DOI: 10.5061/dryad.bcc2fqzqv
title: Iron-driven fast decomposition of soil carbon under periodic anoxia

description: Soil organic carbon (SOC) decomposition underpins soil-atmosphere carbon
 exchange and is regulated by climate change-mediated variations in soil
 redox conditions. Periodic anoxia, commonly occurring following
 precipitation, soil flooding and erosion events, is assumed to preserve
 SOC. Yet, water saturation may also increase SOC decomposition relative to
 unsaturated conditions, and contradictory findings among previous studies
 remain unexplained. Here, using incubation experiments on 20 soils
 collected across a 24° latitude gradient in China, we show that 70% of the
 soils showed higher or similar anoxic decomposition rate of SOC compared
 to oxic treatment, indicating fast SOC loss under relatively short anoxia.
 Methane production was far lower than CO2 due to the presence of
 alternative terminal electron acceptors (TEAs). Variation in alternative
 TEAs and microbial community shows that fast anoxic decomposition was
 primarily driven by iron (Fe) reduction, which accounted for up to 90% of
 anoxic CO2 production. Meanwhile, positive relationships among
 water-extractable organic carbon (OC), hydrochloric acid-extractable
 ferrous Fe, relative abundance of Fe-reducing prokaryotes, and SOC
 decomposition rate suggest the release of readily metabolized substrates
 following Fe reduction. This release provided substrates for anoxic
 metabolism and potentially leading to the loss of OC protected by Fe
 (Fe-bound OC; a slow-cycling OC pool under oxic conditions). Mass balance
 calculation confirms that Fe-bound OC loss was mostly similar to elevated
 anoxic SOC decomposition in magnitude, and random forest modeling
 indicates that soils rich in reducible Fe, SOC, and Fe-reducing
 prokaryotes most likely experience elevated SOC decomposition under
 periodic anoxia. Overall, our findings demonstrate that fast anoxic
 decomposition of SOC is a potentially important pathway that may stimulate
 SOC loss under climate change-mediated intense hydrologic regimes,
 particularly for soils rich in reducible Fe and SOC.
description: # Iron-driven fast decomposition of soil carbon under periodic anoxia ---
 We have submitted our data on the following: soil properties (**Dataset 1
 Soil properties of non-incubated soils.csv**), mean decomposition rate
 (**Dataset 2 Mean decomposition rate and response ratios within 90
 days.csv**), instantaneous decomposition rate (**Dataset 3 Instantaneous
 CO2 and CH4 production rates.csv**), alternative TEA variations (**Dataset
 4 Variation in alternative terminal electron acceptors and their reductive
 products.csv**), soil property variation (**Dataset 5 Variation in soil
 properties over time under different redox treatments.csv**), Fe-bound OC
 variation (**Dataset 6 Variation in Fe-bound OC after different redox
 treatments.csv**), random forest for rate response (**Dataset 7 Random
 forest for predicting mean RRCO2+CH4.csv**), random forest for Fe-bound OC
 loss (**Dataset 8 Random forest for predicting Fe-bound OC loss.csv**),
 and microbial communities (**Dataset_9_Microbial_community and their
 categorization_detected_in_each_soil.csv**). ## Descriptions ### Dataset
 1: Soil properties of non-incubated soils.csv Data * *Soil*: Currently
 valid scientific name * *Site name*: full name of each soil *
 *Vegetation*: grassland or forest * *soil type*: derived from the
 Harmonized World Soil Database * *SOC (%)*: soil organic carbon content in
 unit of % * *SIC (mg g–1 soil)*: soil inorganic carbon content (mg) per
 unit of soil (g) * *pH*: soil pH measured by pH meter * *Clay and silt
 (%)*: proportion of clay and silt (%) * *Fe-bound OC (mg C g–1 soil)*: OC
 content protected by iron (mg) per unit of soil (g) * *Nitrate (mg g–1
 soil)*: nitrate content (mg N) per unit of soil (g) * *MAT (°C)*: mean
 annual temperature (°C) * *Maximum temperature (°C)*: maximum temperature
 of each site (°C) * *MAP (mm)*: mean annual precipitation (mm) *
 *Precipitation of wettest month (mm)*: precipitation of wettest month (mm)
 * *PET of wettest month (mm)*: potential evapo-transpiration of wetted
 month (mm) * *WEOC (mg C g–1 soil)*: water-extractable organic carbon
 content (mg C) per unit of soil (g) * *Spectral slope ratio of WEOM*:
 indicating molecular weight of water-extractable organic matter detected
 by ultraviolet-visible spectra * *Fed (mg g–1 soil)*:
 dithionite-extractable iron content (mg) per unit of soil (g) * *Mnd (mg
 g–1 soil)*: dithionite-extractable manganese content (mg) per unit of soil
 (g) * *Relative abundance of alkyls (%)*: proportion of alkyls in all
 compounds detected by pyrolysis-gas chromatography/mass spectrometry *
 *Relative abundance of lignin (%)*: proportion of lignin in all compounds
 detected by pyrolysis-gas chromatography/mass spectrometry * *Relative
 abundance of lipids (%)*: proportion of lipids in all compounds detected
 by pyrolysis-gas chromatography/mass spectrometry * *Relative abundance of
 aromatics (%)*: proportion of aromatics in all compounds detected by
 pyrolysis-gas chromatography/mass spectrometry * *Relative abundance of
 N-containing compounds (%)*: proportion of N-containing compounds in all
 compounds detected by pyrolysis-gas chromatography/mass spectrometry *
 *Relative abundance of phenols (%)*: proportion of phenols in all
 compounds detected by pyrolysis-gas chromatography/mass spectrometry *
 *Relative abundance of polysaccharides (%)*: proportion of polysaccharides
 in all compounds detected by pyrolysis-gas chromatography/mass
 spectrometry * *Relative abundance of protein (%)*: proportion of protein
 in all compounds detected by pyrolysis-gas chromatography/mass
 spectrometry * *Relative abundance of iron-reducing prokaryotes (%)*:
 proportion of iron-reducing prokaryotes in all detected OTU * *Relative
 abundance of nitrate-reducing prokaryotes (%)*: proportion of
 nitrate-reducing prokaryotes in all detected OTU ### Dataset 2: Mean
 decomposition rate and response ratios within 90 days.csv Data * *Soil*:
 Currently valid scientific name * *Treatment*: incubation conditions
 *Oxic*: incubation under oxic conditions *Anoxic*: incubation under anoxic
 conditions * *Mean CO2 rate within 90 days (μg C g–1 soil day–1)*:
 cumulative CO2 production divided by 90 days * *Mean CH4 rate within 90
 days (μg C g–1 soil day–1)*: cumulative CH4 production divided by 90 days
 * *Response ratio of anoxic-to-oxic CO2+CH4 rate*: anoxic CO2+CH4 rate
 divided by oxic CO2+CH4 rate ### Dataset 3: Instantaneous CO2 and CH4
 production rates.csv Data * *Soil*: Currently valid scientific name *
 *Treatment*: incubation conditions *Oxic*: incubation under oxic
 conditions *Anoxic*: incubation under anoxic conditions * *Gas*: gas type
 *CO2*: carbon dioxide *CH4*: methane * *Time (day)*: gas sampling time *
 *Rate (μg C g–1 soil h–1)*: gas production rate (μg C) per unit of soil
 (g) per hour (h) ### Dataset 4: Variation in alternative terminal electron
 acceptors and their reductive products.csv Data * *Site*: Currently valid
 scientific name * *Time (Day)*: destructive sampling time * *Treatment*:
 incubation conditions *Oxic*: incubation under oxic conditions *Anoxic*:
 incubation under anoxic conditions * *Type of alternative TEAs or
 reductive products*: alternative TEA name *NO₃⁻*: nitrate *Mn(II)*:
 reductive products of tetravalent manganese *Fe(III)*: ferric iron
 *Fe(II)*: ferrous iron *SO₄²⁻*: sulfate * *Contents of Alternative TEAs or
 reductive products (μmol g–1 soil)*: content per unit of soil (g) ###
 Dataset 5: Variation in soil properties over time under different redox
 treatments.csv Data * *Site*: Currently valid scientific name *
 *Treatment*: incubation conditions *Oxic*: incubation under oxic
 conditions *Anoxic*: incubation under anoxic conditions * *Time (Day)*:
 destructive sampling time * *pH*: soil pH measured by pH meter * *ORP
 (mV)*: oxidative reduction potentials of soil (mV) * *WEOC (mg C g–1
 soil)*: water-extractable organic carbon content (mg C) per unit of soil
 (g) * *SR*: spectral slope ratio of WEOM, indicating molecular weight of
 water-extractable organic matter detected by ultraviolet-visible spectra *
 *Response ratio of WEOC*: WEOC content after anoxic incubation divided by
 its content after oxic incubation at a specific time in a specific soil *
 *Response ratio of SR*: spectral slope ratio of WEOM after anoxic
 incubation, divided by its value after oxic incubation at a specific time
 in specific soil * *Relative abundance of iron-reducing prokaryotes (%)*:
 proportion of iron-reducing prokaryotes in all detected OTU * *Relative
 abundance of nitrate-reducing prokaryotes (%)*: proportion of
 nitrate-reducing prokaryotes in all detected OTU * *ΔRelative abundance of
 iron-reducing prokaryotes (%)*: Relative abundance of iron-reducing
 prokaryotes after anoxic incubation minus relative abundance of
 iron-reducing prokaryotes in non-incubated soils * *ΔRelative abundance of
 nitrate-reducing prokaryotes (%)*: Relative abundance of nitrate-reducing
 prokaryotes after anoxic incubation minus relative abundance of
 nitrate-reducing prokaryotes in non-incubated soils ### Dataset 6:
 Variation in Fe-bound OC after different redox treatments.csv Data *
 *Site*: Currently valid scientific name * *Treatment*: incubation
 conditions *Oxic*: incubation under oxic conditions *Anoxic*: incubation
 under anoxic conditions * *Time (day)*: prolonged incubation for &gt;200
 days * *Fe-bound OC (mg g–1 soil)*: the content of OC protected by iron at
 the end of incubation * *Fe-bound OC loss (mg g–1 soil)*: Fe-bound OC
 content in non-incubated soils minus that after incubation for &gt;200
 days * *ΔCumulative CO2+CH4 (mg C g–1 soil)*: the cumulative CO2 and CH4
 production (mg C g−1 soil) estimated over an extended incubation period
 (&gt; 200 days) under the respective redox conditions. Gas sampling beyond
 90 days was reduced to once every 30–40 days due to logistical
 constraints, and cumulative CO2 + CH4 was estimated from these data to
 align with Fe-bound OC measurements. ### Dataset 7: Random forest for
 predicting mean RRCO2+CH4.csv Data * *LnRRCO2+CH4*: log-transformed
 response ratio of mean anoxic to oxic CO2+CH4 rate * *Fed (mg g–1 soil)*:
 dithionite-extractable iron content (mg) per unit of soil (g) * *Mnd (mg
 g–1 soil)*: dithionite-extractable manganese content (mg) per unit of soil
 (g) * *Nitrate (mg g–1 soil)*: nitrate content (mg N) per unit of soil (g)
 * Sulfate (mg g–1 soil): sulfate content (mg N) per unit of soil (g) *
 *Fe-bound OC (mg C g–1 soil)*: OC content protected by iron (mg) per unit
 of soil (g) * *SOC (%)*: soil organic carbon content in unit of % *
 *Clay+Silt (%))*: proportion of clay and silt (%) * *pH*: soil pH measured
 by pH meter * *MAT (°C)*: mean annual temperature (°C) * *MAP (mm)*: mean
 annual precipitation (mm) * *Lipids (%)*: proportion of lipids in all
 compounds detected by pyrolysis-gas chromatography/mass spectrometry *
 *Relative abundance of iron-reducing prokaryotes (%)*: proportion of
 iron-reducing prokaryotes in all detected OTU * *Relative abundance of
 nitrate-reducing prokaryotes (%)*: proportion of nitrate-reducing
 prokaryotes in all detected OTU ### Dataset 8: Random forest for
 predicting Fe-bound OC loss.csv Data * *Fed (mg g–1 soil)*:
 dithionite-extractable iron content (mg) per unit of soil (g) * *Mnd (mg
 g–1 soil)*: dithionite-extractable manganese content (mg) per unit of soil
 (g) * *Nitrate (mg g–1 soil)*: nitrate content (mg N) per unit of soil (g)
 * *Sulfate (mg g–1 soil)*: sulfate content (mg N) per unit of soil (g) *
 *Fe-bound OC (mg C g–1 soil)*: OC content protected by iron (mg) per unit
 of soil (g) * *SOC (%)*: soil organic carbon content in unit of % *
 *Clay+Silt (%))*: proportion of clay and silt (%) * *pH*: soil pH measured
 by pH meter * *MAT (°C)*: mean annual temperature (°C) * *MAP (mm)*: mean
 annual precipitation (mm) * *RRFe-bound OC loss*: equaling Fe-bound OC
 loss divided by Fe-bound OC in non-incubated soils * *Relative abundance
 of iron-reducing prokaryotes (%)*: proportion of iron-reducing prokaryotes
 in all detected OTU * *Relative abundance of nitrate-reducing prokaryotes
 (%)*: proportion of nitrate-reducing prokaryotes in all detected OTU ###
 Dataset 9: Microbial\_community and their
 categorization\_detected\_in\_each\_soil.csv Data * *Name in NCBI*: soil
 name in sequencing files saved in NCBI * *Soil*: Currently valid
 scientific name * *Time (days)*: time of sequencing analysis *
 *Treatment*: incubation conditions *Oxic*: incubation under oxic
 conditions *Anoxic*: incubation under anoxic conditions * *Replicate*:
 replicate names *A*: the first replicate *B*: the second replicate *C*:
 the third replicate * *Column F to Column CQH*: OTU number detected at
 genus level. Iron-reducing prokaryotes contain columns F to L Prokaryotes
 with both iron-reducing and nitrate-reducing functions contain columns M
 to N Nitrate-reducing prokaryotes contain columns O to AC NA indicates not
 applicable or not available due to sample depletion.