Mineral-associated organic matter (MAOM) is considered a stable reservoir for soil nutrients that influences long-term soil carbon (C) and nitrogen (N) dynamics. However, recent experimental and ...theoretical evidence shows that root exudates may mobilize MAOM, thereby providing plants and microbes access to a large and N-rich pool. Given the mechanisms underlying MAOM C and N mobilization remain largely untested, we examined direct and indirect pathways by which root exudates destabilize this nutrient pool in laboratory mesocosms. We simulated root exudation with 13C-labeled oxalic acid to test whether root exudates are directly capable of mobilizing MAOM from mineral surfaces; and with 13C-labeled glucose to test whether indirect stimulation of microbial and extracellular enzyme activity leads to MAOM decomposition. We also tested the potential for oxalic acid and glucose to mobilize MAOM in an additional subset of sterilized soils to clarify the potential for non-microbial pathways of MAOM destabilization.
Over the course of the 12-day MAOM incubation with and without simulated exudates, we measured C cycling (CO2 respiration rates, 13C–CO2 efflux), N cycling (inorganic N pools, gross N mineralization) and related microbial processes (enzyme activities and microbial community composition via phospholipid fatty acid analysis). Both of the simulated root exudates enhanced MAOM-C mineralization, with cumulative respiration increasing 35–89% relative to the water-only control. Likewise, glucose additions enhanced the production of an exo-cellulase and a chitinase by up to 130% and 39%, respectively, while oxalic acid enhanced oxidative enzyme activities up to 91% greater than control rates. We observed a positive association between glucose-induced shifts in enzyme activities, MAOM-C mineralization, and gross ammonification. Oxalic acid additions were associated with initial increases in fungal relative abundance and in sterile soils appeared to stimulate the release of metals and dissolved organic nitrogen into exchangeable pools. Our results indicate that common root exudates, like glucose and oxalic acid, can significantly increase the turnover and potential release of C and N from MAOM through indirect (e.g., enzyme induction) and direct (e.g., mobilization of metal oxides) mechanisms.
•Simulated root exudates mobilized C and N within mineral-associated organic matter.•Glucose and oxalic acid additions enhanced MAOM-C mineralization.•Glucose additions facilitated microbial, enzyme-mediated mobilization of MAOM.•Oxalic acid additions associated with release of organic N and metals from MAOM.
Atmospheric nitrogen (N) deposition is enriching soils with N across biomes. Soil N enrichment can increase plant productivity and affect microbial activity, thereby increasing soil organic carbon ...(SOC), but such responses vary across biomes. Drylands cover ~45% of Earth's land area and store ~33% of global SOC contained in the top 1 m of soil. Nitrogen fertilization could, therefore, disproportionately impact carbon (C) cycling, yet whether dryland SOC storage increases with N remains unclear. To understand how N enrichment may change SOC storage, we separated SOC into plant‐derived, particulate organic C (POC), and largely microbially derived, mineral‐associated organic C (MAOC) at four N deposition experimental sites in Southern California. Theory suggests that N enrichment increases the efficiency by which microbes build MAOC (C stabilization efficiency) if soil pH stays constant. But if soils acidify, a common response to N enrichment, then microbial biomass and enzymatic organic matter decay may decrease, increasing POC but not MAOC. We found that N enrichment had no effect on C fractions except for a decrease in MAOC at one site. Specifically, despite reported increases in plant biomass in three sites and decreases in microbial biomass and extracellular enzyme activities in two sites that acidified, POC did not increase. Furthermore, microbial C use and stabilization efficiency increased in a non‐acidified site, but without increasing MAOC. Instead, MAOC decreased by 16% at one of the sites that acidified, likely because it lost 47% of the exchangeable calcium (Ca) relative to controls. Indeed, MAOC was strongly and positively affected by Ca, which directly and, through its positive effect on microbial biomass, explained 58% of variation in MAOC. Long‐term effects of N fertilization on dryland SOC storage appear abiotic in nature, such that drylands where Ca‐stabilization of SOC is prevalent and soils acidify, are most at risk for significant C loss.
We sampled soils at four long‐term N‐fertilization sites to understand how atmospheric N deposition may affect SOC dynamics in Southern Californian drylands under acidifying versus non‐acidifying conditions. Non‐acidified soils were expected to build soil organic carbon (SOC) via increased plant growth and microbial C stabilization efficiency, but they did not accumulate C. In contrast, acidified soils lost Ca that destabilized mineral‐associated organic C in one of the sites, suggesting that long‐term effects of N fertilization on dryland C storage are of abiotic nature, such that drylands where Ca‐stabilization of SOC is prevalent may be most at risk for significant C losses.
Carbon sequestration in agricultural soils can play a pivotal role in the mitigation of accelerating climate change. Our research evaluated a continuum of agricultural cropping systems, including ...innovative perennial grain cropping, to assess which systems promote increases in and stabilization of soil organic matter (SOM). In comparison with conventional annual cropping systems, perennial grain cropping may be conducive to increased C accrual resulting from no-tillage management, longer growing seasons, and extensive root growth associated with these novel systems. Furthermore, the effects of N fertilizer addition on SOM dynamics under contrasting cropping systems were examined. We conducted physical SOM fractionation into particulate (POM) and mineral-associated organic matter (MAOM) in samples taken over two years from two experimental sites in Central Alberta, Canada. Five contrasting cropping systems (perennial-forage, perennial-grain, fall-grain, spring-grain and fallow) both with and without N fertilizer were tested. Our findings demonstrate that perennial-grain cropping was consistently superior in sequestering SOM-C compared to annual-grain crops at the surface soil layer (0–15 cm depth, Ps< 0.05). Over the duration of this experiment, perennial-grain cropping considerably boosted C accumulation in the recalcitrant SOM pools as represented by increasing MAOM, particularly at the Edmonton site, which is characterized by a clay-rich, Black Chernozemic soil (MAOM: 41.5 and 45.3 g C kg−1 in annual and perennial crops, respectively, P < 0.05). However, recurrent N fertilizer additions diminished C sequestration by perennial-grain cropping in both POM and MAOM fractions (Ps< 0.05). Correlation analysis indicated that accrual and allocation of C within the soil profile was more closely related to aboveground crop biomass productivity as opposed to root growth, particularly for generating more intermediate-labile POM. Our results shed light on how to achieve greater soil C sequestration as a function of cropping system options, N fertilizer addition and underlying soil texture.
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•Perennial grain crops enhance C accrual into labile and stable organic matter pools.•N fertilizer addition attenuates soil C sequestration in perennial grain crops.•Particulate organic matter C in surface soil is related to aboveground biomass C.
Managing ecosystems to sequester soil carbon requires a thorough understanding of complex soil processes. Here, we integrate these soil processes through the metaphor of a game—one that moves through ...multiple dimensions (from macro‐aggregates to micropores and clay particles) and scales (from centimeters to nanometers) of the soil. The rules of the game are based on current understanding of soil carbon persistence, which differs from the classic humus concept of molecular complexity. The game's objective is to win points, by keeping “tokens” (plant‐derived organic compounds) within the soil organic matter for as long as possible. The game begins when tokens enter different “pool‐levels” (plant litter, particulate organic matter, dissolved organic matter, and mineral‐associated organic matter) of the soil, either directly or after metabolic transformation by soil biota. Points are lost through either respiration by soil biota or leaching. We invite readers to play this game and explore different natural ecosystems and land‐use scenarios to better comprehend complex soil processes.
We investigated the bio- and photo-lability of dissolved organic matter (DOM) from the head, mixing zone, and mouth of the Pearl River estuary. At all three sites, bio- and photo-refractory dissolved ...organic carbon (DOC) and biorefractory chromophoric DOM (CDOM) dominated over the corresponding bio- and photo-labile constituents, while photolabile CDOM dominated over photo-refractory CDOM. Relative to the mixing-zone and mouth waters, the headwater was enriched with bio- and photo-labile DOC and photolabile CDOM and depleted with biolabile CDOM. Biolabile DOC was richer than photolabile DOC in the headwater, while photolabile CDOM was richer than biolabile CDOM at all three sites. Pre-biotransformation inhibited, stimulated, or had little impact on DOM photodegradation, depending on site. Ultra-violet absorption coefficients are indicators of bio- and photo-refractory DOC. The relative proportions of transparent and chromophoric DOM control the turnover of biolabile DOC and the effect of pre-biotransformation on DOM photodegradation.
•Bio- and photo-labile dissolved organic matter fractions changes along estuary.•Biodegradation preferentially consumes transparent dissolved organic matter.•Photodegradation mainly consumes colored dissolved organic matter.•UV absorbance is a proxy for bio- and photo-degradability of dissolved organic matter.•Effect of pre-biotransformation on photolability varies along estuary.
Soil organic matter (OM) can be stabilized against decomposition by association with minerals, by its inherent recalcitrance and by occlusion in aggregates. However, the relative contribution of ...these factors to OM stabilization is yet unknown. We analyzed pool size and isotopic composition (¹⁴C, ¹³C) of mineral-protected and recalcitrant OM in 12 subsurface horizons from 10 acidic forest soils. The results were related to properties of the mineral phase and to OM composition as revealed by CPMAS ¹³C-NMR and CuO oxidation. Stable OM was defined as that material which survived treatment of soils with 6 wt% sodium hypochlorite (NaOCl). Mineral-protected OM was extracted by subsequent dissolution of minerals by 10% hydrofluoric acid (HF). Organic matter resistant against NaOCl and insoluble in HF was considered as recalcitrant OM. Hypochlorite removed primarily ¹⁴C-modern OM. Of the stable organic carbon (OC), amounting to 2.4-20.6 g kg⁻¹ soil, mineral dissolution released on average 73%. Poorly crystalline Fe and Al phases$(\text{Fe}_{\text{o}},\text{Al}_{\text{o}})$and crystalline Fe oxides$(\text{Fe}_{\text{d}-\text{o}})$explained 86% of the variability of mineral-protected OC. Atomic$\text{C}_{\text{p}}/(\text{Fe}+\text{Al})_{\text{p}}$ratios of 1.3-6.5 suggest that a portion of stable OM was associated with polymeric Fe and Al species. Recalcitrant OC (0.4-6.5 g kg⁻¹ soil) contributed on average 27% to stable OC and the amount was not correlated with any mineralogical property. Recalcitrant OC had lower Δ¹⁴C and δ¹³C values than mineral-protected OC and was mainly composed of aliphatic (56%) and O-alkyl (13%) C moieties. Lignin phenols were only present in small amounts in either mineral-protected or recalcitrant OM (mean 4.3 and 0.2 g kg⁻¹ OC). The results confirm that stabilization of OM by interaction with poorly crystalline minerals and polymeric metal species is the most important mechanism for preservation of OM in these acid subsoil horizons.
Microorganisms regulate soil organic matter (SOM) formation through accumulation and decomposition of microbial necromass, which is directly and indirectly affected by elevated CO
2
and N ...fertilization. We investigated the role of microorganisms in SOM formation by analyzing
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C recovery in microorganisms and carbon pools in paddy soil under two CO
2
levels, with and without N fertilization, after continuous
13
CO
2
labelling was stopped. Microbial turnover transferred
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C from living microbial biomass (determined by the decrease in phospholipid fatty acids) to necromass (determined by the increase in amino sugars).
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C incorporation in fungal living biomass and necromass was higher than that in bacteria. Bacterial turnover was faster than necromass decomposition, resulting in net necromass accumulation over time; fungal necromass remained stable. Elevated CO
2
and N fertilization increased the net accumulation of bacterial, but not fungal, necromass. CO
2
levels, but not N fertilization, significantly affected
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C incorporation in SOM pools. Elevated CO
2
increased
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C in particulate organic matter via the roots, and in the mineral-associated organic matter (MAOM) via bacterial, but not fungal, necromass. Overall, bacterial necromass plays a dominant role in the MAOM formation response to elevated CO
2
because bacteria are sensitive to elevated CO
2
.
Tidal wetlands sequester vast amounts of organic carbon (OC) and enhance soil accretion. The conservation and restoration of these ecosystems is becoming increasingly geared toward “blue” carbon ...sequestration while obtaining additional benefits, such as buffering sea‐level rise and enhancing biodiversity. However, the assessments of blue carbon sequestration focus primarily on bulk SOC inventories and often neglect OC fractions and their drivers; this limits our understanding of the mechanisms controlling OC storage and opportunities to enhance blue carbon sinks. Here, we determined mineral‐associated and particulate organic matter (MAOM and POM, respectively) in 99 surface soils and 40 soil cores collected from Chinese mangrove and saltmarsh habitats across a broad range of climates and accretion rates and showed how previously unrecognized mechanisms of climate and mineral accretion regulated MAOM and POM accumulation in tidal wetlands. MAOM concentrations (8.0 ± 5.7 g C kg−1) (±standard deviation) were significantly higher than POM concentrations (4.2 ± 5.7 g C kg−1) across the different soil depths and habitats. MAOM contributed over 51.6 ± 24.9% and 78.9 ± 19.0% to OC in mangrove and saltmarsh soils, respectively; both exhibited lower autochthonous contributions but higher contributions from terrestrial or marine sources than POM, which was derived primarily from autochthonous sources. Increased input of plant‐derived organic matter along the increased temperature and precipitation gradients significantly enriched the POM concentrations. In contrast, the MAOM concentrations depended on climate, which controlled the mineral reactivity and mineral–OC interactions, and on regional sedimentary processes that could redistribute the reactive minerals. Mineral accretion diluted the POM concentrations and potentially enhanced the MAOM concentrations depending on mineral composition and whether the mineral accretion benefited plant productivity. Therefore, management strategies should comprehensively consider regional climate while regulating sediment supply and mineral abundance with engineering solutions to tap the OC sink potential of tidal wetlands.
Tidal wetlands sequester organic carbon (OC) and enhance soil accretion, aligning with the growing focus on “blue” carbon sequestration. Despite this, assessments often overlook OC fractions and their drivers, limiting our understanding of OC storage mechanisms. In our study spanning Chinese mangrove and saltmarsh habitats, we showed that elevated temperature and precipitation enriched particulate organic matter (POM), while mineral‐associated organic matter (MAOM) relied on climate and regional sedimentary processes. Mineral accretion diluted POM and potentially enhanced MAOM based on mineral composition and its impact on plant productivity. Managing tidal wetlands as OC sinks should consider regional climate and control sediment and mineral abundance for effective conservation.
The stability and decomposition of biochar are fundamental to understand its persistence in soil, its contribution to carbon (C) sequestration, and thus its role in the global C cycle. Our current ...knowledge about the degradability of biochar, however, is limited. Using 128 observations of biochar‐derived CO2 from 24 studies with stable (13C) and radioactive (14C) carbon isotopes, we meta‐analyzed the biochar decomposition in soil and estimated its mean residence time (MRT). The decomposed amount of biochar increased logarithmically with experimental duration, and the decomposition rate decreased with time. The biochar decomposition rate varied significantly with experimental duration, feedstock, pyrolysis temperature, and soil clay content. The MRTs of labile and recalcitrant biochar C pools were estimated to be about 108 days and 556 years with pool sizes of 3% and 97%, respectively. These results show that only a small part of biochar is bioavailable and that the remaining 97% contribute directly to long‐term C sequestration in soil. The second database (116 observations from 21 studies) was used to evaluate the priming effects after biochar addition. Biochar slightly retarded the mineralization of soil organic matter (SOM; overall mean: −3.8%, 95% CI = −8.1–0.8%) compared to the soil without biochar addition. Significant negative priming was common for studies with a duration shorter than half a year (−8.6%), crop‐derived biochar (−20.3%), fast pyrolysis (−18.9%), the lowest pyrolysis temperature (−18.5%), and small application amounts (−11.9%). In contrast, biochar addition to sandy soils strongly stimulated SOM mineralization by 20.8%. This indicates that biochar stimulates microbial activities especially in soils with low fertility. Furthermore, abiotic and biotic processes, as well as the characteristics of biochar and soils, affecting biochar decomposition are discussed. We conclude that biochar can persist in soils on a centennial scale and that it has a positive effect on SOM dynamics and thus on C sequestration.
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