.4 kg N ha(-1) yr(-1 ) compared with 0.8 kg N ha(-1) yr(-1) on native vegetation. Losses were fr om N in new cattle tissue and N volatilized from urine as ammonia. Increase d losses due to grazing seeded forages were short-lived as the forage produ ction of seeded pastures was equal to that of native vegetation by the seve nth year. During this period, the losses due to grazing seeded forages may be balanced by increased capture of labile N by the forages.