Organic Matter Fractions Contributing to Soil Nitrogen Mineralization Potential
- Denis Curtin and
- Guang Wen
Organic matter pools contributing to N mineralization potential have not been adequately defined. We analyzed 61 soil samples (0–15 cm depth) representing all agroecological regions of Saskatchewan, Canada to determine the relationships between mineralization parameters [i.e., potentially mineralizable N (N0) and the rate coefficient k, measured by aerobic incubation at 35°C for 24 wk] and N fractions defined by a series of chemical and physical extraction procedures. Light-fraction (LF) organic matter, isolated by flotation on NaI solution at specific gravity 1.7, soluble organic matter measured in saturated paste extracts, and NH4-N released by two chemical procedures [digestion in 2 M KCl (100°C for 4 h) and steam distillation inphosphate-borate (PB) buffer at pH 11.2] were evaluated as indices of mineralizable N. Nitrogen released by hot KCl represented 2.5 to 16% of N0, which ranged from 71 to 631 mg kg-1. Hot KCl N was poorly correlated with N0 (R2 = 0.13, P < 0.01), but it was reasonably well related to N mineralized in the first 2 wk of incubation (R2 = 0.65, P < 0.001), and to dissolved organic matter (R2 = 0.58, P < 0.001). The value of k, which ranged from 0.025 to 0.18 wk-1, increased as hot KCl N increased as a proportion of N0 (KCl N/N0 accounted for 64% of the variability in k). These results suggest that hot KCl may be selective for the most labile (possibly water-soluble) organic N. Phosphate-borate extracted an average of 3.5 times as much N as hot KCl, but relationships with mineralization parameters were generally similar to those found for hot KCl. Light-fraction N was the largest N pool measured (33–652 mg N kg-1). It was well correlated with N0 (R2 = 0.83, P < 0.001), but not with N mineralized in the early part of the incubation or with k, indicating that LF N mineralizes slowly compared with chemically extracted N. When LF N was added to PB N, there was roughly enough N in the combined pool to account for N0.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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