Effects of Detrital Inputs and Roots on Carbon Saturation Deficit of a Temperate Forest Soil

Abstract

Soil C sequestration has been proposed as a tool for addressing climate change. However, models used to predict soil C sequestration do not account for C saturation and functional differences among soil C pools. In this study, we examined differences in soil C pool content of a forest soil in Pennsylvania following 20 yr of detrital manipulation (i.e., control, no roots, no leaf litter, no inputs, double leaf litter). Detrital input treatments had a highly significant (ANOVA, F = 10.6, p < 0.0001) effect on soil C in the 0‐ to 5‐cm soil depth. However, soil C pools responded differently to the different treatments: the intra‐microaggregate silt + clay, intra‐microaggregate particulate organic matter (POM), and silt + clay within large macroaggregates in plots receiving no detrital inputs had significantly lower C concentrations (1.1, 2.5, and 0.4 g C kg−1 soil, respectively) than the control, double‐leaf‐litter, and no‐roots plots. Carbon in coarse POM within small macroaggregates was significantly lower in double‐leaf‐litter (4.5 g C kg−1 soil) and no‐leaf‐litter plots (6.6 g C kg−1 soil) than all other treatments (15.3–28 g C kg−1 soil). The intra‐microaggregate fractions in double‐leaf‐litter plots had greater C concentrations than in the control plots; along with moderate C concentrations in all bulk soil samples, this suggests that the soil is not yet approaching C saturation. This study reinforces the use of microaggregates within large macroaggregates as a diagnostic fraction for changes in soil C content and demonstrates that altering the organic matter source and quantity significantly affects soil C sequestration dynamics. Consideration of these factors will enable improved assessment of soil C sequestration capacity and management.

The Path From Litter to Soil: Insights Into Soil C Cycling From Long‐Term Input Manipulation and High‐Resolution Mass Spectrometry

“…The potential to manipulate the stable C pools to offset ongoing climate change has driven extensive debate (Dungait et al, ), but the evidence thus far indicates that adding C to the most stable pools would not be straightforward (Bowden et al, ). Our results largely support this conclusion, as increasing aboveground input quantity for 20 years had no effect on soil C quantity, as had been previously reported for this site (Bowden et al, ; Crow et al, ; Mayzelle et al, ). There was also no indication that C concentration increased in any density fraction (Figure ), similar to results reported after 12 years of litter addition (Crow et al, ), or that the microbial community's catabolic profile was altered (Figure ).…”

“…Roots and their associated microbial communities drive soil aggregation and thus protection of soil C (Six et al, ), and there is evidence that root exclusion resulted in movement of C into the nonaggregated mineral fractions, potentially due to a decrease in aggregation. The soil C concentration of the no inputs treatment fine, heavy fraction increased versus the control treatment (Figure ), possibly due to a decrease in soil aggregation (not measured in this study) as observed by Mayzelle et al () in the same soil, though this was not apparent in the no roots treatment. There was also an increase in the C concentration of the coarse heavy fraction in the no roots treatment, though this was not apparent in the no inputs treatment (Figure ).…”

“…Thus far, these experiments have revealed indirect and nonlinear responses (Bowden et al, ; Lajtha, Bowden, & Nadelhoffer, ; Lajtha, Townsend, et al, ) in total C and mineral‐associated SOM pools, including evidence for context‐specific dynamics and priming. Overall, the mineral‐associated fractions are reported to be less vulnerable to decomposition and less responsive to C accumulation from aboveground inputs, indicating that direct litter addition does not necessarily result in rapid or direct accumulation of additional C in the putatively “stable” pools (Bowden et al, ; Lajtha, Bowden, & Nadelhoffer, ; Lajtha, Townsend, et al, ) even when soils are not “carbon saturated” (Mayzelle et al, ). Moreover, the variability of responses among sites may be due to differences in litter decomposition rate as controlled by climate (Fekete et al, ), litter quality (Bowden et al, ), and soil mineralogy that in turn drive the litter‐to‐soil pathway.…”

Litter and Root Manipulations Provide Insights into Soil Organic Matter Dynamics and Stability

“…The light fraction (<1.85 g cm −3 ) of soil has generally been found to be the most reactive fraction to management or disturbance (Bremer et al, 1994;Liao et al, 2006;Spielvogel et al, 2006;Throop et al, 2013), although other studies have suggested that C accumulation occurs primarily in heavy-fraction, slowturnover pools (Grandy and Robertson, 2007). As expected, decreased litter inputs caused decreases in light-fraction pools, showing that this pool is more rapidly depleted than higher density fraction pools, which are composed of soil C stabilized in aggregates (intermediate density fractions) or by strong organomineral associations (heaviest fractions) (Hatton et al, 2012;Mayzelle et al, 2014). Litter additions, however, did not result in parallel increases in either total soil C or any of the density pools; rapid respiration of O or mineral soil horizon organic matter, due to enhanced microbial activity or to increased rhizospheric activity, may have prevented soil C increases (Spears and Lajtha, 2004).…”

“…The response of soil organic matter (SOM) to changes in forest biomass and litter inputs might have a significant temporal lag; Holub et al (2005) found no effect of above-and belowground organic inputs on SOM in two different forest soils after 4 or 11 yr. Although most ecosystem C models assume a strong linkage between net primary productivity, litter inputs, and soil C accumulation (Gottschalk et al, 2012), soils have finite capacities to sequester C and might "saturate" or achieve maximum equilibrium levels under different combinations of soil texture, mineralogy, and climatic regimes (Chung et al, 2010;Stewart et al, 2009;Six et al, 2002;Mayzelle et al, 2014). The C saturation deficit is the difference between current C content and the point of C saturation (Hassink, 1997), and thus C accumulation potential depends on litter input rates, decomposition rates, soil mineralogy, soil C content, and soil C saturation capacity.…”