Table 1.
The five systems at the Salinas Organic Cropping Systems (SOCS) experiment evaluated in this analysis.
Table 2.
Total nitrogen concentrations of inputs at the Salinas Organic Cropping Systems (SOCS) experiment.
Fig 1.
Soil total nitrogen stocks at the 0 to 30 cm depth by system and year.
Systems differed by annual compost additions, cover crop type and frequency. The horizontal dashed line at 2.1 Mg ha-1 is the mean soil total nitrogen between years 1 and 8 for System 1 and is included on each graph as a reference. Error bars are 95% confidence intervals (CI) that are connected from year to year by the yellow band. Individual data points for replicates 1 through 4 of each system are clustered around the mean in order from left to right so as not to obscure the CI. The F and CC labels along the x-axis indicate when the system was fallow (F) or cover cropped (CC). The soil was sampled in the fall each year right before the winter cover cropping or fallow period (beginning at time 0); therefore, the 4th and 8th sampling dates for Systems 1 and 2 occurred right before they received their first and second cover crops.
Fig 2.
Soil total nitrogen stocks over seven years of vegetable production.
Mean soil total nitrogen stocks (A) and mean paired differences between systems (B). Systems differed by annual compost additions, cover crop type and cover cropping frequency. Error bars are 95% confidence intervals (CI) around the mean (horizontal line). Samples were taken at the 0 to 30 cm depth after the 2nd vegetable crop before winter fallow or cover cropping for years 1 to 7 when two vegetable crops were grown annually. Data points are the 4 replicates each year and are clustered around the mean so as not to obscure the mean and CI. Different lower-case letters above the x-axis indicate that means are significantly different based on the Tukey-Kramer adjusted family-wise error rate of P≤0.05. The standardized effect size (Cohen’s unbiased d, dunb) is shown below the x-axis in plot B.
Fig 3.
Post-harvest soil nitrate nitrogen concentration over 8 years of vegetable production.
Samples were taken after incorporating the last vegetable crop each year and before cover cropping or fallow. Systems differed by annual compost additions, cover crop type and cover cropping frequency. Samples were taken at the 0 to 30 cm depth after the 2nd vegetable crop before winter fallow or cover cropping for years 1 to 7 when two vegetable crops were grown annually (A). When broccoli was grown after lettuce (Years 2 to 7), the relationship between nitrate nitrogen concentrations and days after broccoli incorporation is shown (B); the individual data points represent the mean value for each system and the year(s) that the data correspond to is shown above the x axis. The error bars in plot B are 95% confidence intervals for the mean values.
Fig 4.
Post-harvest soil nitrate concentrations over seven years of vegetable production.
Mean soil nitrate concentrations (A) and mean paired differences between systems (B). Samples were taken at the 0 to 30 cm depth after the 2nd vegetable crop before winter fallow or cover cropping for years 1 to 7 when two vegetable crops were grown annually. Systems differed by annual compost additions, cover crop type and cover cropping frequency. Error bars are 95% confidence intervals (CI) around the mean (horizontal line). Data points are the 4 replicates each year and are clustered around the mean so as not to obscure the mean and CI. Different lower-case letters above the x-axis in plot A indicate that means are significantly different based on the Tukey-Kramer adjusted family-wise error rate of P≤0.05. The standardized effect size (Cohen’s unbiased d, dunb) is shown below the x-axis in plot B.
Fig 5.
Cumulative above ground and estimated below ground cover crop nitrogen uptake over eight years of vegetable production.
Mean nitrogen uptake (A) and mean paired differences between systems (B) in 5 organic vegetable systems. Systems differed by annual compost additions, cover crop type and cover cropping frequency. Nitrogen uptake data does not include nitrogen from legume nitrogen fixation in Systems 1, 2 and 3; see Table 3 for nitrogen fixation data. Error bars are 95% confidence intervals (CI) around the mean (horizontal line). Data points are replicates 1 through 4 averaged across years and are clustered around the mean left to right so as not to obscure the mean and CI. Different lower-case letters above the x-axis in plot A indicate that means are significantly different based on the Tukey-Kramer adjusted family-wise error rate of P≤0.05. The standardized effect size (Cohen’s unbiased d, dunb) is shown below the x-axis in plot B.
Table 3.
Cumulative nitrogen inputs, exports and mass balances for 5 organic vegetable systems over eight years in Salinas, CA.
Systems differed by annual compost additions (0 vs. 7.6 Mg ha-1, oven-dry basis, before each vegetable crop), cover crop type (legume-rye, mustard, or cereal rye alone) and cover cropping frequency (quadrennially vs. annually planted). System 1 = no compost+legume-rye 4th year; System 2 = annual compost+legume-rye 4th year; System 3 = annual compost+legume-rye annually; System 4 = annual compost+mustard annually; System 5 = annual compost+rye annually.
Fig 6.
Cumulative above ground and estimated below ground lettuce nitrogen uptake over eight years of vegetable production.
Mean nitrogen uptake (A) and mean paired differences between systems (B) in 5 organic vegetable systems. Systems differed by annual compost additions, cover crop type and cover cropping frequency. Error bars are 95% confidence intervals (CI) around the mean (horizontal line). Data points are replicates 1 through 4 averaged across years and are clustered around the mean left to right so as not to obscure the mean and CI. Different lower-case letters above the x-axis in plot A indicate that means are significantly different based on the Tukey-Kramer adjusted family-wise error rate of P≤0.05. The standardized effect size (Cohen’s unbiased d, dunb) is shown below the x-axis in plot.
Fig 7.
Cumulative above ground and estimated below ground broccoli nitrogen uptake over six years of vegetable production.
Mean cumulative nitrogen uptake (A) and mean paired differences between systems (B) in 5 organic vegetable systems. Systems differed by annual compost additions, cover crop type and cover cropping frequency. Error bars are 95% confidence intervals (CI) around the mean (horizontal line). Data points are replicates 1 through 4 averaged across years and are clustered around the mean left to right so as not to obscure the mean and CI. Different lower-case letters above the x-axis in plot A indicate that means are significantly different based on the Tukey-Kramer adjusted family-wise error rate of P≤0.05. The standardized effect size (Cohen’s unbiased d, dunb) is shown below the x-axis in plot B.
Fig 8.
Cumulative, estimated, marketable lettuce yield over eight years.
Mean cumulative lettuce yield (A) and mean paired differences between systems (B) in 5 organic vegetable systems. Yields are oven-dry biomass. Systems differed by annual compost additions, cover crop type and cover cropping frequency. Error bars are 95% confidence intervals (CI) around the mean (horizontal line). Data points are replicates 1 through 4 averaged across years and are clustered around the mean left to right so as not to obscure the mean and CI. Different lower-case letters above the x-axis in plot A indicate that means are significantly different based on the Tukey-Kramer adjusted family-wise error rate of P≤0.05. The standardized effect size (Cohen’s unbiased d, dunb) is shown below the x-axis in plot B.
Fig 9.
Cumulative, estimated, marketable broccoli yield over six years.
Mean cumulative broccoli yield (A) and mean paired differences between systems (B) in 5 organic vegetable systems. Yields are oven-dry biomass. Systems differed by annual compost additions, cover crop type and cover cropping frequency. Error bars are 95% confidence intervals (CI) around the mean (horizontal line). Data points are replicates 1 through 4 averaged across years and are clustered around the mean left to right so as not to obscure the mean and CI. The standardized effect size (Cohen’s unbiased d, dunb) is shown below the x-axis in plot B. Broccoli was only grown during 6 of the first 8 years of the experiment.
Fig 10.
Cumulative above ground and estimated below ground vegetable residue nitrogen over eight years of vegetable production.
Mean cumulative vegetable (lettuce + broccoli) residue nitrogen (A) and mean paired differences between systems (B) in 5 organic vegetable systems. Systems differed by annual compost additions, cover crop type and cover cropping frequency. Error bars are 95% confidence intervals (CI) around the mean (horizontal line). Data points are replicates 1 through 4 averaged across years and are clustered around the mean left to right so as not to obscure the mean and CI. Different lower-case letters above the x-axis in plot A indicate that means are significantly different based on the Tukey-Kramer adjusted family-wise error rate of P≤0.05. The standardized effect size (Cohen’s unbiased d, dunb) is shown below the x-axis in plot B.
Fig 11.
Average annual nitrogen inputs, exports and storage or losses in five organic vegetable systems over 8 years.
Values are in kg ha-1 and were calculated from the cumulative values in Table 2. Systems differed by annual compost additions, cover crop type and cover cropping frequency.
Fig 12.
Mean paired difference in cumulative total nitrogen inputs between systems over eight years of vegetable production.
Measurements were taken in five organic vegetable systems. Systems differed by annual compost additions, cover crop type and cover cropping frequency. System 1 = no compost+legume-rye 4th year; System 2 = annual compost+legume-rye 4th year; System 3 = annual compost+legume-rye annually; System 4 = annual compost+mustard annually; System 5 = annual compost+rye annually. The difference between Systems 4 and 5 is not presented because all inputs were fixed, thus there was no variation among replicates. Error bars are 95% confidence intervals (CI) around (horizontal line). Data points are replicates 1 through 4 averaged across years and are clustered around the mean left to right so as not to obscure the mean and CI. The standardized effect size (Cohen’s unbiased d, dunb) is shown below the x-axis.
Fig 13.
Mean paired difference in cumulative total harvest nitrogen exports between systems over eight years of vegetable production.
Measurements were taken in five organic vegetable systems. Systems differed by annual compost additions, cover crop type and cover cropping frequency. System 1 = no compost+legume-rye 4th year; System 2 = annual compost+legume-rye 4th year; System 3 = annual compost+legume-rye annually; System 4 = annual compost+mustard annually; System 5 = annual compost+rye annually. Error bars are 95% confidence intervals (CI) around the mean (horizontal line). Data points are replicates 1 through 4 averaged across years and are clustered around the mean left to right so as not to obscure the mean and CI. The standardized effect size (Cohen’s unbiased d, dunb) is shown below the x-axis.
Fig 14.
Mean paired difference in cumulative soil surface nitrogen balance between systems over eight years of vegetable production.
Measurements were taken in five organic vegetable systems. Systems differed by annual compost additions, cover crop type and cover cropping frequency. System 1 = no compost+legume-rye 4th year; System 2 = annual compost+legume-rye 4th year; System 3 = annual compost+legume-rye annually; System 4 = annual compost+mustard annually; System 5 = annual compost+rye annually. Error bars are 95% confidence intervals (CI) around the mean (horizontal line). Data points are replicates 1 through 4 averaged across years and are clustered around the mean left to right so as not to obscure the mean and CI. The standardized effect size (Cohen’s unbiased d, dunb) is shown below the x-axis.
Fig 15.
Mean paired difference in cumulative soil nitrogen storage change between systems over eight years of vegetable production.
Measurements were taken in five organic vegetable systems. Systems differed by annual compost additions, cover crop type and cover cropping frequency. System 1 = no compost+legume-rye 4th year; System 2 = annual compost+legume-rye 4th year; System 3 = annual compost+legume-rye annually; System 4 = annual compost+mustard annually; System 5 = annual compost+rye annually. Error bars are 95% confidence intervals (CI) around the mean (horizontal line). Data points are replicates 1 through 4 averaged across years and are clustered around the mean left to right so as not to obscure the mean and CI. The standardized effect size (Cohen’s unbiased d, dunb) is shown below the x-axis.
Fig 16.
Mean paired difference in cumulative soil system nitrogen balance between systems over eight years of vegetable production.
Measurements were taken in five organic vegetable systems. Systems differed by annual compost additions, cover crop type and cover cropping frequency. System 1 = no compost+legume-rye 4th year; System 2 = annual compost+legume-rye 4th year; System 3 = annual compost+legume-rye annually; System 4 = annual compost+mustard annually; System 5 = annual compost+rye annually. Error bars are 95% confidence intervals (CI) around the mean (horizontal line). Data points are replicates 1 through 4 averaged across years and are clustered around the mean left to right so as not to obscure the mean and CI. The standardized effect size (Cohen’s unbiased d, dunb) is shown below the x-axis.