Meta-analysis requires the following steps:
1- Determination of selection criteria for including results of a given study
- a. Peer-reviewed; or reviewed and considered acceptable by at least two other research scientists who have published papers on N20 emission.
- b. Duration sufficient to capture the growing-season emissions and preferably the full year.
- c. Adequate methodology as per GRACENET.
- d. Two groups can be formed:
- Studies with more than one rate of N applied, from which a fertilizer-induced emission can be calculated from difference in emission divided by difference in rate. Units for analysis would include emissions per unit of N applied, and emissions per unit of yield response to N.
- Studies in which N fertilizer was applied and reported. These would include the foregoing, along with many more studies of tillage, crop rotation, etc. Units of analysis would include emissions per unit area, and emissions per unit of crop yield.
2- Develop an EndNote database with details of all above studies.
3- Develop a spreadsheet database containing values for the following list of dependent and independent variables. Note that for Group i, variables are repeated for control N rate and control dependent variables to allow calculation per unit of applied N.
- Mean sum of measured emissions (kg N20-N/ha) over the whole year OR over the growing season.
- Standard deviation of sum of measured emissions (kg N20-N/ha)
- Crop yield (kg/ha)
- Standard deviation of crop yield (kg/ha)
- N source (class variable)
- N rate (kg N/ha)
- N timing (class variable- fall, spring, sidedress, split)
- N placement (class variable- surface, incorporated, band, broadcast, etc.)
- Total above-ground plant N uptake (where data are available)
- Study# (linked to EndNote database)
- Year (of growing season)
- Site name
- Latitude (degrees to 4 decimal places)
- Longitude (degrees to 4 decimal places)
- Crop type/rotation (species and product, e.g. corn for grain)
- Tillage practice
- Previous crop
- Mean annual precipitation
- Mean annual temperature
- Aridity Index based on Holdridge
- Soil classification down to series level
- Soil texture class
- Soil % sand
- Soil % clay
- Soil pH
- Soil organic matter/soil C
- Mean air temperature during measurement period.
- Sum of precipitation during measurement period.
- Sum of precipitation for 30 days following fertilizer application.
- Emissions measurement start date
- Emissions measurement end date
4- Statistical analysis of the dataset