Soil Organic Matter Priming Research Articles

Soil organic matter priming: The pH effects.

Priming of soil organic matter (SOM) decomposition by microorganisms is a key phenomenon of global carbon (C) cycling. Soil pH is a main factor defining priming effects (PEs) because it (i) controls microbial community composition and activities, including enzyme activities, (ii) defines SOM stabilization and destabilization mechanisms, and (iii) regulates intensities of many biogeochemical processes. In this critical review, we focus on prerequisites and mechanisms of PE depending on pH and assess the global change consequences for PE. The highest PEs were common in soils with pH between 5.5 and 7.5, whereas low molecular weight organic compounds triggered PE mainly in slightly acidic soils. Positive PEs up to 20 times of SOM decomposition before C input were common at pH around 6.5. Negative PEs were common at soil pH below 4.5 or above 7 reflecting a suboptimal environment for microorganisms and specific SOM stabilization mechanisms at low and high pH. Short-term soil acidification (in rhizosphere, after fertilizer application) affects PE by: mineral-SOM complexation, SOM oxidation by iron reduction, enzymatic depolymerization, and pH-dependent changes in nutrient availability. Biological processes of microbial metabolism shift over the short-term, whereas long-term microbial community adaptations to slow acidification are common. The nitrogen fertilization induced soil acidification and land use intensification strongly decrease pH and thus boost the PE. Concluding, soil pH is one of the strongest but up to now disregarded factors of PE, defining SOM decomposition through short-term metabolic adaptation of microbial groups and long-term shift of microbial communities.

Quantifying apparent and real priming effects based on inverse labelling

Organic inputs to soils can accelerate soil organic matter (SOM) decomposition via so-called priming effects, but at the same time microbial biomass turnover can be accelerated – that will be termed as apparent priming effect. However, only a few studies have been set up to quantify the contribution of extra CO2 production from apparent priming. Here, we labeled the microbial biomass by 14C-glucose in pre-incubation and then added labile carbon (C; 12C or 14C glucose) and nitrogen (N; NH4+) in soil and incubated over 120 days to investigate the contribution of apparent priming to total SOM priming. After 120 days of pre-incubation, 48 % of added glucose was released as CO2, and 34 % of added glucose was recovered in microbial biomass. After glucose addition, microbial biomass and salt extractable organic C were similar between glucose and water addition. However, glucose addition increased the contribution of 14C-glucose to microbial biomass by 2.5-folds and to CO2 by 10-folds. This increased contribution of 14C-glucose indicated accelerated microbial biomass turnover by labile C. Furthermore, 10 and 47 μg C g−1 of previously added and incorporated into microbial biomass 14C and 12C were replaced by new 14C, which contributed to 10 % and 33 % of primed CO2 emissions, respectively. On the contrary, N addition reduced previously added 14C in both microbial biomass pool and released CO2. This may reflect that the faster microbial turnover contributes to microbial necromass and further to stable SOM formation. Similar total apparent priming (∼1.5 μg C g−1 in 120 days, mainly in the first 20 days) was observed after glucose and N addition, which contributed around 1–4 % of total priming. Unlike after glucose and N additions, the 14C released as CO2 (8 % of the remaining previously added 14C-glucose) after water addition was mainly derived from reutilization of microbial necromass. This was supported by the absence of changes in either the 14C or total C in microbial biomass. Overall, the turnover of the microbial biomass pool – the apparent priming – should not be ignored, since microbial biomass acts not only as a major determinant of SOM turnover but also as a C pool.

Soil organic carbon priming co-regulated by labile carbon input level and long-term fertilization history

Labile carbon (C) input and fertilization have important consequences for soil organic matter (SOM) decomposition via the priming effect (PE), thereby impacting soil fertility and C sequestration. However, it remains largely uncertain on how the labile C input levels interact with long-term fertilization history to control PE intensity. To clarify this question, soil samples were collected from a 38-year fertilization field experiment (including five treatments: chemical nitrogen fertilizer, N; chemical fertilizer, NPK; manure, M1; 200 % manure, M2; NPK plus M2, NPKM2), with strongly altered soil physiochemical properties (i.e., soil aggregation, organic C and nutrient availability). These soil samples were incubated with three input levels of 13C-glucose (without glucose, control; low, 0.4 % SOC; high, 2.0 % SOC) to clarify the underlying mechanisms of PE. Results showed that the PE significantly increased with glucose input levels, with values increasing from negative or weak (−2.21 to 3.55 mg C g−1 SOC) at low input level to strongly positive (5.62 to 8.57 mg C g−1 SOC) at high input level across fertilization treatments. The increased PE intensity occurred along with decreased dissolved total nitrogen (DTN) contents and increased ratios of dissolved organic C to DTN, implying that the decline in N availability largely increased PE via enhanced microbial N mining from SOM. Compared to N and NPK treatments, the PE was significantly lower in the manure-amendment treatments, especially for low input level, due to more stable SOM by aggregate protection and higher N and phosphorus availability. These results suggested that manure application could alleviate SOM priming via increased soil C stability and nutrient availability. Collectively, our findings emphasize the importance of long-term fertilization-driven changes in labile C inputs, SOM stability, and nutrient availability in regulating PE and soil C dynamics. This knowledge advances our understanding of the long-term fertilization management for soil C sequestration.

The presence of soybean, but not soybean cropping frequency has influence on SOM priming in crop rotation systems

Legume crops are advocated for integration into crop rotation systems, and cereal-based rotations with the presence of legumes have a substantial effect on improving soil fertility and health. It is not yet clear whether the frequency of legume inclusion in crop rotation systems influences soil biochemical properties and soil organic matter (SOM) mineralization. An incubation experiment was conducted with 13C-glucose addition to evaluate the influences of soybean (Glycine max L.) cropping frequency on SOM mineralization under long-term wheat (Triticum aestivum L.)- and maize (Zea mays L.)-based rotation systems. Phospholipid fatty acids (PLFAs) and 13C-PLFAs were measured to explore microbial biomass, community structure and microbial utilization of glucose in wheat and maize systems. Glucose addition increased native SOM mineralization, i.e. positive priming effect. Compared with less soybean cropping frequency under long-term wheat- and maize-based rotation systems, wheat-soybean-soybean-soybean rotation and maize-soybean-soybean rotation increased the total biomass (PLFAs), fungal biomass and decreased the ratio of bacteria to fungi. Furthermore, the ratio of bacteria to fungi was negatively correlated with PE intensity, indicating that greater fungal biomass played a key role in stimulating SOM priming. That the proportion of 13C-glucose in G- and fungi had a positive relationship with PE intensity also supported this conclusion. The presence of soybean in wheat- and maize-based rotations increased SOM priming, while the soybean cropping frequency had no significant influence on SOM priming. However, in contrast to a maize-based rotation system, the same frequency of soybean in a wheat-based rotation system had lower soil C/N ratio and higher B/F ratio, and resulted in lower PE intensity. Our findings indicated that the presence of soybean in wheat- and maize-based rotation systems increased PE intensity because of higher soil C/N ratio and lower B/F ratio, while the soybean cropping frequency had no significant influence on SOM priming. Furthermore, the presence of soybean in maize system induced more SOM priming than that in wheat system with glucose addition.

Effects of nitrogen and phosphorus fertilization on soil organic matter priming and net carbon balance in alpine meadows

AbstractFertilizers‐induced priming effects of soil organic matter (SOM) decomposition influences net carbon balance and nutrient release. We hypothesize that very strong limitation of plant productivity and microbial activities by nitrogen (N) and phosphorus (P), common in Tibetan meadows, retard SOM decomposition and turnover. Consequently, N and/or P fertilization will induce priming effects of SOM and have implications for carbon balance. Soils from a nine‐year fertilization experiment (N alone, P alone, NP together, and control) from a Tibetan alpine meadow were used to investigate priming effect of SOM and carbon balance after addition of 13C labeled glucose. N and/or P fertilization acidified soil by 0.5 pH unit, decreased SOM content, and increased total and available N, total P. Regardless of fertilization, glucose addition accelerated SOM decomposition with priming effects of 30–60 μg C g−1 soil during 78 days. Alleviation of N and P limitation by N and NP fertilization lowered the priming effect by 17% and 14%, respectively, but P fertilization increased priming effect by 67%. The negative correlation of priming effect intensity with SOM, nitrate or total N, and microbial biomass contents indicated that fertilization‐induced differences in soil N and the microbial community are responsible for the priming effects. Positive correlation of carbon balance with total N and ammonium contents suggested that soil N accounts for carbon sequestration. Therefore, long‐term N and/or P fertilization accelerate SOM decomposition and reduce SOM storage in alpine meadows, of which P fertilization induces the highest priming effect and the lowest SOM storage.

The quality and quantity of SOM determines the mineralization of recently added labile C and priming of native SOM in grazed grasslands

Continuous and pulsed inputs of labile organic carbon (LOC) into soil are common. However, because soil microbial responses to LOC input are rapid, the relative contributions of respiration derived from LOC to total microbial respiration and their influencing factors remain elusive. Furthermore, although numerous studies have explored the priming effect (PE) of soil organic matter (SOM) mineralization induced by LOC addition, few studies have focused on its short-term effects. There are some indications that the response of soil microbes to LOC input depends on the microbial demand of nutrients, especially C and N. Therefore, variations in soil C and N characteristics may further influence microbial response processes to LOC inputs and the decomposition of LOC and SOM, but a comprehensive understanding of this is lacking. To address this gap, 13C-labeled glucose was added to six temperate grassland soils with different C/N ratios (9.98–12.0), which were collected from areas with different grazing exclusion durations and at different soil depths. The microbial respiration was measured at 9-min intervals across a 105-h period. We found that soil microbes responded rapidly to LOC input, and microbial biomass controlled by soil organic C (SOC) and C/N was the most important factor directly influencing the intensity of the microbial response to LOC input. Grazing and deeper soil layers decreased the respiration derived from LOC and their relative contribution to total respiration, mainly attributed to variations in soil C/N and fungal/bacterial ratio (Fu/Ba). LOC addition stimulated SOM decomposition in all soils and increased respiration of SOM by 11.3–92.4 mg C/g SOC, equivalent to 18.7–266.1 % priming. Grazing and increased soil depth resulted in a greater PE and soil C loss, with soil C/N and SOC content being the most important regulators. Overall, this study revealed the important influence of LOC on soil C fluxes and highlighted the important role of SOM quality and quantity in regulating grassland soil C cycling.

Species shifts induce soil organic matter priming and changes in microbial communities

Invasion of plant species with functional traits that influences the rhizosphere can have significant effects on soil organic matter (SOM) dynamics if the invasive species stimulates soil microbial communities with, for example, an enhanced supply of labile carbon and oxygen. We evaluated these effects along a Phragmites invasion chronosequence spanning over 40 years. Using a δ13C and δ15N enriched substrate, we separated SOM-derived and substrate-derived carbon (C) and nitrogen (N) mineralization in surface (top 15 cm), shallow (30–45 cm), and deep (65–80 cm) soils collected from established, newly invaded, and native plant communities. We found all soils were susceptible to SOM priming, but priming profiles differed between vegetation communities, being highest at the surface in native assemblage soils, whereas highest at depth under invasive plants. Changes in functional microbial community composition at depth in Phragmites soils, evidenced by an increase in relative fungal laccase abundance, explained the SOM priming in these deep invaded soils. Our results show that invasive Phragmites maintains a microbial community at depth able to degrade SOM faster than that under native vegetation, evidencing that plant species shifts can fundamentally change soil biogeochemistry, altering element cycling and decreasing SOM residence time. Furthermore, our experimental design allowed to quantify real-time SOM-C and SOM-N gross mineralization, resulting in a new model relating C and N mineralization in these wetland soils and providing new insights on how SOM decomposition impacts N availability and cycling across wetland N pools.

Frequent carbon input primes decomposition of decadal soil organic matter

Soil organic matter (SOM) decomposition in response to global change represents a critical uncertainty in coupled carbon (C) cycle-climate models. Much of this uncertainty arises from our limited mechanistic knowledge of the effects of organic C input frequency on SOM decomposition. Based on a three-source-partitioning isotopic approach (14C glucose addition to soil continuously labeled by C4 plants over 21 years) and literature review of 86 observations, we assessed the priming of fast- and decadal-SOM decomposition after occasional and frequent (every 12 vs 60 days) labile C input in a grassland soil. Frequent glucose input accelerated SOM decomposition over 200 days, but the occasional input reduced this positive priming by 2.6 times. The positive priming by occasional C input resulted in 139 μg C g−1 soil net C sequestration corresponding to 38% of C input. During the 200 days incubation, the primed fast-cycling C (younger than 21 years) under occasional C addition was 52–94% greater compared with frequent addition. Conversely, the priming of decadal SOM (older than 21 years) by frequent C input was 463 μg C g−1 after the 200 days incubation, which represents 5.3% of the initial decadal-cycling soil C pool. This was 1.5 times higher than by occasional input because of the stronger N mining caused by the continuously high activity of microbial K strategists under frequent C input. Our global scale assessment showed that occasional C input reduces the priming of decadal SOM decomposition by 89–127 Tg per year compared to frequent input and thus, sequestrated an additional 102 Tg C per year. Overall, the vulnerability of decadal SOM to increasing C input frequency thus weakening the C sink of grassland, whilst reduced C input frequency (e.g. by drought) suppresses the SOM priming and reduces the positive feedback of SOM decomposition to global change. This points out the necessity to assess the age of soil C when predicting the consequences of altered soil C input frequency under global change.

Organic matter contributions to nitrous oxide emissions following nitrate addition are not proportional to substrate-induced soil carbon priming

The addition of carbon (C) substrate often modifies the rate of soil organic matter (SOM) decomposition. This is known as the priming effect. Nitrous oxide (N2O) emissions from soil are also linked to C substrate dynamics; however, the relationship between the priming effect and N2O emissions from soil is not understood. This study aimed to investigate the effects of C and N substrate addition on the linkages between SOM priming and N2O emissions. We applied 13C-labelled substrates (acetate, butyrate, glucose; 80 μg C g−1), with water as a control, and 15N-labelled N (300 μg N g−1 soil, potassium nitrate) to three different soils, and, after 3 days, we measured the effects on the priming of SOM and sources of N2O emission. Carbon substrate addition increased both CO2- and SOM-derived N2O emissions in the presence of exogenous N. Emissions of CO2 and N2O from soils with added glucose (mean ± standard deviation, 0.73 ± 0.13 μmol m−2 s−1 and 21.4 ± 12.1 mg N m−2 h−1) were higher (p < 0.05) than those from soils treated with acetate (0.64 ± 0.11 μmol m−2 s−1 and 10.9 ± 6.5 mg N m−2 h−1) or butyrate (0.61 ± 0.11 μmol m−2 s−1 and 11.0 ± 6.6 mg N m−2 h−1), respectively. Acetate addition induced a stronger (p < 0.05) priming effect on soil C (0.07 ± 0.09 μmol C m−2 s−1) than that for glucose (0.02 ± 0.10 μmol C m−2 s−1), while butyrate addition resulted in negative priming (−0.09 ± 0.05 μmol C m−2 s−1). SOM-derived N2O emissions were relatively low from soils with butyrate addition (1.4 ± 1.5 mg N m−2 h−1) compared with acetate (2.9 ± 2.3 mg N m−2 h−1) or glucose (9.2 ± 4.5 mg N m−2 h−1). There was no clear relationship between the priming effect and SOM-derived N2O emissions. The observed priming effect related to the potential electron donor supply of the C substrates was not observed. There is a need to further examine the role of soil priming in relation to soil N2O emissions.

Faster carbon turnover in topsoil with straw addition is less beneficial to carbon sequestration than subsoil and mixed soil

AbstractIn arable soils, the topsoil and subsoil are commonly mixed in the operation of deep tillage. It is not yet clear whether the influence of crop straw on soil organic matter (SOM) mineralization of the mixed soil (from the topsoil and subsoil) is different from that of solely the topsoil or subsoil. We conducted a 120‐d incubation experiment with the addition of 13C‐labeled maize (Zea mays L.) straw to study SOM mineralization and straw decomposition in the topsoil (0–15 cm), subsoil (15–30 cm), and their mixture (0–30 cm). Straw addition promoted SOM mineralization and thus induced a positive priming effect. The mineralization of SOM in topsoil with straw addition was 122% higher than in subsoil, and 45% higher than in mixed soil. After the 120‐d incubation, 12–16% of straw C was mineralized as CO2, contributing approximately 50–70% to the total CO2 production. Furthermore, straw C was also incorporated into SOM fractions, with 34, 7, and 59% in the light fraction, particulate organic C (coarse and fine), and mineral‐associated organic C (MAOC) in topsoil, respectively. The SOM mineralization and priming were greater in the topsoil with higher dissolved organic C and NO3–N throughout the incubation. Despite more straw C retained in MAOC in topsoil, the significant decrease in total MAOC with straw addition indicated that SOM turnover induced by fresh C application was probably faster in topsoil than the subsoil and mixed soil. Our findings indicated that both subsoil and mixed soil tended to sequester C with straw addition, whereas topsoil C turnover was faster than subsoil in the presence of straw addition.

Negative priming of soil organic matter following long-term in situ warming of sub-arctic soils

Priming is the change of microbial soil organic matter (SOM) decomposition induced by a labile carbon (C) source. It is recognised as an important mechanism influencing soil C dynamics and C storage in terrestrial ecosystems. Microbial nitrogen (N) mining in SOM and preferential substrate utilisation, i.e., a shift in microbial carbon use from SOM to more labile energy sources, are possible, counteracting, mechanisms driving the priming effect. Climate warming and increased N availability might affect these mechanisms, and thus determine the direction and magnitude of the priming effect. Hence, these abiotic factors can indirectly affect soil C stocks, which makes their understanding crucial for predicting the soil C feedback in a warming world. We conducted a short-term incubation experiment (6 days) with soils from a subarctic grassland that had been subjected to long-term geothermal warming (>55 years) by 2-4°C above unwarmed soil. Soil samples were amended with 13C-labelled glucose and 15N-labelled NH4NO3. We found a significantly negative relationship between in situ warming and cumulative primed C, with negative priming in the warmed soils. The negative priming suggests that preferential substrate utilisation was a key mechanism in our experiment. Our results indicate that changes in SOM characteristics associated with the in situ warming gradient can play a major role in determining the rate and direction of the priming effect. Additionally, we found that neither microbial N limitation nor N addition affected the priming effect, providing evidence that in our experiment, N mining did not lead to positive priming.

Three source-partitioning of CO2 fluxes based on a dual-isotope approach to investigate interactions between soil organic carbon, glucose and straw

Inputs of available organic materials into soil alter the decomposition of soil organic matter (SOM), a process called priming effect. Organic carbon (C) inputs in terrestrial ecosystems are common from various sources (e.g. rhizodeposits, plant residues, microbial necromass) simultaneously, but their interactions as well as mutual effects on SOM decomposition are unknown because multisource partitioning of pools and fluxes was not available. A dual-isotope approach (identical materials except for straw being possessed two 13C abundances) was adopted to partition total CO2 emission from three C sources: SOM, glucose and straw. Cumulative CO2 efflux was quantified into straw-derived (558 μg C g−1), glucose-derived (480 μg C g−1) and SOM-derived (58 μg C g−1) CO2 during the first 7 days of incubation. Glucose or straw addition induced positive SOM priming, whereas glucose combined with straw resulted in higher SOC loss than that induced by single addition of glucose or straw after day 7. The Spearman's correlation showed that the interactions between glucose and straw shifted from increased CO2 evolved during their intensive decomposition (days 1 to 3) to mutual constraint on mineralization during the late stage (days 5 to 7). This study provides evidences for the suitability of the dual-isotope approach to partition multiple sources of CO2 fluxes and C pools, and evaluates their individual or mutual contributions to SOM priming, thus, implicating C sequestration in terrestrial ecosystems.

Soil structure as a significant indirect factor affecting crop yields

Soil structure as a significant indirect factor affecting crop yields

Altered litter inputs modify carbon and nitrogen storage in soil organic matter in a lowland tropical forest

Soil organic matter (SOM) in tropical forests is an important store of carbon (C) and nutrients. Although SOM storage could be affected by global changes via altered plant productivity, we know relatively little about SOM stabilisation and turnover in tropical forests compared to temperate systems. Here, we investigated changes in soil C and N within particle size fractions representing particulate organic matter (POM) and mineral-associated organic matter (MAOM) after 13 years of experimental litter removal (L−) and litter addition (L+) treatments in a lowland tropical forest. We hypothesized that reduced nitrogen (N) availability in L− plots would result in N-mining of MAOM, whereas long-term litter addition would increase POM, without altering the C:N ratio of SOM fractions. Overall, SOM-N declined more than SOM-C with litter removal, providing evidence of N-mining in the L− plots, which increased the soil C:N ratio. However, contrary to expectations, the C:N ratio increased most in the largest POM fraction, whereas the C:N ratio of MAOM remained unchanged. We did not observe the expected increases in POM with litter addition, which we attribute to rapid turnover of unprotected SOM. Measurements of ion exchange rates to assess changes in N availability and soil chemistry revealed that litter removal increased the mobility of ammonium-N and aluminium, whereas litter addition increased the mobility of nitrate-N and iron, which could indicate SOM priming in both treatments. Our study suggests that altered litter inputs affect multiple processes contributing to SOM storage and we propose potential mechanisms to inform future work.

How does management legacy, nitrogen addition, and nitrification inhibition affect soil organic matter priming and nitrous oxide production?

Long-term management of croplands influences the fluxes and sources of nitrous oxide (N2 O). We examined this premise in a greenhouse study by using soils collected from a 38-yr-old field experiment. The sampled treatments were continuous barley (Hordeum vulgare L.; CB), continuous fescue (Festuca rubra L., F.arundinacea Schreb; CF), and two phases of an 8-yr rotation: faba bean (Vicia faba L.; FB) and alfalfa (Medicago sativa L.)-bromegrass (Bromus inermis Leyss) hay. Barley was grown as a test crop in the greenhouse in each soil. The ranking of N2 O emissions was hay>FB>CB>CF (P<.001). We quantified the 15 N-site preference to assess the N2 O-producing processes. Denitrification was the predominant source, contributing 77.4% of the N2 O production. We also evaluated nitrogen (N) additions: urea alone or urea with a nitrification inhibitor (nitrapyrin or DMPSA). Compared with urea alone, nitrapyrin and DMPSA reduced N2 O emissions by 16 and 25%, respectively. We used urea labeled with 15 N to trace N to N2 O emissions, aboveground plant N uptake, and N retention by soils. Total 15 N-recovery (N2 O+plant+soil) was highest under FB (86%) and lowest under CB (29%). We further separated the N2 O derived from urea versus N2 O from soil organic matter (SOM). The inhibitor DMPSA reduced the N2 O derived specifically from added urea-N by more than half (P<.001). With the addition of urea, N2 O production from mineralization of SOM-N accelerated over the control (without urea), termed the priming effect. This priming of SOM-N contributed with 13% of the total N2 O production when averaged across the four management legacies. The CB soil had the highest proportion of priming-derived N2 O (24%). Management legacies clearly differed in soil carbon and N, which governed N2 O production from denitrification and SOM priming.

Strong priming of soil organic matter induced by frequent input of labile carbon

Labile carbon (C) inputs to soil (e.g., litter and root exudation) can prime soil organic matter (SOM) decomposition, and strongly influence SOM dynamics. The direction and intensity of priming, as well as the net C balance in soil, depend on the amount and frequency of labile C inputs. Most recent priming studies are based on single C additions, which are not truly representative of common litter inputs or root exudation in terrestrial ecosystems. Here, we evaluated the effects of 14C-labeled glucose addition to soil in the same final amounts (360 μg C g−1) split into two temporal patterns: seldom (20% of microbial biomass every two months) and frequent addition (4% of microbial biomass every 10 days) on the dynamics of CO2 production and SOM priming over a 200-day incubation. For the first time, we combined enzyme kinetics with substrate-induced growth respiration and fungal diversity to monitor microbially mediated SOM mineralization in response to the labile C input frequency. Frequent glucose addition decreased 14C incorporation into microbial biomass and almost doubled cumulative priming compared to seldom addition, resulting in a net loss of SOM for seldom and frequent C additions of −94 and −367 μg C g−1 respectively. Larger priming loss of SOM with frequent C inputs was accompanied by increased activities of β-glucosidase, chitinase, and acid phosphatase, and by a shift in fungal community towards increased abundance of K-strategist fungal species (mainly Mortierellales sp. and Trichoderma sp.) capable of SOM mineralization. In conclusion, frequent labile C inputs (e.g., rhizodeposits in rhizosphere or litterfall in disturephere) to soil will stimulate a shift in fungal community structure and functions, resulting in intensive priming of SOM decomposition and CO2 losses.

The stoichiometric C-Fe ratio regulates glucose mineralization and stabilization via microbial processes

The association of soil organic matter (SOM) with iron (Fe) oxides by adsorption and/or co-precipitation contributes to long term C stabilization in soil. While there is an understanding of the relationship between soil carbon (C) and the biogeochemical cycling of Fe, a lack of information exists on the role of Fe oxides on the accumulation of C in paddy soils. This study aimed to assess the role of Fe (oxyhydr)oxides on mineralization and stabilization processes following amendment of paddy soil with a labile C substrate (99 atom % 13C-glucose). The study utilized 4 paddy soils with a total Fe concentration ranging from 13.7 to 55.8 g kg−1. In soils with 42.7 and 55.8 g kg−1 Fe, the addition of glucose resulted in an Fe bound organic C: Fe molar ratio (C:Fe molar ratio) ≥6, suggesting the formation of Fe-OM complexes mainly via co-precipitation. The highest portion of 13C (13.8%) protected in Fe-OM complexes was found in soil containing 55.8 g kg−1 of Fe. The stabilization of the added labile C substrate was shown, using random forest analysis, to be controlled by the C: Fe molar ratio, while substrate mineralization was regulated by the core genera Sphingomonas and Devosa (r-strategists) (affiliated to Proteobacteria) and C:N ratio. Substrate mineralization was 47% lower in soil containing 55.8 g Fe kg−1 compared to 13.7 g Fe kg−1, with a concomitant reduction in SOM priming of 37%. This reduction in substrate mineralization and the priming effect was likely due to lower C substrate availability via the formation of Fe-OM complexes, thereby protecting the C from mineralization. In conclusion, the Fe concentration in paddy soils plays a central role in the abiotic stabilization of ‘new’ C through the formation of Fe-OM complexes via co-precipitation, thereby limiting the availability of this C substrate for microbial mineralization, and at the same time modulating the microbial community structure.

Sources and priming of soil N2O and CO2 production: Nitrogen and simulated exudate additions

Identifying the sources of nitrous oxide (N2O) and carbon dioxide (CO2) production from soil is central to enhancing the understanding and prediction of these emissions to the atmosphere. The magnitude of N2O and CO2 production derived from soil organic matter (SOM) can asymmetrically change due to stimuli from root exudation and nitrogen additions ‒ a response termed the priming effect. We conducted an incubation to examine the effects of N and artificial root exudate (ARE) additions on the priming of SOM. We also evaluated the changes in N2O production from nitrification and denitrification by measuring 15N–N2O site preference (SP). ARE consisted of a mixture of 99 atom% 13C labelled compounds at three rates (0, 6.2, 12.5 mg C kg−1 soil day−1) applied daily to microcosms with or without urea, a subset of which was also labelled with 5 atom% 15N. Additions of ARE or urea alone caused positive priming effects; however, addition of ARE and urea concurrently resulted in an antagonistic interactive effect that diminished the N2O production derived from SOM mineralization (P < 0.05). Moreover, CO2 production from SOM decreased in urea-treated microcosms (P < 0.01) such that all soils receiving ARE and urea exhibited reduced positive priming relative to their unfertilized counterparts. Based on SP results, the contributions of denitrification and nitrification to total N2O production were both amplified due to the combined inputs of ARE and urea compared to the untreated control (49.9 ± 10.1 and 28.3 ± 8.0 μg N2O–N kg−1, respectively). In soils receiving only ARE, N2O derived from denitrification decreased relative to a control, thus reducing overall N2O production (−9.5 ± 12.3 μg N2O–N kg−1); conversely, nitrification-derived N2O was differentially augmented (+17.2 ± 9.0 μg N2O–N kg−1). Results indicate that a combination of elevated root exudation with N fertilization has the potential to asymmetrically amplify N2O emissions due to increases in both nitrification and denitrification sources.

Quantifying microbial metabolism in soils using calorespirometry — A bioenergetics perspective

Microbial carbon use efficiency (CUE) measures the partitioning between anabolic and catabolic processes. While most work on CUE has been based on carbon (C) mass flows, the roles of organic C energy contents and microbial energy demand on CUE have been rarely considered. Thus, a bioenergetics perspective could provide new insights on how microorganisms utilize C and ultimately allow evaluating their role in C stabilization in soils. Recently, the calorespirometric ratio (CR)—the ratio of heat dissipation and respiration—has been used to characterize the efficiency of microbial growth in soils. Here, we formulate a coupled mass and energy balance model for microbial growth and provide a generalized relationship between CUE and CR. In the model, we consider two types of organic C in soils: an added substrate (e.g., glucose) and the native soil organic matter (SOM), to also account for priming effects. Furthermore, we consider both aerobic and fermentation metabolic pathways. We use this model as a framework to generalize previous formulations and generate hypotheses on the expected variations in CR as a function of substrate quality, metabolic pathways, and microbial traits (specifically CUE). In turn, the same equations can be used to estimate CUE from measured CR.Our results confirm previous findings on CR and show that without microbial growth, CR depends only on the rates of the different metabolic pathways, while CR is also a function of the growth yields for these metabolic pathways when microbial growth occurs. Under strictly aerobic conditions, CUE increases with increasing CR for substrates with a higher degree of reduction than that of the microbial biomass, while CUE decreases with increasing CR for substrates with a lower degree of reduction than the microbial biomass. When aerobic reactions and fermentation occur simultaneously, the relation between CUE and CR is mediated by (i) the degree of reduction of the substrates, (ii) the rates and growth yields of all metabolic pathways, and (iii) the contribution of SOM priming to microbial growth. Using the proposed framework, calorespirometry can be used to evaluate CUE and the role of different metabolic pathways in soil systems.

Impact of manure on soil biochemical properties: A global synthesis

Manure application mitigates land degradation and improves soil fertility. Despite many individual studies on manure effects, a comprehensive overview of its consequences for a broad range of soil properties is lacking. Through a meta-analysis of 521 observations spanning the experiments from days after pulse addition up to 113 years with continues manure input, we quantified and generalized the average responses of soil biochemical properties depending on climate factors, management, soil, and manure characteristics. Large increase of pools with fast turnover (microbial carbon (C) and nitrogen (N): +88% and +84%, respectively) compared to stable organic matter pools (+27% for organic C, and +33% for total N) reflects acceleration of C and N cycles and soil fertility improvement. Activities of enzymes acquiring C-, energy-, N-, phosphorus- and sulfur were 1.3–3.3 times larger than those in soil without manure for all study durations included. Soil C/N ratio remained unaffected, indicating the stability of coupled C and N cycles. Microbial C/N ratio decreased, indicating a shift towards bacterial domination, general increase of C and N availability and acceleration of element cycling. Composted manure or manure without mineral fertilizers induced the greatest increase compared to non-composted manure or manure with mineral fertilizers, respectively, in most biochemical properties. The optimal manure application rate for adjusting proper soil pH was 25 Mg ha−1 year−1. Among manure types, swine manure caused the greatest increase of N-cycle-related properties: microbial N (+230%), urease (+258%) and N-acetyl-β-D-glucosaminidase (+138%) activities. Manure application strategies should avoid P and N losses and pollution via runoff, leaching or gaseous emissions due to fast mineralization and priming of soil organic matter. In conclusion, manure application favors C accumulation and accelerates nutrient cycling by providing available organic substances and nutrients and thus increasing enzyme activities.