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

Abstract

Clearcuts in interior British Columbia are often aerially seeded with domestic forages such as orchardgrass and alsike clover to prevent erosion and help control understorey vegetation during establishment of lodgepole pine plantations. These seeded clearcuts provide valuable mid-summer range for cattle and with careful management there is little damage to planted trees. There is growing concern about N losses due to clearcutting and site preparation and what effect these might have on long-term forest productivity. The literature indicates that without cattle or forage seeding natural N inputs from the atmosphere and N fixation are barely sufficient to provide adequate nutrition for a lodgepole pine stand. This literature review addresses what effect introducing domestic forages and cattle grazing can have on N cycling 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 grazing were calculated using summary data from a long-term grazing trial conducted in interior British Columbia where experimental pastures were either aerially seeded with clover-orchardgrass mixtures or left unseeded. Using forage yields, cattle gains, and a number of assumptions, N losses due to cattle 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 from N in new cattle tissue and N volatilized from urine as ammonia. Increased losses due to grazing seeded forages were short-lived as the forage production of seeded pastures was equal to that of native vegetation by the seventh year. During this period, the losses due to grazing seeded forages may be balanced by increased capture of labile N by the forages. Key words: N cycling, seeded clearcuts, cattle grazing