Based on data result, the mean dry biomass
of tuber responded to the different nitrogen rates with different crop residue.
The highest tuber yield with all crop residues (canola, fababean and wheat) was
obtained at 75 kg N ha-1 N rate than other rates. The highest N rate
did not produce the highest tuber yield which may be due to the tuber response
to nitrogen fertilizer treatment, soil nutrient or climate condition. Most
crops respond well to N fertilizer when the available soil nitrogen levels are
low. N fertilizer is effective in increasing yield of crops on deficient soils.
On soil low in nitrogen availability, applications of moderate N rates usually
result in yield increases compared to high N rate (Agriculture | Province of Manitoba, n.d.). On
the contrary, the mean dry biomass of root and vine in Figure 3 had their
highest yield at the highest N rate (225 kg N ha-1). The result
indicated increased yield with increased N rate which could mean that
environmental conditions were favourable to the specific plots. Where the
supply of nutrients in the soil is sufficient, crops are more likely to grow
well and produce large amounts of biomass. Nitrogen fertilizers are needed in
those cases where nutrients in the soil are lacking and cannot produce
sufficient biomass. Unbalanced nitrogen fertilization may also be the reason
for different plant part (vines, roots and tubers) response to N rate and loss
of biomass yield.  The efficiency of N
fertilizer use will be high where the organic matter content of the soil is
also high. In very poor or depleted soils, crops use fertilizer applications
inefficiently. Therefore, N fertilizers should be applied in sufficient quantities
and in balanced proportions. The difference biomass yield among different crop
residue could have resulted from their different level of carbon to nitrogen
level and N content. The C: N ratio of crop residue plays an important role
driving the rate at which nitrogen is cycled by soil micro-organisms. If the C:
N ratio is low then mineralization or the release of nitrogen occurs. If the C:
N ratio is high then immobilization or nitrogen is tied up within the plot. The
C: N ratio of crop residue declines as the residue decomposes (i.e. C is
released as CO2). Nitrogen is temporarily tied up in residues having high C: N
ratios however, as the C: N ratio declines mineralization (N release) can
occur. The magnitude of immobilization is directly related to the quantity of
crop residue (Agriculture | Province of Manitoba, n.d.). A low carbon-to-nitrogen ratio means the material is high in
nitrogen. Crop residues with a high C: N ratios (low nitrogen) decompose slowly
and may trigger nitrogen deficiency in plants as they decompose. In most cases
for vines, roots and tubers, there was a general increase in mean dry biomass
yield across each crop residues (canola, fababean and wheat). In figure 4, canola
appeared to have a high C: N ratio (low nitrogen content) which might have been
the reason for the lowest yield in vines, roots and tuber dry biomass. Roots
and tubers each had their highest biomass yield with wheat as the preceding
crop which explains that wheat had a low C: N ratio (high nitrogen content). For
vines dry biomass, the lowest C: N ratio with high nitrogen content was
observed to be with fababean residue as it produced the highest biomass yield. However,
the mean dry biomass yield response to crop residues and N rates suggests that
crop residue had different carbon to nitrogen ratio and nitrogen contents
(Figure 5). The preceding crop may mop up soil nitrate or fix molecular
nitrogen, hence potentially contributing to the N supply of tubers. Crop nitrogen
uptake and crop residues low in N and hence high in C: N ratio can decrease the
N losses.

From a fertilizer use
efficiency standpoint, 75 kg ha_1 is most efficient. However, the recommended
fertilization rate for potato production in this region would depend on the
availability of nitrogen in the soil and the value of biomass relative to the
cost of additional fertilizer. The rate of fertilizer application required to
achieve optimum yield varies with site, growing conditions, crop management,
and incidence of disease and insects. This complex response of biomass yield
reflects the fact that variation in soil N supply can often be as important as
crop N demand in determining the optimal fertilizer N rate. For example, in Figure
5, tuber dry biomass with canola and fababean residue had different maximum
tuber yields but similar optimal fertilizer N rates. Fertilizer N rate also has
important an impact on potato plant quality. This potato field received lots of
management and yield improvement by simply applying standard agronomic
management practices of nitrogen fertilization, weed control, and a single
harvest per year. Simple management alone, without addition of any nitrogen, increased
biomass yields of potato plant in the controlled plots. All crop residue
(canola, fababean and wheat) were also observed to improve the biomass of potato.
Although canola residue has the lowest impact, it also had a significant effect
on mean dry biomass. However, mean dry biomass increased constantly over the
different crop residue except for the vine’s biomass which decreased with wheat
residue. Thus, while management operations other than nitrogen fertilization
can make certain biomass improvements initially, the beneficial effects of crop
residues are continuing and increasing. The potential for continued increase in
biomass yields appears favorable to both N rates and crop residues. Despite a
combination of variable weather, site limitations (e.g., the field might
contain soil with high nitrogen level), soil fertility and/or deficiencies
(e.g. nitrogen, phosphorus, potassium), this potato study have produced tuber
biomass yields as high as 9968.93 and 9001.30 8.5 with N rate and crop residue respectively.
Management of nitrogen fertilizer and crop residue for crop production would
improve the cropping options available to farmers. However, not all crop
residue available N is utilized by the potato crop. Both the rate of fertilizer
N application and the relative efficiency of the potato crop in taking up the
applied fertilizer can also influence the potential for loss of N to the
environment. For N rate not recovered in crop residues during plant growth,
there are two pathways of N loss to the environment which are of greatest
concern: nitrate leaching to groundwater and emissions of nitrous oxide (N2O), a greenhouse gas. Fertilizer represents a significant cost to potato growers
and selecting the correct fertilizer N rate is of significant economic
importance to growers. The economic risk associated with insufficient N
fertilization, due to loss of tuber yield is of far greater concern than the
economic risk associated with excessive N fertilization. It is therefore common
for growers to apply a sufficiently high fertilizer N to ensure that the crop N
demand is met under most growing conditions. The optimal fertilizer N rate for
potato crop varies widely among experimental reps. For this study, the optimal
fertilizer N rate from 4 reps ranges from 75 kg N ha ?1 to 225 kg N
ha ?1 and it reflects the crop biological response.

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Summary and ConclusionThis study was conducted at CMCDC offsite
field to determine the impacts of N rates and crop residues on the potato
yield. Fertilizer N management is necessary to
achieve economic goals associated with biomass yield whereas over-fertilization
greatly increases the risk of environmental losses of N. Nitrogen (N)
fertilizer applications required to optimize crop yield will be a function of
the difference between crop N demand and the amount of N supplied by the soil
and residues. Therefore, from the result in figure 5, It is better to
apply 75 kg N ha-1 nitrogen rate to potato production for reasonable
yield at sites similar to study area.  However, assessment of the effect of past
management practices on crop yield potential and soil N dynamics is important
to accurately predict crop fertilizer N requirements. In
conclusion from this experiment, nitrogen fertilization and crop
residues had significant impacts on potato dry biomass and were used by each
plant parts at all rates. 

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