Global Change Biology Global Change Biology (2015) 21, 1358–1367, doi: 10.1111/gcb.12786
Litter type control on soil C and N stabilization dynamics in a temperate forest P I E R R E - J O S E P H H A T T O N 1 , C R I S T I N A C A S T A N H A 2 , M A R G A R E T S . T O R N 2 and JEFFREY A. BIRD1 1 School of Earth & Environmental Sciences, Queens College, CUNY, New York, NY, USA, 2Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA
Abstract While plant litters are the main source of soil organic matter (SOM) in forests, the controllers and pathways to stable SOM formation remain unclear. Here, we address how litter type (13C/15N-labeled needles vs. fine roots) and placement-depth (O vs. A horizon) affect in situ C and N dynamics in a temperate forest soil after 5 years. Litter type rather than placement-depth controlled soil C and N retention after 5 years in situ, with belowground fine root inputs greatly enhancing soil C (x1.4) and N (x1.2) retention compared with aboveground needles. While the proportions of added needle and fine root-derived C and N recovered into stable SOM fractions were similar, they followed different transformation pathways into stable SOM fractions: fine root transfer was slower than for needles, but proportionally more of the remaining needle-derived C and N was transferred into stable SOM fractions. The stoichiometry of litter-derived C vs. N within individual SOM fractions revealed the presence at least two pools of different turnover times (per SOM fraction) and emphasized the role of N-rich compounds for long-term persistence. Finally, a regression approach suggested that models may underestimate soil C retention from litter with fast decomposition rates. Keywords:
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C, 15N, fine root, forest, litter, litter decomposition, needle, soil organic matter, stabilization
Received 22 July 2014 and accepted 18 October 2014
Introduction While the factors controlling plant litter decomposition rates in soils are increasingly well understood (Gholz et al., 2000; Silver & Miya, 2001; Adair et al., 2008; Zhang et al., 2008; Garcia-Palacios et al., 2013), large uncertainties remain concerning the factors that control the long-term retention of litter-derived carbon (C) and nitrogen (N) in soils (Prescott, 2010; Cotrufo et al., 2013). Those factors that effectively predict initial mass loss of plant litter (e.g., litter chemistry) may not be major controllers on the amount and form of litterderived C and N stabilized in soils by physical and chemical mechanisms (K€ ogel-Knabner & Kleber, 2011; Schmidt et al., 2011). To effectively model long-term C and N cycling in forest ecosystems and assess the potential of forest soils to act as a C sink in a changing environment, we need to better understand how litter inputs influence the proportion and persistence of litter-derived C and N remaining in soils on the time scale of decades and centuries (Cotrufo et al., 2013). Fine roots (2 mm from the soil 99.5%), adjusted to 2 mm soil fraction was greater for roots placed in the O horizon than roots placed to the A horizon (31%; P = 0.017), but statistically equivalent to needles placed in O and A horizon (P > 0.05; Table 1). In contrast to roots, needle-derived C and N recovery in the >2 mm soil fraction was unaffected by placement-depth in soil. Litter-derived C : N ratios in >2 mm soil fraction were consistently greater for roots than for needles (+107% averaged across depths; P < 0.001). While needlederived C : N ratio was unaffected by the placementdepth in soil, root-derived C : N ratio were 29% wider for roots placed to the A horizon compared with those applied to the O horizon (P = 0.001). Litter type affected C (although not significant; P = 0.071) and N (P = 0.045) recovered in the
Litter type control on soil C and N stabilization dynamics in a temperate forest P I E R R E - J O S E P H H A T T O N 1 , C R I S T I N A C A S T A N H A 2 , M A R G A R E T S . T O R N 2 and JEFFREY A. BIRD1 1 School of Earth & Environmental Sciences, Queens College, CUNY, New York, NY, USA, 2Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA
Abstract While plant litters are the main source of soil organic matter (SOM) in forests, the controllers and pathways to stable SOM formation remain unclear. Here, we address how litter type (13C/15N-labeled needles vs. fine roots) and placement-depth (O vs. A horizon) affect in situ C and N dynamics in a temperate forest soil after 5 years. Litter type rather than placement-depth controlled soil C and N retention after 5 years in situ, with belowground fine root inputs greatly enhancing soil C (x1.4) and N (x1.2) retention compared with aboveground needles. While the proportions of added needle and fine root-derived C and N recovered into stable SOM fractions were similar, they followed different transformation pathways into stable SOM fractions: fine root transfer was slower than for needles, but proportionally more of the remaining needle-derived C and N was transferred into stable SOM fractions. The stoichiometry of litter-derived C vs. N within individual SOM fractions revealed the presence at least two pools of different turnover times (per SOM fraction) and emphasized the role of N-rich compounds for long-term persistence. Finally, a regression approach suggested that models may underestimate soil C retention from litter with fast decomposition rates. Keywords:
13
C, 15N, fine root, forest, litter, litter decomposition, needle, soil organic matter, stabilization
Received 22 July 2014 and accepted 18 October 2014
Introduction While the factors controlling plant litter decomposition rates in soils are increasingly well understood (Gholz et al., 2000; Silver & Miya, 2001; Adair et al., 2008; Zhang et al., 2008; Garcia-Palacios et al., 2013), large uncertainties remain concerning the factors that control the long-term retention of litter-derived carbon (C) and nitrogen (N) in soils (Prescott, 2010; Cotrufo et al., 2013). Those factors that effectively predict initial mass loss of plant litter (e.g., litter chemistry) may not be major controllers on the amount and form of litterderived C and N stabilized in soils by physical and chemical mechanisms (K€ ogel-Knabner & Kleber, 2011; Schmidt et al., 2011). To effectively model long-term C and N cycling in forest ecosystems and assess the potential of forest soils to act as a C sink in a changing environment, we need to better understand how litter inputs influence the proportion and persistence of litter-derived C and N remaining in soils on the time scale of decades and centuries (Cotrufo et al., 2013). Fine roots (2 mm from the soil 99.5%), adjusted to 2 mm soil fraction was greater for roots placed in the O horizon than roots placed to the A horizon (31%; P = 0.017), but statistically equivalent to needles placed in O and A horizon (P > 0.05; Table 1). In contrast to roots, needle-derived C and N recovery in the >2 mm soil fraction was unaffected by placement-depth in soil. Litter-derived C : N ratios in >2 mm soil fraction were consistently greater for roots than for needles (+107% averaged across depths; P < 0.001). While needlederived C : N ratio was unaffected by the placementdepth in soil, root-derived C : N ratio were 29% wider for roots placed to the A horizon compared with those applied to the O horizon (P = 0.001). Litter type affected C (although not significant; P = 0.071) and N (P = 0.045) recovered in the
