Chemical and physical changes in standing cereal straw residues in no-till fallow and its relationship to nitrogen and phosphorus availability
Adjustments to nitrogen and phosphorus fertilization rates may be required in fields which receive a no-till (herbicide) fallow treatment as compared to the conventional tillage fallow. For example, there is potentially greater immobilization of nutrient in no-till fields.
IPNI-1993-CAN-SK11
30 Aug 2001
Methodology
- Field
Samples of cereal straw (wheat and barley) have been taken from chemical fallow plots at the University Research Farm (Kernen) at Saskatoon. These include straw samples taken in the fall of 1992 and the spring of 1993. These samples will be used to determine changes in straw composition over the second winter. Another set of straw samples was taken this spring from plots which had been chemical fallowed in 1992 (2 year old straw) for comparison with samples of straw from an adjacent plot which had been cropped in 1992 (1 year old straw). This spring two plots were set up in southwestern Saskatchewan, cone on barley stubble and the other on wheat stubble. These fields will be chemical fallowed in 1993 and seeded to cereal in 1994. Samples of straw will be taken from the plots at six week intervals over the summer, fall, winter and spring to monitor the changes in composition with time. The above plots will be continued in 1994.
Growth Chamber
A growth chamber experiment will be set up in the spring of 1994 to examine the changes in chemical composition of the standing straw residue over time under controlled conditions. In this experiment, wheat plants will be grown to maturity in soil beds, harvested, and the remaining standing straw used in the experiment. The chemical composition of the plants will be measured at time zero and at two week intervals following simulated rainfall - leaching events. One treatment will involve the soil surface covered with plastic but allowing the straw to protrude. Simulated rainfall will then be applied and the chemical composition of the straw runoff measured to determine possible mechanisms by which the chemical nature of the straw is altered.
Another growth chamber - incubation experiment is proposed in which fresh straw and chemical-fallow straw are added to a soil and the changes in available nitrate, ammonium and phosphate measured over time. Ion exchange membranes will be used in the incubation to act as a plant root simulator to accumulate the released ion. Wheat plants will also be grown on the soils and N and P uptake in the plants measured.
Laboratory
Chemical characterization of the straw samples will proceed as follows: Straw samples will be analyzed for total and water soluble carbon, nitrogen, and phosphorus. Carbon, nitrogen and phosphorus will be further divided into organic and inorganic forms. Cargon: nitrogen and carbon : phosphorus ratios will be used to estimate the immobilization potential of the material when it enters the soil. The chemical nature of the dissolved organics in the water soluble fractions will also be examined via high pressure liquid chromatography.
An evaluation of changes in straw strength is proposed in which the cereal straw is subject to a type of modulus of rupture test in which the pressure required to break the straw is measured. This technique will be used to measure the changes in physical strength of the straw over the chemical fallow period. Measurements of straw density will also be carried out. Strength and mass changes in the straw under no-till fallow may have a significant role in the degree of erosion protection. current % cover - erosion protection relationships (eg C in USLE) developed mainly on conventionally tilled summerfallow may not hold for the residues remaining on no-till fallow.