Recent (<4 year old) leaf litter is not a major source of microbial carbon in a temperate forest mineral soil

Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

Replicated samples (n=4) of bulk 14C values for 0-15 cm depth collected in September 2005 had slightly lower 14C values and a similar pattern: 90 ± 4‰ (low) and 92 ± 16‰ (elevated) for Walker Branch and 105 ± 4‰ (low) and 120 ± 15‰ (elevated) for Haw Ridge.

“Replicated samples (n ¼ 4) of bulk 14 C values for the 0e15 cm depth collected in September 2005 had slightly lower 14 C values as they integrated C deeper in the soil, but showed a similar pattern (Hanson, unpublished data): 90 Æ 4& (low) and 92 Æ 16& (elevated) for Walker Branch and 105 Æ 4& (low) and 120 Æ 15& (elevated) for Haw Ridge.”

May combine two claims.

Measured PLFA Δ14C signatures were higher than those measured in bulk mineral soil organic matter in the experiments, but fell within the range of 14C values measured in mineral soil roots.

“Measured PLFA D 14 C signatures were higher than those measured in bulk mineral soil organic matter in our experiments, but fell within the range of 14 C values measured in mineral soil roots.”

Probably not a substantial claim: may not pass S3 (load-bearing).

The plant material picked out included leaves and some roots and had Δ14C values ranging from 177-195‰ for low-14C litter plots to 319-384‰ for elevated-14C litter plots.

“The plant material picked out included leaves and some roots and had D 14 C values ranged from 177e195& for low-14 C litter plots to 319e384& for elevated-14 C litter plots.”

May combine two claims.

Gram-negative bacteria tended to have higher Δ14C values compared to the average, and declined less over the 8-month period.

“Gram-negative bacteria tended to have higher D 14 C values compared to the average, and declined less over the 8-month period.”

• AssumptionRests on an assumption: Saturated PLFA, especially branched compounds, are generally associated with Gram-positive bacteria, while monounsaturated and cyclopropyl-substituted PLFA are associated with Gram-negative bacteria.

Probably not a substantial claim: may not pass S3 (load-bearing).

Replicate and duplicate analyses fell within the 2-sigma error of the calculated mean for all but one of the compounds (WB3; 18cy).

“While the quoted errors are large, especially for small samples, the replicate and duplicate analyses fall within the 2sigma error of the calculated mean for all but one of the compounds (WB3; 18cy; Table 1).”

Probably not a substantial claim: may not pass S3 (load-bearing).

There is some indication from the microbial respiration data that a larger contribution of C (up to 8-12%) occurs in spring compared to later in the growing season.

“There is some indication from the microbial respiration data that a larger contribution of C (up to 8e12%) occurs in spring (Fig. 3 and Cisneros Dozal et al., 2006) compared to later in the growing season.”

• cites Cisneros Dozal et al., 2006Not checked yet

  Not checked in the free run.

May combine two claims.

Differences between individual PLFA biomarkers measured in this study were small and mostly not significant.

“However, the differences between individual biomarkers measured in this study were small and mostly not significant.”

Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

Mean values of Gram-negative bacteria and the fungal PLFA marker 18:2ω6 tend to be greater than Gram-positive PLFA values regardless of treatment, though differences are mostly not significant.

“Mean values of Gram-negative bacteria and the fungal PLFA marker 18:2 u6 tend to be greater than Gram-positive PLFA values regardless of treatment (Table 2), though differences are mostly not significant.”

• AssumptionRests on an assumption: Saturated PLFA, especially branched compounds, are generally associated with Gram-positive bacteria, while monounsaturated and cyclopropyl-substituted PLFA are associated with Gram-negative bacteria.
• AssumptionRests on an assumption: The PLFA 18:2ω6 is characteristic for fungi.

May combine two claims.

Errors were not different between duplicate and replicate samples, and the standard deviation associated with the mean of the replicates was of similar magnitude to the error for each sample.

“Errors were not different between duplicate and replicate samples, and the standard deviation associated with the mean of the replicates was of similar magnitude to the error for each sample.”

Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

The amount of 14C-free C added by the PCGC varied from near zero to ~2 μg C, though the amount was not predictable from the number of injections.

“The amount of 14 C-free C added by the PCGC varied from near zero to w2 mg C, though the amount was not predictable from the number of injections.”

May combine two claims.

Measures of two proxies for microbial C sources in mineral soil both show that <10% of the C in or respired by microbes is derived from surface leaf litter that had been manipulated in 14C content; the manipulations took place either 1-4 years (EBIS plots) or <1 year (mesocosms) ago.

“Measures of two proxies for microbial C sources in mineral soil both show that <10% of the C in or respired by microbes is derived from surface leaf litter that had been manipulated in 14 C content; the manipulations took place either 1e4 years (EBIS plots) or <1 year (mesocosms) ago.”

May combine two claims.

Depending on the compound isolated, there is either no difference in the 14C of PLFA over the course of 8 months, or there is a decline of up to 40-60‰.

“Depending on the compound isolated, there is either no difference in the 14 C of PLFA over the course of 8 months, or there is a decline of up to 40e60& (Figs. 3 and 4).”

May combine two claims.

Differences in PLFA result in F values <12%, most are not different from zero given the large analytical errors.

“Differences in PLFA result in F values <12% (Table 2), most are not different from zero given the large analytical errors.”

May combine two claims.

Both measures of microbial C source 14C signatures-microbial respiration and individual PLFA biomarkers-show either small or no differences between high- and low-14C litter addition plots.

“Both measures of microbial C source 14 C signatures e microbial respiration and individual PLFA biomarkers e show either small or no differences between high-and low-14 C litter addition plots.”

May combine two claims.

Although the 14C associated with labeled litter additions resulted in a cumulative difference in the 14C of Oa horizons over three years, there was no apparent accumulation of litter-derived C products in soil A horizon microbes over time.

“Although the 14 C associated with labeled litter additions resulted in a cumulative difference in the 14 C of Oa horizons over three years, there was no apparent accumulation of litter-derived C products in soil A horizon microbes over time.”

May combine two claims.

Overall, PLFA averaged 202 ± 19‰ (low-14C) and 236 ± 56‰ (high-14C) for the Haw Ridge sites, not distinguishable from values at the Walker Branch site of 208 ± 39‰ (low-14C) and 215 ± 44‰ (high-14C).

“Overall, PLFA averaged 202 Æ 19& (low-14 C) and 236 Æ 56& (high-14 C) for the Haw Ridge sites, not distinguishable from values at the Walker Branch site of 208 Æ 39& (low-14 C) and 215 Æ 44& (high-14 C).”

Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

There is an overall increasing trend in the 14C signature over time for both plots, though in 2003/2004 this is complicated by more 14C releases at the ORR sites.

“There is an overall increasing trend in the 14 C signature over time for both plots, though in 2003/2004 this is complicated by more 14 C releases at the ORR sites.”

May combine two claims.

The 14C signature of the PLFA sampled in 2005 average 200-220‰, while the litter that fell in autumn of 2004 had Δ14C of 110-120‰ in both plots.

“The 14 C signature of the PLFA sampled in 2005 (Fig. 1) average 200e220&, while the litter that fell in autumn of 2004 had D 14 C of 110e120& in both plots.”

May combine two claims.

Differences between elevated- and low-14C litter addition plots sampled in August 2005 ranged from <20-30‰ in bulk (plant litter removed) soils from which PLFA were extracted to ~120-190‰ for overlying O horizon material, from an original difference of ~750‰ between litter treatments.

“Differences between elevated-and low-14 C litter addition plots sampled in August 2005 thus ranged from <20e30& in the bulk (plant litter removed) soils from which PLFA were extracted to w120e190& for the overlying O horizon material, from an original difference of w750& between litter treatments (Fig. 1).”

May combine two claims.

Complexities of vertical dissolved C transfer, especially sorption and desorption in the O horizon, alter the nature and age of DOC entering mineral soil and may delay the appearance of litter-derived C in A horizon organic matter for up to several years.

“Complexities associated with the vertical transfer of dissolved C within soil profiles, especially significant sorption and desorption within the O horizon, alters the nature and age of DOC that enters the mineral soil (Fröberg et al., 2003), and may add a time lag to delay the appearance of litter-derived C in A horizon organic matter for up to several years.”

• cites Fröberg et al., 2003Not checked yet

  Not checked in the free run.

Classified as probably not a substantial claim.

Carbon isotopic signatures of PLFA have been used to identify microbial C sources in a number of studies.

“Carbon isotopic signatures of PLFA have been used to identify microbial C sources in a number of studies (Bird et al., 2008;Brant et al., 2006b;Denef et al., 2009;Kramer and Gleixner, 2006;Moore-Kucera and Dick, 2008;Rethemeyer et al., 2004;Rubino et al., 2009;Waldrop et al., 2000;Waldrop and Firestone, 2006).”

• cites Bird et al., 2008Not checked yet

  Not checked in the free run.

• cites Brant et al., 2006bNot checked yet

  Not checked in the free run.

• cites Denef et al., 2009Not checked yet

  Not checked in the free run.

Classified as probably not a substantial claim. May combine two claims.

Mass balancing of C and 14C inputs and losses suggested that sequestration of DOC in the A horizon would have increased Δ14C in soil organic matter by only about 5‰, too small to detect.

“Mass balancing of both C and 14 C inputs and losses suggested that sequestration of DOC in the A horizon would have increased the D 14 C in soil organic matter by only about 5&.Such a small change was not large enough to detect (Fröberg et al., 2009).”

• cites Fröberg et al., 2009Not checked yet

  Not checked in the free run.

May combine two claims.

Large contributions of C from litter <1 year old or >4 years old to microbial PLFA are not supported.

“Large contributions of C from litter < 1 year (or >4 years) old (which fell after (or prior to) the manipulation and therefore did not differ between plots) are not supported because the 14 C signatures of the PLFA compounds (averaging 200e220&) is much higher that of the 2004e5 leaf litter (115&) or pre-2000 litter.”

Classified as probably not a substantial claim. May combine two claims.

As defined in Eq. (3), F indicates only the influence of leaf litter carbon that is >1 up to 4 years old in the PLFA sampled from the EBIS plots and for microbially respired CO2.

“As defined in Eq. ( 3), F indicates only the influence of leaf litter carbon that is >1 up to 4 years old in the PLFA we sampled from the EBIS plots; for microbially respired CO 2 .”

Classified as probably not a substantial claim.

Modern and fossil 14C contamination introduced in combustion and graphitization was assessed by measuring samples of different sizes derived from an oxalic acid standard or coal.

“We assessed the amount of 'modern' and 'fossil' (i.e.radiocarbon-free) 14 C contamination introduced in the combustion and graphitization procedures by measuring a series of samples of different sizes (spanning 5 mg to 1 mg) derived from a combusted and graphitized oxalic acid standard (modern) or coal ( 14 C-free material).”

Probably not a substantial claim: may not pass S3 (load-bearing).

The precision and accuracy for measurements of graphite targets by AMS are determined by measuring known-age standards; for standards that are not small samples, these are ±2-3‰ at the WM Keck Carbon Cycle AMS.

“The precision and accuracy for measurements of graphite targets by AMS are determined by measuring of known-age standards e for standards that are not small samples, these are Æ2e3& at the WM Keck Carbon Cycle AMS (Xu et al., 2007).”

• cites Xu et al., 2007Not checked yet

  Not checked in the free run.

Classified as probably not a substantial claim. May combine two claims.

PLFA are compounds found in living microbial cell walls and are frequently used as characteristic biomarkers for microbial community structure.

“Phosphlipid Fatty Acids (PLFA) are compounds found in living microbial cell walls and are frequently used as characteristic biomarkers for microbial community structure (see review by Leckie, 2005).”

• cites Leckie, 2005Not checked yet

  Not checked in the free run.

May combine two claims.

The F calculation ignores that litterfall in 2004 and prior to 2000 did not differ in 14C between plots, so it could be underestimating the contribution of recent (<1 year) or pre-treatment (>4 years) litter C to PLFA-C.

“This ignores the fact that litterfall in 2004 (and prior to 2000) did not differ in its 14 C signature between the two plots.We are thus not strictly calculating the only from leaf litter added in the experiment e and therefore could be underestimating the contribution of recent (<1 year) or pre-treatment (>4 years) litter C to PLFA-C.”

May combine two claims.

Much of the bulk organic matter is in forms that turn over slowly and may therefore mask changes in more labile C fractions in the total C pool.

“However, much of the bulk organic matter is in forms that turn over slowly, and may therefore mask changes in more labile C fractions in the total C pool.”

Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

The 2004 litterfall 14C values are higher than the background atmosphere in 2004, reflecting either 14C residual in plant storage pools and/or additional 14C releases at the sites.

“These values are higher than the background atmosphere in 2004, reflecting either 14 C residual in plant storage pools (Gaudinski et al., 2009), and/or additional 14 C releases at the sites.”

• cites Gaudinski et al., 2009Not checked yet

  Not checked in the free run.

May combine two claims.

There was no measurable change in the 14C of A horizon soil C in Fröberg et al. (2009), because overall DOC input to the A horizon was small compared to C stocks.

“There was no measurable change in the 14 C of A horizon soil C, however, as the overall DOC input to the A horizon was small compared to C stocks.”

• cites Fröberg et al., 2009Not checked yet

  Not checked in the free run.

May combine two claims.

Fröberg et al. (2009) showed that DOC was isotopically altered but not retained in the O horizon, and that some high-14C C derived from surface litter was sequestered in the A horizon.

“That study showed that DOC was isotopically altered (but not retained) in the O horizon, and that some of the high-14 C C derived from surface litter was sequestered in the A horizon.”

• cites Fröberg et al., 2009Not checked yet

  Not checked in the free run.

May combine two claims.

Radiocarbon signatures of plant material picked out of the soil used for PLFA analysis were similar to those of the O horizon and likely represent O horizon material incorporated into the very large sample.

“Radiocarbon signatures of plant material picked out of the soil used for PLFA analysis had 14 C signatures similar to those of the O horizon, and likely represent O horizon material that was incorporated into the very large sample.”

Classified as probably not a substantial claim.

Because the PCGC correction factor varied with time and the cause was unknown, uncertainty in this correction factor was increased for each sample wheel run until all standards overlapped at 1-sigma with the radiocarbon value measured for a 1 mg C sample of the same standard not passed through the PCGC.

“Because the correction factor for the PCGC varied with time and we were unable to ascertain why, we increased the uncertainty in this correction factor for each sample wheel run until all standards overlapped at the 1-sigma level with the radiocarbon value measured for a 1 mg C sample of the same standard that was not passed through the PCGC (Santos et al., 2007;in press).”

• cites Santos et al., 2007;in pressNot checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing).

A picture is emerging in which root-derived C is apparently the predominant source of mineral soil organic matter in forest soils.

“Nonetheless, a picture is emerging in which rootderived C is apparently the predominant source of mineral soil organic matter in forest soils (Bird and Torn, 2006;Brant et al., 2006b;Swanston et al., 2005).”

• cites Bird and Torn, 2006Not checked yet

  Not checked in the free run.

• cites Brant et al., 2006bNot checked yet

  Not checked in the free run.

• cites Swanston et al., 2005Not checked yet

  Not checked in the free run.

May combine two claims.

Recent studies using differential isotope labeling suggest that downward transport of litter C is not a major source of C in soil A horizon microbial biomarkers, heterotrophically respired CO2-C, or soil organic matter C sources.

“Recent studies taking advantage of differential isotope labeling of these sources in forest soils suggest that downward transport of litter C is not a major source of C in soil A horizon microbial biomarkers (Bird et al., 2008;Brant et al., 2006a,b), heterotrophically respired CO 2 -C (Cisneros Dozal et al., 2006), or soil organic matter C sources (Fröberg et al., 2007;Hanson et al., 2005;Swanston”

• cites Bird et al., 2008Not checked yet

  Not checked in the free run.

• cites Brant et al., 2006a,bNot checked yet

  Not checked in the free run.

• cites Cisneros Dozal et al., 2006Not checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

It is possible that the EBIS plot PLFA results underestimate the overall contribution from leaf litter C because there was no difference in 14C for leaf litter that is <1 year old or >4 years old, and this could be diluting the difference between plots.

“It is thus possible that the EBIS plot PLFA results underestimate the overall contribution from leaf litter C because there was no difference in 14 C for leaf litter that is <1 year old or >4 years old, and this could be diluting the difference between plots.”

Probably not a substantial claim: may not pass S3 (load-bearing). The quote only partly states this claim.

Samples with very low-14C values that showed clear excess CO2 yield on combustion were excluded from the data set.

“These samples (normally very small ones, e.g.<30 mg C) always had very low-14 C values and have been excluded from the data set.”

Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

The authors have chosen to report data with large analytical errors; duplicate and replicate analyses indicate that these large errors are appropriate.

“We have thus chosen to report data with large analytical errors; duplicate and replicate analyses indicate that these large errors are appropriate (Table 1).”

Probably not a substantial claim: may not pass S3 (load-bearing).

Due to very large errors calculated for samples under 10 micrograms C in size (up to ±120‰), those samples were excluded from subsequent analyses.

“Due to the very large errors calculated for samples under 10 micrograms C in size (up to Æ120&), we have excluded them from subsequent analyses.”

May combine two claims.

While 14C signatures of PLFA were between those of bulk organic matter from the Oa and A horizons, the lack of a difference in PLFA-14C between high- and low-14C treatments in the litter addition experiment limits contribution from the Oa horizon to be <33% of microbial C.

“While 14 C signatures of PLFA were between those of bulk organic matter from the Oa and A horizons, the lack of a difference in PLFA-14 C between high-and low-14 C treatments in our litter addition experiment limits contribution form the Oa horizon to be <33% of microbial C.”

May combine two claims.

F may underestimate overall leaf litter contribution because there was no difference in 14C in litter that fell in the fall of 2004, prior to sample collection, or in the years prior to the litter manipulation experiment.

“F may underestimate overall leaf litter contribution because there was no difference in 14 C in for litter that fell in the fall of 2004, prior to our sample collection, or in the years prior to our litter manipulation experiment.”

May combine two claims.

While the authors cannot completely rule out the possibility that the EBIS plot results underestimate the contribution of C from leaf litter that fell before (>4 years) or after (<1 years) the experimental period, three additional lines of evidence make a major contribution from such a source unlikely.

“While we cannot completely rule out the possibility that our EBIS plot results underestimate the contribution of C from leaf litter that fell before (>4 years) or after (<1 years) the experimental period, three additional lines of evidence make a major contribution from such a source unlikely.”

May combine two claims.

Given that root inputs were excluded in the mesocosm experiment, the decline in C content in the A horizon, and in radiocarbon signature of PLFA, tend to corroborate the idea that roots, rather than Oa horizon carbon, are the major source of C for microbes.

“Given that root inputs were excluded in the mesocosm experiment, the decline in C content in the A horizon, and in radiocarbon signature of PLFA, tend to corroborate the idea that roots, rather than Oa horizon carbon, are the major source of C for microbes.”

The quote only partly states this claim.

Analytical uncertainties associated with PLFA measurements limit interpretations of which microbial groups incorporated more surface litter C.

“However, analytical uncertainties associated with PLFA measurements limit these interpretations.”

May combine two claims.

While the Δ14C of leaf litter in 2000 was 245-260‰ at Walker Branch and 109-195‰ at Haw Ridge, the Δ14C of leaf litter that fell in prior years was much lower, closer to 100‰.

“While the D 14 C of leaf litter in 2000 was 245e260& at Walker Branch and 109e195& at Haw Ridge (leaf litter in this part of the ORR was the source for the low-14 C litter addition), the D 14 C of leaf litter that fell in prior years was much lower, closer to 100& (Hanson et al., 2005).”

• cites Hanson et al., 2005Not checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing).

The authors experienced trouble in developing the PLFA analysis with the incomplete evaporation of solvents used to transfer the purified compounds.

“We experienced some trouble in developing our PLFA analysis with the incomplete evaporation of solvents used to transfer the purified compounds.”

May combine two claims.

The lack of a strong signal from recent leaf litter in microbial respiration (in years when <1 year old leaf litter had very different 14C signatures), and the lack of a litter-derived 14C signature in the mesocosms in the 8 months following litter addition, indicate that a major contribution from <1 year litter is unlikely.

“The lack of a strong signal from recent leaf litter in microbial respiration (in years when <1 year old leaf litter had very different 14 C signatures), and the lack of a litter-derived 14 C signature in the mesocosms in the 8 months following litter addition, indicate that a major contribution from <1 year litter is unlikely.”

Probably not a substantial claim: may not pass S3 (load-bearing).

Single compound radiocarbon measurements, particularly of very small (<30 mg C) samples, have large associated uncertainties because of contamination of the sample when processed by the PCGC.

“Single compound radiocarbon measurements, particularly of very small (<30 mg C) samples, have large associated uncertainties because of contamination of the sample when processed by the PCGC.”

Probably not a substantial claim: may not pass S3 (load-bearing).

Samples that when combusted yielded more C than expected based on PLFA peak areas and number of injections were excluded as likely contaminated with residual solvent.

“We also excluded samples which when combusted yielded more C than expected based on PLFA peak areas and the number of injections as likely contaminated with residual solvent (see Discussion).”

May combine two claims.

The lack of difference in PLFA between plots cannot be exclusively due to incorporation of <1 year leaf litter carbon; if current year leaf litter were a major source of PLFA-C, another source of PLFA-C with much higher 14C signatures than measured in the other soil components would be needed.

“Hence the lack of difference in PLFA between plots cannot be exclusively due to incorporation of <1 year leaf litter carbon; indeed if current year leaf litter were a major source of PLFA-C, we would need to find another source of PLFA-C that had much higher 14 C signatures than we measure in the other soil components (see Fig. 1).”

May combine two claims.

While DOC carbon is sequestered in the mesocosm A horizons during this time period, it is not primarily derived from recent leaf litter given the decline or lack of change in 14C in microbial biomarkers.

“While DOC carbon is sequestered in the mesocosm A horizons during this time period (Fröberg et al., 2009), it is not primarily derived from recent leaf litter given the decline or lack of change in 14 C in microbial biomarkers.”

• cites Fröberg et al., 2009Not checked yet

  Not checked in the free run.

May combine two claims.

In both treatment plots, the 14C signature of fine roots reported by Joslin et al. (2006) is closest to the 14C signatures recorded in the PLFA compounds, suggesting that roots are the major source of C in the mineral soil microbial community.

“In both treatment plots, the 14 C signature of fine roots reported by Joslin et al. (2006) is closest to the 14 C signatures recorded in the PLFA compounds, suggesting that roots are the major source of C in the mineral soil microbial community (Fig. 3).”

• cites Joslin et al. (2006)Not checked yet

  Not checked in the free run.

May combine two claims.

Carbon contents in the mesocosm A horizons declined during the experiment, though there was no measurable change in the radiocarbon signature of bulk A horizon organic matter.

“Carbon contents in the mesocosm A horizons declined during the experiment (Fröberg et al., 2009), though there was no measurable change in the radiocarbon signature of bulk A horizon organic matter.”

• cites Fröberg et al., 2009Not checked yet

  Not checked in the free run.

May combine two claims.

The radiocarbon content of DOC draining litter and organic layers and entering the A horizon was 406‰ in April and 531‰ in November.

“The radiocarbon content of DOC draining litter and organic layers and entering the A horizon was 406& in April (after labeled litter addition and following our March sampling) and 531& in November (our second sampling; Fig. 3).”

• cites Fröberg et al., 2009Not checked yet

  Not checked in the free run.