Organic Matter Fractions Contributing to Soil Nitrogen Mineralization Potential

Abstract

AbstractOrganic 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.