Strategies for achieving self sufficiency in nitrogen on a mixed farm in eastern Canada based on use of the faba bean

Abstract

To what extent can a mixed farm be self sufficient in nitrogen by growing nitrogen-fixing legumes and recycling animal manures? An opportunity to examine this question was provided by an upland farm in the Annapolis Valley of Nova Scotia where faba beans (Vicia faba L. minor) were grown on 1/3rd of the cultivated land, grains from cereals and faba bean were fed to laying hens and the manure was applied to cereals. The farmer had ceased to apply commercial fertilizers and herbicides after 1975, thinking that inputs of N (nitrogen) via legume nitrogen fixation should be sufficient, and that weeds could be adequately controlled by mechanical means. His cereal yields fell abruptly, apparently because of insufficient N. Was the apparent N deficiency due to low nitrogen fixation in faba beans and/or to high losses via leaching, export of N from the farm in eggs, ammonia volatilization, denitrification and/or to excessive weeds? To address this question, in 1978-79 we examined patterns of faba bean root+nodule nitrogenase activity, leaf nitrate reductase activity and N accumulation and removals in faba bean on this upland farm and on a nearby dykeland farm. The total N input via N2 fixation was estimated by applying a reported ratio of acetylene reduction to N2 fixation, and by comparison of N accumulation in faba bean with that in non-legume plots. Observations on crop and weed biomass and N content, manure N, soil nitrate and ammonia volatilized after application of manure to microplots were conducted on the upland farm; denitrification in soil cores on the dykeland farm was estimated using the acetylene blockage technique. The composition, seed banks and biomass of weeds on these farms and two other farms and their response to added fertilizer-N were also examined. The observations indicate that atmospheric N2 and soil-N were the major sources of N for faba beans during vegetative and reproductive growth successively. For both farms, N removed in faba bean seeds exceeded N fixation, indicating a negative soil N balance for the faba beans. However, for the upland farm (where the a large portion of the seed-N is recycled via hen manure), a nitrogen budget indicated a large excess of inputs of N via nitrogen fixation over losses of N and suggested that the excess N was accumulating in the soil organic matter (SOM). In essence, SOM (approx. 5% N by weight) was increasing at the expense of increased crop production. It could be expected that as SOM accumulates over time, the amount of N available to crops via soil N mineralization will increase. Rough calculations indicated that once a steady state is reached, the N mineralized from SOM would be sufficient to support yields equivalent to or better than those attained when commercial fertilizers were used. but also that it would take of the order of 100 years for yields to approach desired levels. Thus a strategy for increasing yields of "wait and let the organic matter accumulate" is not practical. It was concluded that a faster increase in yields could be achieved by "restructuring" the farm system so that more of the N circulates through the crops and less through weeds and soil organic matter. To this end, a regular crop rotation was implemented in 1980: faba beans - oats - clover - winter wheat. The clover is seeded with oats in year 2, and is turned under in year 3 prior to planting winter wheat. Manure is applied to the oats and wheat. Maintenance of nearly continuous plant cover is a key consideration. Thus it is desired to control annual weeds during the crop season, but not to eliminate them so that they provide a self-seeding cover when crops are not present.