Review: Factors affecting rhizosphere priming effects

Living plants change the local environment in the rhizosphere and consequently affect the rate of soil organic matter (SOM) decomposition. The rate may increase for 3‐ to 5‐folds, or decrease by 10 % to 30 % by plant cultivation. Such short‐term changes of rate (intensity) of SOM decomposition are due to the priming effect. In the presence of plants, a priming effect occurs in the direct vicinity of the living roots, and it is called rhizosphere priming effect (RPE). Plant‐mediated and environmental factors, such as, plant species, development stage, soil organic matter content, photosynthesis intensity, and N fertilization which affect RPE are reviewed and discussed in this paper. It was concluded that root growth dynamics and photosynthesis intensity are the most important plant‐mediated factors affecting RPE. Environmental factors such as amount of decomposable C in soil and N_min content are responsible for the switch between following mechanisms of RPE: concurrence for N_min between roots and microorganisms, microbial activation or preferential substrate utilization. Succession of mechanisms of RPE along the growing root in accordance with the rhizodeposition types is suggested. Different hypotheses for mechanisms of filling up the C amount loss by RPE are suggested. The ecosystematic relevance of priming effects by rhizodeposition relates to the connection between exudation of organic substances by roots, the increase of microbial activity in the rhizosphere through utilization of additional easily available C sources, and the subsequent intensive microbial mobilization of nutrients from the soil organic matter.     

Moisture modulates rhizosphere effects on C decomposition in two different soil types

While it is well known that soil moisture directly affects microbial activity and soil organic matter (SOM) decomposition, it is unclear if the presence of plants alters these effects through rhizosphere processes. We studied soil moisture effects on SOM decomposition with and without sunflower and soybean. Plants were grown in two different soil types with soil moisture contents of 45% and 85% of field capacity in a greenhouse experiment. We continuously labeled plants with depleted ¹³C, which allowed us to separate plant-derived CO₂-C from original soil-derived CO₂-C in soil respiration measurements. We observed an overall increase in soil-derived CO₂-C efflux in the presence of plants (priming effect) in both soils. On average a greater priming effect was found in the high soil moisture treatment (up to 76% increase in soil-derived CO₂-C compared to control) than in the low soil moisture treatment (up to 52% increase). Greater plant-derived CO₂-C and plant biomass in the high soil moisture treatment contributed to greater priming effects, but priming effects remained significantly higher in the high moisture treatment than in the low moisture treatment after correcting for the effects of plant-derived CO₂-C and plant biomass. The response to soil moisture particularly occurred in the sandy loam soil by the end of the experiment. Possibly, production of root exudates increased with increased soil moisture content. Root exudation of labile C may also have become more effective in stimulating microbial decomposition in the higher soil moisture treatment and sandy loam soil. Our results indicate that moisture conditions significantly modulate rhizosphere effects on SOM decomposition.     

Rhizosphere priming effect: Its functional relationships with microbial turnover, evapotranspiration, and C–N budgets

Understanding soil organic matter (SOM) decomposition and its interaction with rhizosphere processes is a crucial topic in soil biology and ecology. Using a natural ¹³C tracer method to separately measure SOM-derived CO₂ from root-derived CO₂, this study aims to connect the level of rhizosphere-dependent SOM decomposition with the C and N balance of the whole plant–soil system, and to mechanistically link the rhizosphere priming effect to soil microbial turnover and evapotranspiration. Results indicated that the magnitude of the rhizosphere priming effect on SOM decomposition varied widely, from zero to more than 380% of the unplanted control, and was largely influenced by plant species and phenology. Balancing the extra soil C loss from the strong rhizosphere priming effect in the planted treatments with C inputs from rhizodeposits and root biomass, the whole plant–soil system remained with a net carbon gain at the end of the experiment. The increased soil microbial biomass turnover rate and the enhanced evapotranspiration rate in the planted treatments had clear positive relationships with the level of the rhizosphere priming effect. The rhizosphere enhancement of soil carbon mineralization in the planted treatments did not result in a proportional increase in net N mineralization, suggesting a possible de-coupling of C cycling with N cycling in the rhizosphere.     

A novel method for separating root‐derived organic compounds from root respiration in non‐sterilized soils

A novel method of separating exudates from root respiration in non‐sterilized soils has been developed. The method is based on a simultaneous elution of exudates from rhizosphere and the blowout of CO₂ originating from root respiration. The innovation of the method lies in the function of a membrane pump to drive the movement of air and simultaneously the circulation of water according to the Siphon principle. The separation method was tested by means of ¹⁴C pulse labeling of Lolium perenne to track the C dynamics in the production of rhizosphere CO₂ and of exudates, which were eluted. The total ¹⁴C activity of rhizosphere CO₂ and of eluted exudates was found to be 8.5 % and 2.3 % of total assimilated ¹⁴C, respectively. Thus, at least 19 % of root‐derived C can be accounted to root exudation. However, the suggested Siphon method underestimates the amount of exudates and shows only a minimum of organic substances exuded by roots. The diurnal dynamics of exudation was detected, but no significant day‐night changes were measured in root and microbial respiration. Tight coupling of assimilation with exudation, but not with root and microbial respiration, was observed. The advantages, shortcomings, and possible applications of the Siphon method are discussed.     

Effect of active roots on the decomposition of soil organic materials

Summary The effect of one form of soil organic matter, such as living roots or root exudates on another form of soil organic matter, such as dead roots or incorporated litter and litter leachates, has been studied from various perspectives over the last 25 years. The effect seems to be either positive (priming) or negative (conserving). The present review concentrates on the conserving effect, measured as a decrease in ¹⁴CO₂ released, in both field and greenhouse/growth chamber studies. The field experiments suggested that certain physical conditions in the soil, such as less available moisture or restricted aeration which led to lower microbial activity, explained the conserving effect of living roots on soil organic matter. Although more detailed greenhouse/growth chamber studies confirmed the conserving effect per se, it appears that biological rather than physical factors could better explain the reduction in the rate of decomposition of ¹⁴C-labelled plant residues in the presence of roots. However, a complex picture has emerged through a variety of postulates, all proposed in attempts to explain the conserving effect. Finally, the most recent studies have argued that the decrease in decomposition of labelled organic matter in planted soil is probably more apparent than real. A decrease in respired ¹⁴CO₂ could be explained by an incorporation of ¹⁴C derived from old roots into the rhizosphere microbial populations of the living roots. To make any further progress on the fundamental question of how soil organic matter moves along its continuum from a living to a refractory state, the microenvironment needs to be examined at periodic intervals. New developments in improved histochemical and electron-probe microanalyses look promising.LRS Contribution no. 3878970     

Cattle grazing drives nitrogen and carbon cycling in a temperate salt marsh

We examined the impact of long-term cattle grazing on soil processes and microbial activity in a temperate salt marsh. Soil conditions, microbial biomass and respiration, mineralization and denitrification rates were measured in upper salt marsh that had been ungrazed or cattle grazed for several decades. Increased microbial biomass and soil respiration were observed in grazed marsh, most likely stimulated by enhanced rates of root turnover and root exudation. We found a significant positive effect of grazing on potential N mineralization rates measured in the laboratory, but this difference did not translate to in situ net mineralization measured monthly from May to September. Rates of denitrification were lowest in the grazed marsh and appeared to be limited by nitrate availability, possibly due to more anoxic conditions and lower rates of nitrification. The major effect of grazing on N cycling therefore appeared to be in limiting losses of N through denitrification, which may lead to enhanced nutrient availability to saltmarsh plants, but a reduced ability of the marsh to act as a buffer for land-derived nutrients to adjacent coastal areas. Additionally, we investigated if grazing influences the rates of turnover of labile and refractory C in saltmarsh soils by adding ¹⁴C-labelled leaf litter or root exudates to soil samples and monitoring the evolution of ¹⁴CO₂. Grazing had little effect on the rates of mineralization of ¹⁴C used as a respiratory substrate, but a larger proportion of ¹⁴C was partitioned into microbial biomass and immobilized in long- and medium-term storage pools in the grazed treatment. Grazing slowed down the turnover of the microbial biomass, which resulted in longer turnover times for both leaf litter and root exudates. Grazing may therefore affect the longevity of C in the soil and alter C storage and utilization pathways in the microbial community.     

Carbon turnover in the rhizosphere under continuous plant labeling with <Superscript>13</Superscript>CO<Subscript>2</Subscript>: Partitioning of root,microbial, and rhizomicrobial respiration

The input of labeled C into the pool of soil organic matter, the CO₂ fluxes from the soil, and the contribution of root and microbial respiration to the CO₂ emission were studied in a greenhouse experiment with continuous labeling of oat plants with ¹³CO₂ using the method of the natural ¹³C abundance in the air. The carbon of the microbial biomass composed 56 and 39% of the total amounts of ¹³C photoassimilates in the rhizosphere and in the bulk soil, respectively. The contribution of root respiration to the CO₂ emission from the soil reached 61–92%, including 4–23% of the rhizomicrobial respiration. The contribution of the microbial respiration to the total CO₂ emission from the soil varied from 8 to 39%. The soil organic matter served as the major carbon-containing substrate for microorganisms in the bulk soil and in the rhizosphere: 81–91% of the total amount of carbon involved in the microbial metabolism was derived from the soil organic matter.     

Short and long-term effects of elevated CO2 on Lolium perenne rhizodeposition and its consequences on soil organic matter turnover and plant N yield

It is still unclear whether elevated CO₂ increases plant root exudation and consequently affects the soil microbial biomass. The effects of elevated CO₂ on the fate of the C and nitrogen (N) contained in old soil organic matter pools is also unclear. In this study the short and long-term effects of elevated CO₂ on C and N pools and fluxes were assessed by growing isolated plants of ryegrass (Lolium perenne) in glasshouses at elevated and ambient atmospheric CO₂ and using soil from the New Zealand FACE site that had >4 years exposure to CO₂ enrichment. Using ¹⁴CO₂ pulse labelling, the effects of elevated CO₂ on C allocation within the plant-soil system were studied. Under elevated CO₂ more root derived C was found in the soil and in the microbial biomass 48 h after labelling. The increased availability of substrate significantly stimulated soil microbial growth and acted as priming effect, enhancing native soil organic matter decomposition regardless of the mineral N supply. Despite indications of faster N cycling in soil under elevated CO₂, N availability to plants stayed unchanged. Soil previously exposed to elevated CO₂ exhibited a higher N cycling rate but again there was no effect on plant N uptake. With respect to the difficulties of extrapolating glasshouse experiment results to the field, we concluded that the accumulation of coarse organic matter observed in the field under elevated CO₂ was probably not created by an imbalance between C and N but was likely to be due to more complex phenomena involving soil mesofauna and/or other nutrients limitations.     

Qualitative and quantitative analysis of water‐soluble root exudates in relation to plant species and development

The composition of root‐derived substances is of great importance for the understanding of processes in the rhizosphere. Therefore, methods allowing a comprehensive collection and chemical analysis of the organic root exudates are necessary. In this study, we compare different methods with regard to their suitability to collect and characterize root exudates. Because the percolation or water logging method failed to quantitatively extract root exudates, a dipping method was developed which allowed an almost complete sampling of coldwater‐soluble root exudates. By ¹⁴CO₂ labeling of the shoots the composition of root exudates was found to be influenced by plant species and growth stage. In comparison to pea plants maize plants had a higher share of carboxylic acids and a lower share of sugars. Younger maize plants exuded considerably higher amounts of ¹⁴C labeled organic substances per g root dry matter than older ones. During plant development the relative amount of sugars decreased at the expense of carboxylic acids. The described methods are well suited for the elucidation of the influence of growth factors on root exudation.     

Effect of barley plants on the decomposition of 14C-labelled soil organic matter

The effect of barley plants on the rate of decomposition of soil organic matter over a 6-week period was studied using soil that had been previously labelled by incubation with ¹⁴C-labelled ryegrass for 1 year. The plants reduced the loss of ¹⁴CO₂, from soil by 70 per cent over 42 days. About half of the reduction was accounted for by the uptake of labelled C by the plant roots, very little ¹⁴C label being associated with the shoot. Chemical fractionation of the root showed that the ¹⁴C was chemically incorporated into cell wall materials such as cellulose and holocellulose. The reduction in organic matter decomposition in the presence of plants has been explained by earlier workers in terms ofa reduction in microbial activity as a result of a soil moisture deficit caused by plant transpiration. This explanation does not account for all the reduction in decomposition noted in the present experiments. Control soil (without a plant, but amended with glucose or yeast extract to simulate the effect of root exudates) showed a small positive priming effect, the release of ¹⁴CO₂, being increased. Thus the mechanism by which plants conserve organic matter is complex and cannot be explained merely by analogy to an increased level of nutrients available for microbial metabolism.     

Interaction between rhizosphere microorganisms and plant roots: 13C fluxes in the rhizosphere after pulse labeling

The input dynamics of labeled C into pools of soil organic matter and CO₂ fluxes from soil were studied in a pot experiment with the pulse labeling of oats and corn under a ¹³CO₂ atmosphere, and the contribution of the root and microbial respiration to the emission of CO₂ from the soil was determined from the fluxes of labeled C in the microbial biomass and the evolved carbon dioxide. A considerable amount of ¹³C (up to 96% of the total amount of the label found in the rhizosphere soil) was incorporated into the biomass of the rhizosphere microorganisms. The diurnal fluctuations of the labeled C pools in the microbial biomass, dissolved organic carbon, and CO₂ released in the rhizosphere of oats and corn were related to the day/night changes, i.e., to the on and off periods of the photosynthetic activity of the plants. The average contribution of the corn root respiration (70% of the total CO₂ emission from the soil surface) was higher than that of the oats roots (44%), which was related to the lower incorporation of rhizodeposit carbon into the microbial biomass in the soil under the corn plants than in the soil under the oats plants.     

Role of arbuscular mycorrhizal network in carbon and phosphorus transfer between plants

Intercropping with aerobic rice or arbuscular mycorrhizal fungi (AMF) colonization alleviated watermelon wilt disease, which is likely attributed to rice root exudates or AMF depressing watermelon wilt pathogen. However, it is unclear whether rice root exudates transfers to watermelon rhizosphere soil and whether AMF affects the transfer of rice root exudates to watermelon rhizosphere soil. A rhizobox experiment, with aerobic rice under ¹⁴?CO₂, was conducted to investigate the effect of AMF colonization on carbon (C) transfer from rice to watermelon and on phosphorus (P) uptake by both watermelon and rice. The rhizobox was separated into labelling side (L side) and sampling side (S side) by inserting nylon mesh in the middle of the box. The L side was planted with aerobic rice, and the S side was aerobic rice (monocropping) or watermelon (intercropping). When ¹⁴?CO₂ was added to rice canopy at the L side, ¹⁴?C activities of rice roots and rhizosphere soils in the L side were increased by intercropping with watermelon or AMF colonization. The ¹⁴?C was detected in roots and rhizosphere soils of rice and watermelon in the S side, but no differences were found among different treatments. ¹⁴?C activities in leaves were improved by AMF inoculation in the S side, regardless of rice or watermelon. Mycorrhizal colonization stimulated P absorption and translocation to rice in intercropping system. These findings suggest that AMF colonization could increase C transfer from rice to watermelon while intercropping with watermelon could promote AMF colonization and P uptake by rice.     

Determination of the fate of 13C labelled maize and wheat exudates in an agricultural soil during a short-term incubation

A broader knowledge of the contribution of carbon (C) released by plant roots (exudates) to soil is a prerequisite for optimizing the management of organic matter in arable soils. This is the first study to show the contribution of constantly applied ¹³C‐labelled maize and wheat exudates to water extractable organic carbon (WEOC), microbial biomass‐C (MB‐C), and CO₂‐C evolution during a 25‐day incubation of agricultural soil material. The CO₂‐C evolution and respective δ¹³C values were measured daily. The WEOC and MB‐C contents were determined weekly and a newly developed method for determining δ¹³C values in soil extracts was applied. Around 36% of exudate‐C of both plants was recovered after the incubation, in the order WEOC < MB‐C < CO₂‐C for maize and MB‐C < WEOC < CO₂‐C for wheat. Around 64% of added exudate‐C was not retrieved with the methods used here. Our results suggest that great amounts of exudates became stabilized in non‐water extractable organic fractions. The amounts of MB‐C stayed relatively constant over time despite a continuous exudate‐C supply, which is the prerequisite for a growing microbial population. A lack of mineral nutrients might have limited microbial growth. The CO₂‐C mineralization rate declined during the incubation and this was probably caused by a shift in the microbial community structure. Consequently, incoming WEOC was left in the soil solution leading to rising WEOC amounts over time. In the exudate‐treated soil additional amounts of soil‐derived WEOC (up to 110 μg g⁻¹) and MB‐C (up to 60 μg g⁻¹) relative to the control were determined. We suggest therefore that positive priming effects (i.e. accelerated turnover of soil organic matter due to the addition of organic substrates) can be explained by exchange processes between charged, soluble C‐components and the soil matrix. As a result of this exchange, soil‐derived WEOC becomes available for mineralization.     

Priming effects: Interactions between living and dead organic matter

In this re-evaluation of our 10-year old paper on priming effects, I have considered the latest studies and tried to identify the most important needs for future research. Recent publications have shown that the increase or decrease in soil organic matter mineralization (measured as changes of CO₂ efflux and N mineralization) actually results from interactions between living (microbial biomass) and dead organic matter. The priming effect (PE) is not an artifact of incubation studies, as sometimes supposed, but is a natural process sequence in the rhizosphere and detritusphere that is induced by pulses or continuous inputs of fresh organics. The intensity of turnover processes in such hotspots is at least one order of magnitude higher than in the bulk soil. Various prerequisites for high-quality, informative PE studies are outlined: calculating the budget of labeled and total C; investigating the dynamics of released CO₂ and its sources; linking C and N dynamics with microbial biomass changes and enzyme activities; evaluating apparent and real PEs; and assessing PE sources as related to soil organic matter stabilization mechanisms. Different approaches for identifying priming, based on the assessment of more than two C sources in CO₂ and microbial biomass, are proposed and methodological and statistical uncertainties in PE estimation and approaches to eliminating them are discussed. Future studies should evaluate directions and magnitude of PEs according to expected climate and land-use changes and the increased rhizodeposition under elevated CO₂ as well as clarifying the ecological significance of PEs in natural and agricultural ecosystems. The conclusion is that PEs - the interactions between living and dead organic matter - should be incorporated in models of C and N dynamics, and that microbial biomass should regarded not only as a C pool but also as an active driver of C and N turnover.     

Corn (Zea mays L.) root exudates and their impact on 14C-pyrene mineralization

Corn (Zea mays L.) root exudates were flushed from a hydrophobic system that allowed the aseptic separation of soluble exudates from the intact plant root. Plants were grown for 90 d, during which time root exudates flowed from the hydroponic setup directly onto columns containing soil previously contaminated with polycyclic aromatic hydrocarbons (PAHs). Mineralization of the PAH, pyrene, was then determined in soil removed from columns. In addition, exudated samples were directly taken from the hydroponic system for estimation of total organic carbon release and for use in microbial studies. In soil from columns that received root exudates from a planted (versus an unplanted) apparatus, there was a significant increase in ¹⁴C-pyrene mineralization. The extent of stimulation was comparable to that measured in rhizosphere soil isolated from plants growing in the same soil. Soil from columns that received solution from apparatuses that were not planted showed no stimulation of ¹⁴C-pyrene mineralization. Separate studies confirmed that some members of the soil microbial community were able to utilize these soluble plant compounds. This indicates that root exudates have the potential to increase the degradation of xenobiotics by the growth of soil microorganisms. Separating the chemical impact of the root exudates from any root surface phenomena is an important step in isolating a potential mechanism of phytoremediation. Many studies have speculated on the involvement of root exudates in rhizosphere degradation of organic contaminants, but very few studies go beyond adding simple carbon substrates in short pulses. This study employed a system that exposed the microbial community to real root exudates in the concentrations and over a time period that mimicked actual conditions.     

Carbon Turnover in the Rhizosphere

Considerable progress has been made during the last decade towards understanding and quantifying the input and turnover of plant carbon in the rhizosphere. This was made possible by the development (partially by the authors) and combination of appropriate new methods, such as:

• –homogeneous labelling of whole plants with ¹⁴C
• –distinction between root and microbial respiration
• –separation of soil zones of known distances from the roots
• –determination of microbial soil biomass.

These methods were applied to study the following aspects:

• –release of organic plant carbon into the soil by growing roots
• –utilization of this plant carbon by the microbial biomass in the rhizosphere
• –related influence on the turnover of soil organic matter, and
• –spatial range of such root influence in the soil.

About 19% of the total photosynthetic production of the investigated plants was released into the rhizosphere as organic material. Most of this (15%) was transformed by the rhizosphere microorganisms into CO₂, while only a small fraction (4%) remained in the soil, mainly as microbial cells (2.5%). As a result, microbial rhizosphere biomass increased considerably. Relative to the organic C-input, however, the incorporation of root derived carbon by the microbial biomass was remarkably low (13%). Along with the increase in microbial rhizosphere biomass, the presence of plant roots also enhanced the decomposition of soil organic matter and affected soil aggregate stability. Root carbon and root influences were even detected up to 20 mm away from the roots. This may be partially attributed to the contribution of root derived volatiles. Accordingly, both the actual volume of the rhizosphere and its metabolic significance is greater than what has so far been assumed. Possible interactions involving root, soil and microbial carbon are discussed.     

Carbon fluxes in soil food webs of increasing complexity revealed by C labelling and C natural abundance

Soil food webs are mainly based on three primary carbon (C) sources: root exudates, litter, and recalcitrant soil organic matter (SOM). These C sources vary in their availability and accessibility to soil organisms, which could lead to different pathways in soil food webs. The presence of three C isotopes (¹²C, ¹³C and ¹⁴C) offers an unique opportunity to investigate all three C sources simultaneously. In a microcosm experiment we studied the effect of food web complexity on the utilization of the three carbon sources. We choose an incomplete three factorial design with (i) living plants, (ii) litter and (iii) food web complexity. The most complex food web consisted of autochthonous microorganisms, nematodes, collembola, predatory mites, endogeic and anecic earthworms. We traced C from all three sources in soil, in CO₂ efflux and in individual organism groups by using maize grown on soil developed under C₃ vegetation and application of ¹⁴C labelled ryegrass shoots as a litter layer. The presence of living plants had a much greater effect on C pathways than food web complexity. Litter decomposition, measured as ¹⁴CO₂ efflux, was decreased in the presence of living plants from 71% to 33%. However, living plants increased the incorporation of litter C into microbial biomass and arrested carbon in the litter layer and in the upper soil layer. The only significant effect of food web complexity was on the litter C distribution in the soil layers. In treatments with fungivorous microarthropods (Collembola) the incorporation of litter carbon into mineral soil was reduced. Root exudates as C source were passed through rhizosphere microorganisms to the predator level (at least to the third trophic level). We conclude that living plants strongly affected C flows, directly by being a source of additional C, and indirectly by modifying the existing C flows within the food web including CO₂ efflux from the soil and litter decomposition.     

Does accelerated soil organic matter decomposition in the presence of plants increase plant N availability?

Plant roots can increase microbial activity and soil organic matter (SOM) decomposition via rhizosphere priming effects. It is virtually unknown how differences in the priming effect among plant species and soil type affect N mineralization and plant uptake. In a greenhouse experiment, we tested whether priming effects caused by Fremont cottonwood (Populus fremontii) and Ponderosa pine (Pinus ponderosa) grown in three different soil types increased plant available N. We measured primed C as the difference in soil-derived CO₂-C fluxes between planted and non-planted treatments. We calculated “excess plant available N” as the difference in plant available N (estimated from changes in soil inorganic N and plant N pools at the start and end of the experiment) between planted and non-planted treatments. Gross N mineralization at day 105 was significantly greater in the presence of plants across all treatments, while microbial N measured at the same time was not affected by plant presence. Gross N mineralization was significantly positively correlated to the rate of priming. Species effects on plant available N were not consistent among soil types. Plant available N in one soil type increased in the P. fremontii treatment but not in the P. ponderosa treatment, whereas in the other two soils, the effects of the two plant species were reversed. There was no relationship between the cumulative amount of primed C and excess plant available N during the first 107 days of the experiment when inorganic N was still abundant in all planted soils. However, during the second half of the experiment (days 108-398) when soil inorganic N in the planted treatments was depleted by plant N uptake, the cumulative amount of primed C was significantly positively correlated to excess plant available N. Primed C explained 78% of the variability in plant available N for five of the six plant-soil combinations. Excess plant available N could not be predicted from cumulative amount of primed C in one species-soil type combination. Possibly, greater microbial N immobilization due to large inputs of rhizodeposits with low N concentration may have reduced plant available N or we may have underestimated plant available N in this treatment because of N loss through root exudation and death. We conclude that soil N availability cannot be determined by soil properties alone, but that is strongly influenced by root-soil interactions.     

Plant biomass influences rhizosphere priming effects on soil organic matter decomposition in two differently managed soils

We used a continuous labeling method of naturally ¹³C-depleted CO₂ in a growth chamber to test for rhizosphere effects on soil organic matter (SOM) decomposition. Two C3 plant species, soybean (Glycine max) and sunflower (Helianthus annus), were grown in two previously differently managed soils, an organically farmed soil and a soil from an annual grassland. We maintained a constant atmospheric CO₂ concentration at 400±5 ppm and δ¹³C signature at −24.4‰ by regulating the flow of naturally ¹³C-depleted CO₂ and CO₂-free air into the growth chamber, which allowed us to separate new plant-derived CO₂-C from original soil-derived CO₂-C in soil respiration. Rhizosphere priming effects on SOM decomposition, i.e., differences in soil-derived CO₂-C between planted and non-planted treatments, were significantly different between the two soils, but not between the two plant species. Soil-derived CO₂-C efflux in the organically farmed soil increased up to 61% compared to the no-plant control, while the annual grassland soil showed a negligible increase (up to 5% increase), despite an overall larger efflux of soil-derived CO₂-C and total soil C content. Differences in rhizosphere priming effects on SOM decomposition between the two soils could be largely explained by differences in plant biomass, and in particular leaf biomass, explaining 49% and 74% of the variation in primed soil C among soils and plant species, respectively. Nitrogen uptake rates by soybean and sunflower was relatively high compared to soil C respiration and associated N mineralization, while inorganic N pools were significantly depleted in the organic farm soil by the end of the experiment. Despite relatively large increases in SOM decomposition caused by rhizosphere effects in the organic farm soil, the fast-growing soybean and sunflower plants gained little extra N from the increase in SOM decomposition caused by rhizosphere effects. We conclude that rhizosphere priming effects of annual plants on SOM decomposition are largely driven by plant biomass, especially in soils of high fertility that can sustain high plant productivity.     

Sources of CO2 efflux from soil and review of partitioning methods

Five main biogenic sources of CO₂ efflux from soils have been distinguished and described according to their turnover rates and the mean residence time of carbon. They are root respiration, rhizomicrobial respiration, decomposition of plant residues, the priming effect induced by root exudation or by addition of plant residues, and basal respiration by microbial decomposition of soil organic matter (SOM). These sources can be grouped in several combinations to summarize CO₂ efflux from the soil including: root-derived CO₂, plant-derived CO₂, SOM-derived CO₂, rhizosphere respiration, heterotrophic microbial respiration (respiration by heterotrophs), and respiration by autotrophs. These distinctions are important because without separation of SOM-derived CO₂ from plant-derived CO₂, measurements of total soil respiration have very limited value for evaluation of the soil as a source or sink of atmospheric CO₂ and for interpreting the sources of CO₂ and the fate of carbon within soils and ecosystems. Additionally, the processes linked to the five sources of CO₂ efflux from soil have various responses to environmental variables and consequently to global warming. This review describes the basic principles and assumptions of the following methods which allow SOM-derived and root-derived CO₂ efflux to be separated under laboratory and field conditions: root exclusion techniques, shading and clipping, tree girdling, regression, component integration, excised roots and insitu root respiration; continuous and pulse labeling, ¹³C natural abundance and FACE, and radiocarbon dating and bomb-¹⁴C. A short sections cover the separation of the respiration of autotrophs and that of heterotrophs, i.e. the separation of actual root respiration from microbial respiration, as well as methods allowing the amount of CO₂ evolved by decomposition of plant residues and by priming effects to be estimated. All these methods have been evaluated according to their inherent disturbance of the ecosystem and C fluxes, and their versatility under various conditions. The shortfalls of existing approaches and the need for further development and standardization of methods are highlighted.