Decoupled carbon and nitrogen mineralization in soil particle size fractions of a forest topsoil Q4 Carolin Bimüller a, * , Carsten W. Mueller a , Margit von Lützow a , Olivia Kreyling a , Angelika K € olbl a , Stephan Haug b , Michael Schloter c , Ingrid K € ogel-Knabner a, d a Lehrstuhl für Bodenkunde, Department of Ecology and Ecosystem Management, Center of Life and Food Sciences, Weihenstephan, Technische Universit€ at München, D-85350 Freising-Weihenstephan, Germany b TUMjStat, Lehrstuhl für Mathematische Statistik, Technische Universit€ at München, D-85748 Garching, Germany c Helmholtz Zentrum München, German Research Center for Environmental Health, Research Unit for Environmental Genomics, D-85764 Neuherberg, Germany d Institute for Advanced Study, Technische Universit€ at München, Lichtenbergstraße 2a, D-85748 Garching, Germany article info Article history: Received 17 February 2014 Received in revised form 29 July 2014 Accepted 2 August 2014 Available online xxx Keywords: Laboratory incubation SOM stabilization Rendzic Leptosol European beech Heterotrophic respiration NMR abstract To better understand how carbon and nitrogen mineralization are linked in soils, we conducted a long- term incubation experiment and compared carbon and nitrogen dynamics in the bulk soil and in soil fractions. Topsoil of a Rendzic Leptosol from a beech forest site near Tuttlingen, Germany, was separated into three particle size classes: sand (2000e20 mm), silt (20e2 mm), and clay (<2 mm). Bulk soil and particle size fractions were incubated in replicate, allowing periodic destructive sampling of triplicates at day 0, 14, 42, 84, 140, 210, and 280. We monitored CO 2 eC respiration, NH 3 eN emissions, nitrogen mineralization, pool sizes of total and salt extractable (0.01 M CaCl 2 ) organic carbon and nitrogen, and microbial biomass carbon and nitrogen. The chemical composition of selected samples was further characterized by 13 C-NMR spectroscopy. Fractionation did not influence carbon mineralization ( P fractions z bulk soil), which decreased in the order sand > clay > silt. The fractions respired between 10.4% (sand fraction), 8.8% (clay fraction) and 4.4% (silt fraction) of total soil organic carbon. However, nitrogen mineralization was affected by the fractionation procedure ( P fractions < bulk soil) and followed the order clay > silt > sand. Frac- tionation increased the surface area and hence provided accessory mineral surfaces, which allowed new binding of especially nitrogen-rich compounds, in addition to ammonium fixation via cation exchange. As indicated by lower metabolic quotients, microbial carbon mineralization was more efficient in the bulk soil compared to the calculated sum of fractions. In the clay fraction, carbon mineralization rates, salt extractable organic carbon contents, and microbial biomass carbon and nitrogen contents declined strongly towards the end of the incubation. This indicates that in the clay fraction, organic carbon was not available for microbial degradation and that microorganisms were strongly carbon-limited causing a subsequent inhibition of nitrogen immobilization. Density fractionation revealed that organic matter in the sand fraction consisted mainly of particulate organic matter present as light material containing partly decomposed plant remnants. The organic matter in the clay fraction was mostly adsorbed on mineral surfaces. Organic matter in the sand and in the clay fraction was dominated by O/N-alkyl C indicating low recalcitrance, but the C/N ratio of organic matter narrowed with decreasing particle size. Our results suggest that carbon and nitrogen mineralization are decoupled in the mineral-associated fractions of the soil. The specific interactions of both carbon and nitrogen containing components with the mineral matrix strongly modulate the mineralization dynamics. Therefore, isolated considerations of C/N or alkyl C to O/N- alkyl C ratios of organic matter are insufficient as indicators for decomposition in plant residues. The combined consideration of C/N and alkyl C to O/N-alkyl C ratios provides a first impression about the degree of decomposition in plant residues. However, bioavailability in fractions where organic matter is mainly stabilized by spatial inaccessibility and by organo-mineral interactions cannot be explained by these ratios, but can be examined by an incubation approach. © 2014 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: þ49 (0)8161 71 5338; fax: þ49 (0)8161 71 4466. E-mail addresses: carolin.bimueller@wzw.tum.de, carolin.bimueller@gmx.de (C. Bimüller). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio http://dx.doi.org/10.1016/j.soilbio.2014.08.001 0038-0717/© 2014 Elsevier Ltd. All rights reserved. Soil Biology & Biochemistry xxx (2014) 1e11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 SBB5946_proof ■ 16 August 2014 ■ 1/11 Please cite this article in press as: Bimüller, C., et al., Decoupled carbon and nitrogen mineralization in soil particle size fractions of a forest topsoil, Soil Biology & Biochemistry (2014), http://dx.doi.org/10.1016/j.soilbio.2014.08.001