Abstract
Adjusting the N fertilization to soil potentially mineralizable N in Histosols is required to secure high vegetable yields while mitigating nitrate contamination of surface waters. However, there is still no soil test N (STN) relating the response of Histosol-grown onion (Allium cepa L.) to added N. Compositional data analysis can integrate soil C and N composition into a STN index computed as Mahalanobis distance (M2) across isometric log ratios (ilr) of diagnosed and reference soil C and N compositions. Our objective was to calibrate onion response to added N against a compositional STN index for Histosols. Reference compositions were computed from high N-mineralizing Histosols reported in the literature. Soil analyses were total C and N, and a residual soil mass (Fv) was computed as 100%-%C-%N to close the compositional vector to 100%. The C, N, and Fv proportions were synthesized into two ilrs. We conducted thirteen onion N fertilization trials in Histosols of south-western Quebec showing contrasting C, N, and Fv proportions. Each crop received four N rates broadcast before seeding or split-applied. We derived two STN classes separating weakly to highly responsive crops about the M2 value of 5.5. Onion crops grown on soils showing M2 values >5.5 required more N and yielded less in control treatments compared with soils showing M2 values 2 2 > 5.5) soils responded significantly (P -1, respectively. Using literature data and the results of this study, we elaborated a provisory N requirement model for Histosol-grown onions in Quebec.
Highlights
Onions (Allium cepa L.) are grown on nearly 9000 ha of Histosols in Quebec, Ontario and New York state
Our objective was to conduct a meta-analysis of multi-year and multi-site trials on yield response of dry onions to added N in Quebec Histosols using a compositional index as soil test N
A compositional soil test N (STN) index that integrates C, N and Fv into a Mahalanobis distance was calibrated against N requirements of onions grown on Histosols
Summary
Onions (Allium cepa L.) are grown on nearly 9000 ha of Histosols in Quebec, Ontario and New York state. Reference [2] showed that combined effects of fertilization, drainage and mineralization produced 40 to 50 times more NO3-N in runoff water during the growing season under cultivated compared to uncultivated marsh in Ontario. Reference [3] reported N losses of 37 - 245 kg N ha−1∙yr−1 from Ontario Histosols with yearly concentrations varying between 15 and 43 mg NO3-N L−1 in surface waters. After mineralization of organic N into nitrate, the net nitrate accumulation reached 850 kg NO3-N ha−1 in New York Histosols [4] and 1400 kg NO3-N ha−1 in the Florida Everglades [5]. When soil N supply capacity is high over-fertilization must contribute to nutrient waste and water contamination [6]-[8], especially for onion crops, due to low capacity of the root system to exploit soil N [9]
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