Phosphate and Nitrate Release from Mucky Mineral Soils

High-organic (mucky) mineral soils make a small proportion of the Canadian agricultural land but are highly productive, especially for organic farming. Although these high-quality soils may release large amounts of nitrate and phosphate to the environment, there is yet no reliable agro-environmental indicator for managing N and P compared to the adjacent mineral and organic soils. Our objective was to quantify the N mineralization and P environmental risks of mucky mineral soils. Nine Canadian soil series (eight Orthic Humic Gleysols and one Terric Humisol with three variants) were analyzed for texture, pH(CaCl2), total C and N, oxalate and Mehlich-III (M-III) extractable P, Al and Fe, and water extractable P (Pw). Soil texture varied from loamy sand to heavy clay, organic carbon (OC) content ranged from 14 to 392 g·OC·kg, total N from 1.21 to 16.38 g·N·kg, and degree of P saturation (DPSM-III) as molar (P/[Al + γFe])M-III percentage between 0.3% and 11.3%. After 100 d of incubation, soils released 31 to 340 mg·N·kg. The N mineralization rate was closely correlated to organic matter content (r = 0.91, p < 0.01). Sandy to loamy soils released 1.2 1.8 kg·N·ha·d compared to 1.6 2.4 kg·N·ha·d for clayey soils, 2.0 2.8 kg·N·ha·d for mucky clayey soils and 2.6 2.7 kg·N·ha·d for Humisol. For (P/[Al + 3Fe])M-III ratios of mucky clayey soils below 4.5%, water-extractable P did not exceed threshold of 9.7 mg Pw L. Mucky clayey soils could be managed for N similarly to Humisol and for P with (P/[Al + 3Fe])M-III percentage not exceeding 4.5%.