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D.G. Patriquin, D. Burton & N.Hill,
Biology Department, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada.
Several farmers in Nova Scotia who are attempting to be self sufficient in feed production for cattle or poultry grow the faba bean as an alternative to importing soybean meal. Although they have noted that the faba bean benefits grain crops following it in rotations, they are uncertain of how this should be taken into account in fertilizer applications. An egg producer who grows faba beans on one third (10 ha) of his cultivated land has not utilized any N fertilizer since 1975. In 1975 he achieved the highest oat yields in the province; the year following~his grain yields (but not his bean yields) declined by about 50%, end currently he purchases about 25% of his poultry feed (egg output is about 100 dozen per day). We examined various facets of N cycling on this farm in an effort to determine what were the limiting factors. Construction of a N budget was simplified by the absence of a large pool of inorganic N associated with fertilizer N.
Nitrogen fixation in faba beans was estimated from acetylene reducing activity of excised nodules, measured weekly, by using a molar Ratio of nodule acetylene-reducing activity-to-whole plant N. fixation of 1.8 (Hudd et al. 1980 Physiol. Plant. 48, 111). The general magnitude of N2 fixation was also estimated by comparing N accumulation in the legume with that in non-legumes. Denitrification was assumed to be insignificant because soil NO3-N was generally less than 5 ppm, and denitrification measured by the acetylene blockage technique was found to be low in other soils with this nitrate concentration. Volatilization or ammonia from sol1 after incorporation of manure was measured by absorption of ammonia in dilute acid. Leaching losses were estimated from data on nitrate content of water from tile drains. N content of manure was determined just before it was put on the fields, and N contents and standing crops of weeds and crops were determined at harvest, and in late fall.
Although the estimated rates of N2 fixation in faba bean were high (to 200 kg N/ha), the N removed from fields when seeds were harvested exceeded these, leaving a net negative N balance. Positive effects of faba bean on succeeding crops may be associated with mineralization of high N residues from faba bean. The application of poultry manure to grain crops recycles some of the N removed in seeds, and results in a positive N balance for the farm as a whole, with the excess of inputs over outputs presently accumulating in the soil. The N budget indicates that there would not be a positive N balance in the absence of plant cover on the fields between crops; this cover is provided by annual weeds, which in this context, function as a self-seeding cover crop. During the crow season, weeds are controlled by cultural techniques. Within fields, and for three different crops (faba beans, winter wheat, and oats), the ratio of crop biomass-to-(weed + crop)biomass increased with increasing (weed + crop) biomass reaching a value of 1.0 on plots with the highest total biomass. The implication is that as soil fertility (organic matter) builds up, weed problems become fewer (the crop is a better competitor); at low levels of soil fertility the high proportion of weeds functions as a negative feedback mechanisms to recycle nutrients into the soil (as opposed to removing them in crop seeds). A model is developed to predict changes in soil N. grain yields, and weed standing crops in the future. It suggests that self sufficiency in feed, and yields similar to the 1975 levels would be achieved in about 25 years under present techniques. "Internal rearrangements" in the systems may permit more rapid achievement of these goals (i.e. result in more rapid approach to input/output equilibrium or steady state). The study demonstrates that for small mixed farms, biological resources now at hand are sufficient to replace industrial N fixation. The limiting factors are ores concerned with management of the N once it is fixed, rather than with how much N2 is fixed.
Copyright © 1975 David Patriquin. All rights reserved.
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