Features Inoculants Seed & Chemical
Impact of crop residue type on phosphorus release

It is generally assumed that crop residues will decompose more slowly under
no-till (NT) than under conventional tillage (CT) management. With a reduced
rate of decomposition, we would expect less nutrients to be released in a given
time period. This may not always be correct, given that the amount of a nutrient
released from crop residue depends not only on the decomposition rate, but also
on the nutrient concentration in the original crop residue. This could be affected
more by crop type than any management input.

To address these questions, a trial was initiated in an established long-term
tillage and crop rotation study. The objective was to quantify how much P is
released from red clover, field pea, canola and wheat residues under CT and
NT seeding systems. The trial was conducted at Fort Vermilion in northwestern
Alberta in 1998-1999 and 1999-2000, using an established study evaluating two
tillage systems: NT and CT, and four different crop rotations that included
red clover green manure, field peas, canola and wheat. In 1998-1999, the red
clover did not survive the winter and was replaced with a field pea green manure
crop. Crop residues of the green manure, field peas, wheat and canola were collected
at harvest, weighed, and analyzed for P to determine the amount of P being returned
to the plot. The residues were then placed in decomposition-resistant litter
bags with 1mm mesh and either buried in the soil (CT), or placed on the soil
surface (NT). The bags were sampled periodically over a 12 month period and
the residues analyzed for P to determine how much P still remained in the decomposing
residues and… by difference from the amounts applied… how much P had been
released.

Crop residue dry matter (DM) returned after the different crops was considerably
higher in 1999-2000, relative to 1998-1999, reflecting the higher crop production
during the 1999 growing season (see tables 1 and 2). However, P concentration
in the crop residues resulted in a large difference in the amount of total P
being returned to the field. While the green manure crops returned the largest
amount of P, spring wheat produced the least crop residue and lowest residue
P returned to the field. The amounts of residues produced and added to the soil
did not differ significantly between tillage treatments, and there were no significant
interactions between tillage and crop residues in residue DM produced or P applied.

During the 46 to 52 weeks that residue samples were monitored in this study,
all the amounts of P released were less than the amounts that had been applied
with the residues (see tables 1 and 2). The green manure crops released the
largest proportion of the residue P (70 to 78 percent), reflecting the ease
of decomposition of this fresh plant material. This release was also rapid,
with most of the P returned to the soil within five weeks of application. The
mature crop residues proved to be more resistant to decomposition and P release.
The slow decomposition and lower P content of the field pea, canola and wheat
residue resulted in some immobilization of P during the 46 to 52 week decomposition
period.

An evaluation of the carbon (C), lignin and P in the plant residues was carried
out and the results indicated a positive correlation between P release from
the residue and percent P, and a negative correlation between P release and
residue C:P ratio (data not shown). Whether the residue was buried with CT or
left on the surface with NT, there was no effect on P release. The more resistant
parts of the residues, including nutrients they contain, become soil organic
matter which decomposes slowly. Therefore, although only the green manure crops
released significant amounts of P, it is advisable that all crop residues be
added to the soil because they maintain or increase soil organic matter. Soil
organic matter is important not only as a slow-release source of nutrients for
crops and soil organisms, but also for maintaining soil structure. Even nutrients
added as fertilizers are not utilized efficiently when soil organic matter is
low.

Results of this study illustrate that the release of P from crop residues is
influenced by not only the P content of the residue, but the ease of decomposition.
The rapid and large release of residue P from green manure crops can be expected
to contribute to plant P supply when these fields are recropped. By contrast,
wheat residues added significantly less P to the soil, and were just as likely
to immobilize as release P in the 46 to 52 week period. Canola and field pea
fell somewhere between wheat and the green manure crop, reflecting a higher
residue P contribution and lower C:P ratio. Tillage system practiced did not
affect amounts of P released by residues.

Dr. Lupwayi (e-mail: LupwayiN@ agr.gc.ca) and Dr. Rice (retired)
are with the Agriculture and Agri-Food Canada Beaverlodge Research Farm
in Alberta. Drs. Clayton, Harker, and Turkington are with the Agriculture
and Agri-Food Canada Research Centre, Lacombe, Alberta. Dr. Johnston is
PPI/PPIC Western Canada Director, located at Saskatoon, Saskatchewan.