Real Priming Effects Research Articles

Microbial response to long-term fertilization of paddy soils: Apparent and real priming effects

Fertilization is crucial for increasing crop productivity and it alters soil microbial biomass and activities. These alterations exert implications for soil carbon (C) stocks, primarily through the priming effect (PE). Here, we investigated the mechanisms underlying PE and their impact on soil C stocks in paddy soils subjected to long-term (31 years) fertilization. Soils from four depths within the upper 40 cm layer, subjected to four fertilization types (no fertilizer, mineral fertilizer, mineral + straw, mineral + chicken manure), were incubated for 60 days with or without 13C-labeled glucose. Following substrate addition, an increase in isotopically unlabeled CO2 efflux may stem from accelerated microbial turnover (apparent PE) or modified soil organic matter (SOM) mineralization (real PE). By analyzing microbial biomass (MBC), CO2 and dissolved organic C (DOC) dynamics, we explored the partitioning of C from glucose into three pools. Following glucose addition (day 2), the SOM-derived MBC decreased by 4.7 % at 0–10 cm in unfertilized and by 6.3 % in mineral-fertilized soils. SOM-derived MBC also decreased in soils fertilized with straw- and chicken manure at 0–20 and 0–30 cm. Hence, the apparent PE closely correlated with fertilization types. At day 2, the SOM-derived DOC increased with the increase in extractable Fe2+ content, indicating high contribution of the released mineral-bound organic C in the DOC pool. Microbial decomposition of SOM-derived DOC and loss of unlabeled C through apparent PE increased the release of unlabeled CO2, resulting in a positive but low PE. After 60 days of incubation, the addition of glucose to the subsoils (30–40 cm) caused a net loss of C (positive PE) of 6.6–19 mg kg−1. In contrast, negative PEs were observed in the topsoil, indicating a decrease in microbial activity and C turnover. These findings suggested that fertilization regimes, whether involving long-term extra C input or not, can alter microbial biomass and turnover rates. Moreover, the C turnover mechanisms in paddy soils are complex, including apparent PE, release of mineral-bound organic C, and negative PE. Collectively, these processes can consequently change soil C stocks.

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.

Positive and negative priming effects induced by freshly added mineral-associated oxalic acid in a Mollisol

Freshly added root exudates carbon (C) can accelerate or delay the mineralization of soil organic carbon (SOC) pool. Such short-term change is a phenomenon known as the ‘priming effect’. However, it is still unclear whether the minerals associated with fresh C will still be able to cause the priming effect given that in the mineral associated fresh carbon complex, minerals protect fresh C from microbial decomposition. The hypothesis of this study is that, although oxalic acid is bound to minerals, the addition of mineral-associated oxalic acid to soils will accelerate the decomposition of SOC. To clarify this, a study was performed using a series of experiments including adsorption experiment, where different concentrations of oxalic acid (OA) were mixed with clay minerals to obtain the mineral-associated oxalic acid (MAOA) including vermiculite-associated oxalic acid (VmAOA) and montmorillonite-associated oxalic acid (MtAOA) at different concentrations. The X-ray diffraction (XRD) analysis was performed before and after the adsorption of oxalic acid by clay minerals. The VmAOA and MtAOA were added onto a Mollisol, incubated for 21 days and thereafter, the priming effects were determined in different treatments. Nuclear Magnetic Resonance (NMR) spectroscopy was performed before and after incubation. The results revealed that the adsorption of oxalic acid onto minerals affected the mineral structure through a decrease in mineral peaks. The OA mineralization increased with incubation time, while SOC mineralization showed the opposite trend to OA after day 7. In all the treatments, the strongest positive priming effect was induced on day 1 and was considered an apparent priming effect. On day 4 of the high treatment, and on day 7 of the low treatment with oxalic acid, the positive priming effect was observed and suggested as a real priming effect. After day 7 of the high treatment and day 11of the low treatment with oxalic acid, the positive priming effect shifted to the negative priming effect and the amount of SOC mineralized decreased till the end of incubation time. The percentage of alkyl groups increased in VmOA while the percentage of carboxyl groups and ketones groups increased in MtOA, indicating that alkyl groups are well protected by vermiculite while carboxyl groups and ketones groups are well protected by montmorillonite. This finding reveals that freshly added carbon, although bound to the mineral, can still accelerate SOC decomposition, and once fresh carbon bound to the mineral becomes available, the preference for SOC decomposition shifts to that of fresh carbon, resulting in a shift from a positive priming effect to a negative effect. This relevant finding will improve the understanding of the key role played by root exudates (oxalic acid) in the C cycle.

Increase in aboveground fresh litter quantity over-stimulates soil respiration in a temperate deciduous forest

In the context of climate change, the amount of carbon allocated to soil, particularly fresh litter, is predicted to increase with terrestrial ecosystem productivity, and may alter soil carbon storage capacities. In this study we performed a 1-year litter-manipulation experiment to examine how soil CO 2 efflux was altered by the amount of fresh litter. Three treatments were applied: litter exclusion (E), control (C, natural amount: 486 g m −2) and litter addition (A, twice the natural amount: 972 g m −2). Litter decomposition rate was not affected by fresh litter amount. However, the addition or exclusion of fresh litter quickly increased or decreased total soil CO 2 efflux ( F S) significantly, but the relative contribution of fresh litter to total soil respiration remained unchanged between the C and A treatments, as determined by laboratory measurements. Variation in F S among treatments was not related to modification of its temperature sensitivity which was not affected by fresh litter amount ( Q 10: 3.5 for E, 3.2 for C, 3.6 for A). While litter exclusion was the main cause of the F S decrease in the E treatment, only 68% of F S was directly attributable to litter addition in the A treatment. The remaining 32% of F S in the A treatment was related to a real priming effect that appeared to be a long-lasting phenomenon. This priming effect lasting over 1 year may be related to a continuous release of organic compounds from litter to soil because of the progressive decomposition of leaf litter. Q 10 estimates and isotopic data lead to the hypothesis that the priming effect corresponded to the activation of the whole soil system. As a consequence, the increase in ecosystem productivity may lead, via an increase in the amount of litter, to an increase in carbon turnover in soil. Further labelling experiments involving high-frequency carbon stable isotope measurements of CO 2 efflux would help to clarify the relative importance of bulk soil and rhizosphere in the priming effect.

Model of apparent and real priming effects: Linking microbial activity with soil organic matter decomposition

The most frequently used models simulating soil organic matter (SOM) dynamics are based on first-order kinetics. These models fail to describe and predict such interactions as priming effects (PEs), which are short-term changes in SOM decomposition induced by easily available C or N sources. We hypothesized that if decomposition rate depends not only on size of the SOM pool, but also on microbial biomass and its activity, then PE can be simulated. A simple model that included these interactions and that consisted of three C pools – SOM, microbial biomass, and easily available C – was developed. The model was parameterized and evaluated using results of 12C–CO 2 and 14C–CO 2 efflux after adding 14C-labeled glucose to a loamy Haplic Luvisol. Experimentally measured PE, i.e., changes in SOM decomposition induced by glucose, was compared with simulated PE. The best agreement between measured and simulated CO 2 efflux was achieved by considering both the total amount of microbial biomass and its activity. Because it separately described microbial turnover and SOM decomposition, the model successfully simulated apparent and real PE. The proposed PE model was compared with three alternative approaches with similar complexity but lacking interactions between the pools and neglecting the activity of microbial biomass. The comparison showed that proposed new model best described typical PE dynamics in which the first peak of apparent PE lasted for 1 day and the subsequent real PE gradually increased during 60 days. This sequential decomposition scheme of the new model, with immediate microbial consumption only of soluble substrate, was superior to the parallel decomposition scheme with simultaneous microbial consumption of two substrates with different decomposability. Incorporating microbial activity function in the model improved the fit of simulation results with experimental data, by providing the flexibility necessary to properly describe PE dynamics. We conclude that microbial biomass should be considered in models of C and N dynamics in soil not only as a pool but also as an active driver of C and N turnover.

Fate of biuret 15N and its effect on net mineralisation of native soil N in forest soils

Biuret (C2H5N3O2) priming effect on mineralisation of native soil N has not been precisely quantified in previous studies, although it is a potential microbial activity regulator and slow-release N fertiliser. Following application of biuret at concentrations of 0 (B0) and 100 (B100) mg/kg (oven-dried) soil, we measured the dynamics of biuret-derived 15N in soil N pools, soil C mineralisation, and microbial biomass C in a sandy loam and a silt loam during a 112-day-long incubation to investigate the fate of biuret 15N and its effect on net mineralisation of native soil N. Biuret was decomposed faster in the sandy loam soil than the silt loam soil. In the sandy loam soil, the stabilised N pool was a strong sink for the biuret-derived 15N and accumulated about half of the applied 15N at the end of incubation. In the silt loam soil, 68% of the 15N applied was recovered in the NO3−-N pool and the stabilised N pool accumulated only about 25% of the applied 15N at the end of incubation. Biuret addition increased the turnover rate constant of soil organic matter and caused a real priming effect on net mineralisation of native soil N in both soils. The additional mineralisation of native soil N was 20.1 mg/kg (equivalent to 27.3 kg N/ha) in the sandy loam soil and 20.5 mg/kg (equivalent to 57.3 kg N/ha) in the silt loam soil. Biuret priming effect was related to the acceleration of soil organic matter decomposition by increased microbial activity at an early stage and the death/decay of microbes at a later stage of incubation. The native soil N released through the priming effect was partially from soil non-biomass organic matter and partially from soil microbial biomass.

Manipulating the N release from 15N labelled celery residues by using straw and vinasses

The aim of this laboratory study was to investigate the effect of straw and vinasses on the nitrogen (N) mineralization–immobilization turnover of celery residues during two periods (each simulating a time period from autumn till spring) under laboratory conditions. During the first period (1–198 d), 15N-labelled celery residues (1.1 g dry matter (DM) kg −1 soil) were incubated together with straw (8.1 g DM kg −1 soil), aiming to immobilize the N released from celery residues, followed by an incorporation of vinasses (1.9 g DM kg −1 soil) after 84 d, with a view to remineralizing the immobilized celery–N. During the second period (198–380 d), the experimental set-up was repeated, except that non-labelled celery residues were used. Total N, mineral N and their 15N enrichments as well as microbial biomass N were determined at regular time intervals. During both periods, mixing celery residues with straw significantly increased microbial biomass N (90.5 and 40.5 mg N kg −1 extra compared to celery only treatment) and decreased the amount of mineral N (reduction of 56.1 and 45.9 mg N kg −1 soil compared to celery only treatment) and the celery-derived mineral 15N (0% of mineral celery-derived 15N in straw treatment compared to 35% of mineral celery-derived 15N in celery only treatment). After maximum immobilization, a natural remineralization (without addition of vinasses) of 32.2 (at day 198) and 11.1 mg N kg −1 soil (at day 380) occurred in the straw treatment, but the mineral N content remained significantly lower than in the celery only treatment during the complete experiment, and the amount of remineralized celery– 15N was very low (5.4% of celery-derived 15N after 380 d). Vinasses caused no real priming effect, although it did slightly increase the amount of remineralized celery– 15N (+6.4% of celery-derived 15N at day 380 compared to the straw treatment), probably due an apparent added N interaction caused by displacement reactions with the soil microbial biomass.

Influence of urease and nitrification inhibitors on N losses from soils fertilized with urea

The aim of this study was to evaluate how the N losses through volatilization and leaching from soils fertilized with urea can be affected by the application of a urease inhibitor or a urease plus a nitrification inhibitor. The experiment was carried out using lysimeters with 15N-labelled urea and N-(n-butyl) thiophosphoric triamide (NBPT) as urease inhibitor and dicyandiamide (DCD) as nitrification inhibitor, comparing three different treatments: urea alone (U), urea + NBPT (UN) and urea + NBPT + DCD (UND). Both volatilization and leaching were significantly different in the soils used, according to their physico-chemical characteristics. However, the pattern of the loss was similar: the volatilization was significantly reduced by NBPT (UN), but the presence of DCD (UND) significantly increased the loss, with respect to UN. Considering leaching, the highest amount of NO3 – was lost with UND, the lowest with U. The greatest amount of N lost by leaching was soil-derived N produced by the N mineralization-immobilization turnover. We suggest that, by maintaining the NH4 + in the soils, the inhibitors, in particular DCD, caused a priming effect with a subsequent increase in the rate of soil organic matter mineralization and an extra release of soil organic N. The priming effect was real in the sandy loam (SL) soil where a net N release was observed, whereas in the clay loam (CL) soil the effect of the inhibitors was less pronounced and an apparent priming effect was observed; however, a real priming effect also cannot be excluded in this soil.

Review of mechanisms and quantification of priming effects

Priming effects are strong short-term changes in the turnover of soil organic matter caused by comparatively moderate treatments of the soil. In the course of priming effects large amounts of C, N and other nutrients can be released or immobilized in soil in a very short time. These effects have been measured in many field and laboratory experiments; however, only a few of the studies were aimed at an extended investigation of the mechanisms of such phenomena. The aim of this overview is to reveal possible causes and processes leading to priming actions using the references on agricultural ecosystems and model experiments. Multiple mechanisms and sources of released C and N are presented and summarized in Tables for positive and negative real and apparent priming effects induced after the addition of different organic and mineral substances to the soil. Soil microbial biomass plays the key role in the processes leading to the real priming effects. The most important mechanisms for the real priming effects are the acceleration or retardation of soil organic matter turnover due to increased activity or amount of microbial biomass. Isotopic exchange, pool substitution, and different uncontrolled losses of mineralized N from the soil are responsible for the apparent N priming effects. Other multiple mechanisms (predation, competition for nutrients between roots and microorganisms, preferred uptake, inhibition, etc.) in response to addition of different substances are also discussed. These mechanisms can be distinguished from each other by the simultaneous monitoring of C and N release dynamics; its comparison with the course of microbial activity; and by the labelling of different pools with 14C or 13C and 15N. Quantitative methods for describing priming effects and their dynamics using 14C and 15N isotopes, as well as for non-isotopic studies are proposed.

Substrate amendments can alter microbial dynamics and N availability from maize residues to subsequent crops

The release and recovery of N from poor-quality (0.35% N), 15N-labelled maize residues was studied over five cropping cycles in a pot experiment in a sandy (7% clay) Quarzipsamment and a sandy–clay (31% clay) Acrorthox from Brazil. During the cropping cycles the soils were amended with bean residues high in N (4.4%), cellulose, or KNO 3. Total recovery of maize-N in plants at the end of the experiment was greater in the sandy soil (23–37%) compared with the sandy–clay soil (19–30%) in all treatments. Additions of bean residues increased the recovery of maize-N consistently in both soils compared with control treatments, especially when applied from the first cropping cycle onwards. At the end of the five cropping cycles, sequential bean residue additions resulted in a 38% increase in maize residue-N recovery in the sandy soil and a 32% increase in the sandy–clay soil. When applied from the second cycle onwards they resulted in only 27% and 9% increases respectively. The increased maize-N recovery resulted from greater microbial activity due to the bean-C application, followed by fast turnover of the newly-formed and largely unprotected microbial biomass. However, rather than producing a “real priming effect” on maize residue decomposition, this seems to have produced a “pool substitution” effect, where microbial metabolites of bean instead of maize origin were stabilized in soil. KNO 3–N applications had the opposite effect to that of bean applications (16% average decrease in maize-N recovery compared with control treatments) because available-C was not added to stimulate microbial activity, and NH 4–N from the decomposing residues was probably recycled in preference to NO 3–N by the soil microorganisms. Cellulose application considerably reduced availability of both soil-N and residue-N to plants but the effect lasted for only one cropping cycle. The results clearly showed the potential for manipulation of long-term recoveries of N from recalcitrant plant residues, particularly when better quality residues are also applied to satisfy part of the N demand of plants and microbial biomass, but this probably occurs at the expense of the recovery of the N from the other substrate.