Soil organic matter fractionation as a tool for predicting nitrogen mineralization in silty arable soils M. A. Kader 1,2 , S. S leutel 1 , S. A. B egum 1,2 , K. D’H aene 3 , K. J egajeevagan 1 & S. D e N eve 1 1 Department of Soil Management, Ghent University, Coupure Links 653, 9000 Gent, Belgium, 2 Department of Soil Science, Bangladesh Agricultural University Mymensingh 2202, Bangladesh, and 3 Institute for Agricultural and Fisheries Research (ILVO), Burg. van Gansberghelaan 115, Box 2, 9820 Merelbeke, Belgium Abstract After decades of searching for a practical method to estimate the N mineralization capacity of soil, there is still no consistent methodology. Indeed it is important to have practical methods to estimate soil nitrogen release for plant uptake and that should be appropriate, less time consuming, and cost effective for farmers. We fractionated soil organic matter (SOM) to assess different fractions of SOM as predictors for net N mineralization measured from repacked (disturbed) and intact (undisturbed) soil cores in 14 weeks of laboratory incubations. A soil set consisting of surface soil from 18 cereal and root-cropped arable fields was physically fractionated into coarse and fine free particulate OM (coarse fPOM and fine fPOM), intra-microaggregate particulate OM (iPOM) and silt and clay sized OM. The silt and clay sized OM was further chemically fractionated by oxidation with 6% NaOCl to isolate an oxidation-resistant OM fraction, followed by extraction of mineral bound OM with 10% HF (HF-res OM). Stepwise multiple linear regression yielded a significant relationship between the annual N mineralization (kg N ⁄ ha) from undisturbed soil and coarse fPOM N (kg N ⁄ ha), silt and clay N (kg N ⁄ ha) and its C:N ratio (R 2 = 0.80; P < 0.01). The relative annual N mineralization (% of soil N) from disturbed soils was related to coarse fPOM N, HF-res OC (% of soil organic carbon) and its C:N ratio (R 2 = 0.83; P < 0.01). Physical fractions of SOM were thus found to be the most useful predictors for estimating the annual N mineralization rate of undisturbed soils. However, the bioavailability of physical fractions was changed due to the disturbance of soil. For disturbed soils, a presumed stable chemical SOM fraction was found to be a relevant predictor indicating that this fraction still contains bio-available N. The latter prompted a revision in our reasoning behind selective oxidation and extraction as tools for characterizing soil organic N quality with respect to N availability. Nonetheless, the present study also underscores the potential of a combined physical and chemical fractionation procedure for isolating and quantifying N fractions which preferentially contribute to bulk soil N mineralization. The N content or C:N ratio of such fractions may be used to predict N mineralization in arable soils. Keywords: Chemical fractionation, nitrogen mineralization prediction, physical fractionation, silty arable loess soil, soil organic matter Introduction Realistic estimates of N mineralized from soil organic matter (SOM) are essential for determining the rate of N fertilizer application required to optimize crop yield and quality and to minimize adverse impacts of excessive N on the environment (Sharifi et al., 2007). It is thought that a significant portion of SOM is chemically and ⁄ or physically stabilized and resistant to microbial degradation, whereas a small part of the organic N is more labile and plays a prominent role as a source of substrate for N mineralization (Stanford & Smith, 1972; Parton et al., 1987). Biologically based methods can successfully assess the labile organic N pool by incubating the soil under aerobic or anaerobic conditions at a specified temperature for a certain length of time. However they are time consuming and cannot fit into the batch-analysis techniques of soil-testing laboratories (Sharifi et al., 2007). Correspondence: M. A. Kader. E-mail: mdabdul.kader@ugent.be Received August 2010; accepted after revision August 2010 Soil Use and Management, December 2010, 26, 494–507 doi: 10.1111/j.1475-2743.2010.00303.x 494 ª 2010 The Authors. Journal compilation ª 2010 British Society of Soil Science Soil Use and Management