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.     

Nodulated soybean enhances rhizosphere priming effects on soil organic matter decomposition more than non-nodulated soybean

The phenomenon that rhizosphere processes significantly control soil organic matter (SOM) decomposition, also termed rhizosphere priming effect (RPE), is now increasingly recognized as significant as the effects of soil temperature and moisture on SOM decomposition. However, the exact mechanisms responsible for RPE remain largely unknown. Particularly, some reports have suggested that the quality of rhizodeposits may play a significant role in causing different levels of RPE among various plant species. However, direct evidence for the “rhizodeposit quality hypothesis” has been lacking. Here we tested the hypothesis by investigating RPE on soil carbon (C) and nitrogen (N) mineralization of two soybean (Glycine max L. Merr.) isolines differing only in their ability to form nodules and to fix N₂, and thus differing in tissue N concentration and rhizodeposit quality. We used a continuous ¹³C-labeling method for measuring RPE on soil organic C decomposition, and employed an N-budgeting method for quantifying RPE on soil net N mineralization. We found that the rhizodeposits from nodulated soybean produced a stronger RPE (53% vs. 26%) on soil organic C decomposition than the rhizodeposits from non-nodulated soybean at the maturity stage when nodulated soybean had significantly higher plant tissue N concentration but similar plant biomass, while both soybean isolines produced a similar RPE (33–34%) at the vegetative stage when there was no difference in plant tissue N concentration or plant biomass. The levels of RPE on soil net N mineralization were similar between the two isolines, ranging from 25% at the vegetative stage to 38–46% at the maturity stage. Moreover, RPE on soil organic C decomposition was not linearly proportional to RPE on soil net N mineralization. These results indicate that higher rhizodeposit quality is one of the most likely causes to the higher RPE of the nodulated soybean compared to the non-nodulated soybean. Further investigations of rhizodeposit quality and quantity between the two soybean isolines are warranted to further test this rhizodeposit quality hypothesis.     

Rhizosphere priming of soil organic matter by bacterial groups in a grassland soil

Plants often impact the rate of native soil organic matter turnover through root interactions with soil organisms; however the role of root-microbial interactions in mediation of the “priming effect” is not well understood. We examined the effects of living plant roots and N fertilization on belowground C dynamics in a California annual grassland soil (Haploxeralf) during a two-year greenhouse study. The fate of ¹³C-labeled belowground C (roots and organic matter) was followed under planted (Avena barbata) and unplanted conditions, and with and without supplemental N (20 kg N ha⁻¹ season⁻¹) over two periods of plant growth, each followed by a dry, fallow period of 120 d. Turnover of belowground ¹³C SOM was followed using ¹³C-phospholipid fatty acid (PLFA) biomarkers. Living roots increased the turnover and loss of belowground ¹³C compared with unplanted soils. Planted soils had 20% less belowground ¹³C present than in unplanted soils after 2 cycles of planting and fallow. After 2 treatment cycles, unlabeled soil C was 4.8% higher in planted soils than unplanted. The addition of N to soils decreased the turnover of enriched belowground ¹³C during the first treatment season in both planted and unplanted soils, however no effect of N was observed thereafter. Our findings suggest that A. barbata may increase soil C levels over time because root and exudate C inputs are significant, but that increase will be moderated by an overall faster C mineralization rate of belowground C. N addition may slow soil C losses; however, the effect was minor and transient in this system. The labeled root-derived ¹³C was initially recovered in gram negative (highest enrichment), gram positive, and fungal biomarkers. With successive growing seasons, the labeled C in the gram negative and fungal markers declined, while gram positive markers continued to accumulate labeled belowground C. The rhizosphere of A. barbata shifted the microbial community composition, resulting in greater abundances of gram negative markers and lower abundances of gram positive, actinobacteria and cyclopropyl PLFA markers compared to unplanted soil. However, the longer-term utilization of labeled belowground C by gram positive bacteria was enhanced in the rhizosphere microbial community compared with unplanted soils. We suggest that the activities of gram positive bacteria may be major controllers of multi-year rhizosphere-related priming of SOM decomposition.     

Seasonal dynamics of leaf- and root-derived C in arctic tundra mesocosms

We investigated contributions of leaf litter, root litter and root-derived organic material to tundra soil carbon (C) storage and transformations. ¹⁴C-labeled materials were incubated for 32 weeks in moist tussock tundra soil cores under controlled climate conditions in growth chambers, which simulated arctic fall, winter, spring and summer temperatures and photoperiods. In addition, we tested whether the presence of living plants altered litter and soil organic matter (SOM) decomposition by planting shoots of the sedge Eriophorum vaginatum in half of the cores. Our results suggest that root litter accounted for the greatest C input and storage in these tundra soils, while leaf litter was rapidly decomposed and much of the C lost to respiration. We observed transformations of ¹⁴C between fractions even when total C appeared unchanged, allowing us to elucidate sources and sinks of C used by soil microorganisms. Initial sources of C included both water soluble (WS) and acid-soluble (AS) fractions, primarily comprised of carbohydrates and cellulose, respectively. The acid-insoluble (AIS) fraction appeared to be a sink for C when conditions were favorable for plant growth. However, decreases in ¹⁴C activity from the AIS fraction between the fall and spring harvests in all treatments indicated that microorganisms consumed recalcitrant C compounds when soil temperatures were below 0 °C. In planted leaf litter cores and in both planted and unplanted SOM cores, the greatest amounts of ¹⁴C at the end of the experiment were found in the AIS fraction, suggesting a high rate of humification or accumulation of decay-resistant plant tissues. In unplanted leaf litter cores and planted and unplanted root litter cores most of the ¹⁴C remaining at the end of the experiment was in the AS fraction suggesting less extensive humification of leaf and root detritus. Overall, the presence of living plants stimulated decomposition of leaf litter by creating favorable conditions for microbial activity at the soil surface. In contrast, plants appeared to inhibit decomposition of root litter and SOM, perhaps because of microbial preferences for newer, more labile inputs from live roots.     

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.     

Scots pine (Pinus sylvestris L.) roots and soil moisture did not affect soil thermal sensitivity

Through their effects on microbial metabolism, temperature and moisture affect the rate of decomposition of soil organic matter. Plant roots play an important role in SOM mineralization and nutrient cycling. There are reports that rhizosphere soil exhibits higher sensitivity to temperature than root-free soil, and this can have implications for how soil CO₂ efflux may be affected in a warmer world. We tested the effects of 1-week incubation under different combinations of temperature (5, 15, 30 ^°C) and moisture (15, 50, 100% WHC) on the respiration rate of soil planted with Scots pine and of unplanted soil. Soil respiration in both soils was the highest at moderate moisture (p < 0.0001) and, increased with temperature (p < 0.0001). There was also marginally significant effect of soil kind on respiration rate (p < 0.055), but the significant interaction of temperature effect with soil kind effect, indicated, that soil respiration of planted soil was higher than unplanted soil only at 5 ^°C (p < 0.05). The soil kind effect was compared also as Q₁₀ coefficients for respiration rate, showing the relative change in microbial activity with increased temperature. However, there was no difference in the thermal sensitivity of soil respiration between planted and unplanted soils (p = 0.99), irrespective of the level of soil moisture. These findings were similar to the latest studies and confirmed, that in various models, being useful tools in studying of soil carbon cycling, there is no need to distinguish between planted and unplanted soil as different soil carbon pools.     

Rhizosphere priming effect of Populus fremontii obscures the temperature sensitivity of soil organic carbon respiration

C efflux from soils is a large component of the global C exchange between the biosphere and the atmosphere. However, our understanding of soil C efflux is complicated by the “rhizosphere priming effect,” in which the presence of live roots may accelerate or suppress the decomposition of soil organic C. Due to technical obstacles, the rhizosphere priming effect is under-studied, and we know little about rhizosphere priming in tree species. We measured the rates of soil-derived C mineralization in root-free soil and in soil planted with cottonwood (Populus fremontii) trees. Live cottonwood roots greatly accelerated (a rhizosphere priming effect) or suppressed (a negative rhizosphere priming effect) the mineralization of soil organic C, depending upon the time of the year. At its maximum, soil organic C was mineralized nine times faster in the presence of cottonwood roots than in the unplanted controls. Over the course of the experiment, approximately twice as much soil organic C was mineralized in pots planted with cottonwoods compared to unplanted control pots. Soil organic C mineralization rates in the unplanted controls were temperature-sensitive. In contrast, soil organic C mineralization in the cottonwood rhizosphere was unresponsive to seasonal temperature changes, due to the strength of the rhizosphere priming effect. The rhizosphere priming effect is of key importance to our understanding of soil C mineralization, because it means that the total soil respiration is not a simple additive function of soil-derived and plant-derived respiration.     

Soil microbial biomass and nitrogen dynamics in a turfgrass chronosequence: A short-term response to turfgrass clipping addition

A mechanistic understanding of soil microbial biomass and N dynamics following turfgrass clipping addition is central to understanding turfgrass ecology. New leaves represent a strong sink for soil and fertilizer N, and when mowed, a significant addition to soil organic N. Understanding the mineralization dynamics of clipping N should help in developing strategies to minimize N losses via leaching and denitrification. We characterized soil microbial biomass and N mineralization and immobilization turnover in response to clipping addition in a turfgrass chronosequence (i.e. 3, 8, 25, and 97 yr old) and the adjacent native pines. Our objectives were (1) to evaluate the impacts of indigenous soil and microbial attributes associated with turf age and land use on the early phase decomposition of turfgrass clippings and (2) to estimate mineralization dynamics of turfgrass clippings and subsequent effects on N mineralization of indigenous soils. We conducted a 28-d laboratory incubation to determine short-term dynamics of soil microbial biomass, C decomposition, N mineralization and nitrification after soil incorporation of turfgrass clippings. Gross rates of N mineralization and immobilization were estimated with ¹⁵N using a numerical model, FLAUZ. Turfgrass clippings decomposed rapidly; decomposition and mineralization equivalent to 20-30% of clipping C and N, respectively, occurred during the incubation. Turfgrass age had little effect on decomposition and net N mineralization. However, the response of potential nitrification to clipping addition was age dependent. In young turfgrass systems having low rates, potential nitrification increased significantly with clipping addition. In contrast, old turfgrass systems having high initial rates of potential nitrification were unaffected by clipping addition. Isotope ¹⁵N modeling showed that gross N mineralization following clipping addition was not affected by turf age but differed between turfgrass and the adjacent native pines. The flush of mineralized N following clipping addition was derived predominantly from the clippings rather than soil organic N. Our data indicate that the response of soil microbial biomass and N mineralization and immobilization to clipping addition was essentially independent of indigenous soil and microbial attributes. Further, increases in microbial biomass and activity following clipping addition did not stimulate the mineralization of indigenous soil organic N.     

Plant roots and species moderate the salinity effect on microbial respiration,biomass, and enzyme activities in a sandy clay soil

The aim of this study was to determine the effects of plant absence or presence on microbial properties and enzyme activities at different levels of salinity in a sandy clay soil. The treatments involved five salinity levels—0.5 (control), 2.5, 5, 7.5, and 10 dS m^?1 which were prepared using a mixture of chloride salts—and three soil environments (unplanted soil, and soils planted with either wheat or clover) under greenhouse conditions. Each treatment was replicated three times. At the end of the experiment, soil microbial respiration, substrate-induced respiration (SIR), microbial biomass C (MBC), and enzyme activities were determined after plant harvest. Increasing salinity decreased soil microbial properties and enzyme activities, but increased the metabolic quotient (qCO₂) in both unplanted and planted soils. Most microbial properties of planted soils were greater than those of unplanted soils at low to moderate salinity levels, depending upon plant species. There was a small or no difference in soil properties between the unplanted and planted treatments at the highest salinity level, indicating that the indirect effects of plant presence might be less important due to significant reduction of plant growth. The lowered microbial activity and biomass, and enzyme activities were due to the reduction of root activity and biomass in salinized soils. The lower values of qCO₂ in planted than unplanted soils support the positive influence of plant root and its exudates on soil microbial activity and biomass in saline soils. Nonetheless, the role of plants in alleviating salinity influence on soil microbial activities decreases at high salinity levels and depends on plant type. In conclusion, cultivation and growing plant in abandoned saline environments with moderate salinity would improve soil microbial properties and functions by reducing salinity effect, in particular planting moderately tolerant crops. This helps to maintain or increase the fertility and quality of abandoned saline soils in arid regions.     

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.     

A field study of gross rates of N mineralization and nitrification and their relationships to microbial biomass and enzyme activities in soils treated with dairy effluent and ammonium fertilizer

Abstract. Gross N mineralization and nitrification rates were measured in soils treated with dairy shed effluent (DSE) (i.e. effluent from the dairy milking shed, comprising dung, urine and water) or ammonium fertilizer (NH₄Cl) under field conditions, by injecting ¹⁵N-solution into intact soil cores. The relationships between gross mineralization rate, microbial biomass C and N and extracellular enzyme activities (protease, deaminase and urease) as affected by the application of DSE and NH₄Cl were also determined. During the first 16 days, gross mineralization rate in the DSE treated soil (4.3–6.1 μg N g^?1 soil day^?1) were significantly (P 14;< 14;0.05) higher than those in the NH₄Cl treated soil (2.6–3.4 μg N g^?1 soil day^?1). The higher mineralization rate was probably due to the presence of readily mineralizable organic substrates in the DSE, accompanied by stimulated microbial and extracellular enzyme activities. The stable organic N compounds in the DSE were slow to mineralize and contributed little to the mineral N pool during the period of the experiment. Nitrification rates during the first 16 days were higher in the NH₄Cl treated soil (1.7–1.2 μg N g^?1 soil day^?1) compared to the DSE treated soil (0.97–1.5 μg N g^?1 soil day^?1). Soil microbial biomass C and N and extracellular enzyme activities (protease, deaminase and urease) increased after the application of the DSE due to the organic substrates and nutrients applied, but declined with time, probably because of the exhaustion of the readily available substrates. The NH₄Cl application did not result in any significant increases in microbial biomass C, protease or urease activities due to the lack of carbonaceous materials in the ammonium fertilizer. However, it did increase microbial biomass N and deaminase activity. Significant positive correlations were found between gross N mineralization rate and soil microbial biomass, protease, deaminase and urease activities. Nitrification rate was significantly correlated to biomass N but not to the microbial biomass C or the enzyme activities. Stepwise regression analysis showed that the variations of gross N mineralization rate was best described by the microbial biomass C and N.     

Depressed gross mineralization and immobilization of nitrogen and changes in microbial population in soil after heat treatment

Partial sterilization causes a change in N mineralization in soil. An increase in the net rate of N mineralization was reported in soil with chloropicrin applied to it (Rovira 1976), and has been well known in soil fumigated with chloroform to measure the microbial biomass N (Jenkinson and Ladd 1981). The gross rate of N mineralization increased in soil inoculated with fresh soil following fumigation with chloroform (Shen et al. 1984). The increased rate of N mineralization has been attributed to the rapid decomposition of organisms killed by partial sterilization (Jenkinson 1966). On the other hand, Nira et al. (1996) reported that the application of a fumigant in a field depressed the gross rates of N mineralization and immobilization in spite of the increase in the net rate of N mineralization. These results suggested that the increase in the net rate of N mineralization by partial sterilization is presumably due to the change in the ratio of N mineralization to immobilization. However, the residues of a fumigant may depress gross N transformation in the field, because the residues may continue to influence microbial activity long after the original treatment (Jenkinson 1966). Some effects of partial sterilization without residues on gross N mineralization remain to be determined.     

Denitrification with and without maize plants (Zea mays L.) under irrigated field conditions

The study was conducted under irrigated field conditions to examine the effect of maize plants on denitrification. Both planted and unplanted field plots received 150kgNha^–1 as urea. In a third treatment, which was also planted and received urea at 150kgNha^–1, the soil nitrate N content was brought up to equal to that in the unplanted plots by applying additional doses of N as calcium nitrate. Soil cores were collected 24 and 72h after irrigation and the denitrification rate was measured by the acetylene inhibition method. Nitrate-N content, aerobically mineralizable C, microbial biomass carrying capacity and denitrification potential were also studied on field-moist soil. Maize plants grown under field conditions always had the potential to increase denitrification in conditions of both high and low water-filled porosity. When nitrate-N content of the planted soil decreased due to plant uptake, denitrification was reduced in the planted soils. However, when nitrate-N uptake by plants was compensated through additional doses of nitrate fertilizer, denitrification was always higher in planted than unplanted soil. The stimulatory effect of plants on denitrification was observed at both high and low soil nitrate-N concentrations, though it was more pronounced at high nitrate-N levels. The effect of plants on denitrification and related parameters was confined to the root zone. Received: 15 April 1996     

Fate of nitrogen from crop residues as affected by biochemical quality and the microbial biomass

Net mineralization of N from a range of shoot and root materials was determined over a period of 6 months following incorporation into a sandy-loam soil under controlled environment conditions. Biochemical “quality” components of the materials showed better correlation with net N mineralization than did gross measures of the respiration and N content of the soil microbial community during decomposition. The quality components controlling net N mineralization changed during decomposition, with water-soluble phenolic content significantly correlated with net N mineralization at early stages, and water-soluble N, followed by cellulose at later stages. C-to-N and total N were correlated with net N mineralization towards the end of the incubation only. Cumulative microbial respiration during the early stages of decomposition was correlated with net N mineralization measured after 2 months, at which time maximum net N mineralization was recorded for most residues. However, there was no relationship between microbial-N and net N mineralization. Biochemical quality factors controlling the C and N content of the residue remaining at the end of the incubation as light fraction organic matter (LFOM) were also investigated. Both C and N content of LFOM derived from the residues were correlated with residue cellulose content, and the chemical characteristics of LFOM were highly correlated with those of the original plant material. Incorporation of low cellulose, high water-soluble N-containing shoot residues resulted in more N becoming mineralized than had been added in the residues, demonstrating that net mineralization of native soil organic matter had occurred. Large amounts of N were lost from the mineral-N pool during the incubation, which could not be accounted for by microbial immobilization.     

Contribution of exudates,arbuscular mycorrhizal fungi and litter depositions to the rhizosphere priming effect induced by grassland species

The presence of plants induces strong accelerations in soil organic matter (SOM) mineralization by stimulating soil microbial activity – a phenomenon known as the rhizosphere priming effect (RPE). The RPE could be induced by several mechanisms including root exudates, arbuscular mycorrhizal fungi (AMF) and root litter. However the contribution of each of these to rhizosphere priming is unknown due to the complexity involved in studying rhizospheric processes. In order to determine the role of each of these mechanisms, we incubated soils enclosed in nylon meshes that were permeable to exudates, or exudates & AMF or exudates, AMF and roots under three grassland plant species grown on sand. Plants were continuously labeled with ¹³C depleted CO₂ that allowed distinguishing plant-derived CO₂ from soil-derived CO₂. We show that root exudation was the main way by which plants induced RPE (58–96% of total RPE) followed by root litter. AMF did not contribute to rhizosphere priming under the two species that were significantly colonized by them i.e. Poa trivialis and Trifolium repens. Root exudates and root litter differed with respect to their mechanism of inducing RPE. Exudates induced RPE without increasing microbial biomass whereas root litter increased microbial biomass and raised the RPE mediating saprophytic fungi. The RPE efficiency (RPE/unit plant-C assimilated into microbes) was 3–7 times higher for exudates than for root litter. This efficiency of exudates is explained by a microbial allocation of fresh carbon to mineralization activity rather than to growth. These results suggest that root exudation is the main way by which plants stimulated mineralization of soil organic matter. Moreover, the plants through their exudates not only provide energy to soil microorganisms but also seem to control the way the energy is used in order to maximize soil organic matter mineralization and drive their own nutrient supply.     

Mineralization of sulfur from organic residues assessed by inverse isotope dilution

Sulfur (S) deficiency in soils is increasingly recognized in agricultural systems. The quantification of S mineralization/immobilization processes after incorporation of organic materials into soils is a key factor to predict the availability of S to growing plants. However, immobilization and mineralization occur simultaneously making the quantification of the magnitude of each process difficult. We used the inverse isotope (³⁵SO₄) dilution technique to quantify immobilization and mineralization fluxes after incorporation of two organic residues with contrasting C/S ratio's (cabbage or wheat straw) into a sandy soil in planted and unplanted soils (pot trial with ryegrass and incubation). The soil was labeled with ³⁵SO₄ and incubated for 63 days prior to the application of residues. The specific activity (SA) of soil-extractable SO₄ did not change significantly in the control soil during the subsequent experimental period despite significant net mineralization, illustrating that labile-S in soil was homogenously labeled. Application of residues decreased the SAs during the incubation due to the dilution with unlabeled-S from the residues. A three-compartment dynamic model was fitted to the SA data predicting that gross mineralization of residue-S was almost complete over 43 days incubation although this release was not matched by the increase in soil SO₄ due to immobilization reactions. Soil-extractable SO₄ was significantly increased in the cabbage-treated soil while the reverse was true in the wheat straw amended soil in which the S-immobilization was almost twice the gross mineralization of residue-S. The SA of S in ryegrass were maximally 15% lower than in corresponding soil extracts suggesting that residue mineralization was similar in planted and unplanted soils. The inverse isotope dilution method offers potential for screening S release of different residues; however the details of the dynamics of soil-S isotopes show that the individual fluxes are not constant during the incubation.     

Calcium and phosphorus interact to reduce mid-growing season net nitrogen mineralization potential in organic horizons in a northern hardwood forest

Acid deposition can deplete soil calcium (Ca) and be detrimental to the health of some forests. We examined effects of soil Ca and phosphorus (P) availability on microbial activity and nitrogen (N) transformations in a plot-scale nutrient addition experiment at the Hubbard Brook Experimental Forest in New Hampshire, USA. We tested the hypotheses that (1) microbial activity and N transformations respond to large but not small changes in soil Ca, (2) soil Ca availability influences net N mineralization via the immobilization of N, rather than via changes in microbial activity, and (3) the response to Ca is constrained by P availability. Seasonality was a primary influence on the microbial response to treatments; N cycling processes varied from May to October and treatment effects were only detectable in the mid-growing season, in July. Neither microbial activity (C mineralization) nor gross N mineralization responded to Ca or to P, in either horizon. In the Oa horizon in July net N mineralization was reduced by high Ca and by Ca + P, and gross nitrification was increased by P addition. In the Oe horizon in July net N mineralization was reduced by Ca + P. These results partially supported our hypotheses, suggesting that soil Ca depletion has the potential to increase mid-growing season N availability via subtle changes in N immobilization, and that this effect is sensitive to soil P chemistry. The horizon-specific nature of the responses that we detected suggests that the proportions of Oe and Oa horizons comprising the surface organic layer will influence the relative importance of these processes at the ecosystem scale. Our results highlight the need for further attention to seasonal changes in controls of microbial mineralization/immobilization processes, to functional differences between organic horizons, and to interactions between Ca and P in soils, in order to learn the specific mechanisms underlying the influence of Ca status on nutrient recycling in these northern hardwood ecosystems.     

Negative priming effect on mineralization in a soil free of vegetation for 80 years

The priming effect (PE) is a complex process corresponding to a modification of mineralization rates of soil organic matter (SOM) following inputs of fresh organic matter (FOM). The priming effect can be either positive or negative (i.e. an acceleration or retardation of SOM decomposition) and is controlled by several factors such as microbial community composition, SOM chemical structure and nutrient availability. The first objective of our experiment was to study negative or positive PE of stabilized SOM. The second was to identify the role of FOM decomposers in the PE of stabilized SOM. We incubated, for 39 days, a fallow soil free of vegetation for 80 years amended with ¹³C‐cellulose and inoculated with a FOM‐decomposing community. The soil contained stabilized SOM. The PE of the stable organic matter was always negative and tended to be more negative when the FOM‐decomposing community was added. This suggests that for this particular soil, SOM mineralization was not limited by energy. Moreover, as the inoculation of a FOM‐decomposing community led to a more negative PE, we assume that the FOM‐decomposing community facilitated the access of FOM to the indigenous bare soil community.     

氮肥施用对玉米根际呼吸温度敏感性的影响

为研究氮肥施用对玉米根际呼吸和土壤基础呼吸温度敏感性的影响,采用动态密闭气室红外CO2分析法,于2010年进行田间试验,该试验设4个处理：裸地不施氮肥（CK）、裸地施氮肥（CK-N）、种植玉米不施加氮肥（M）、种植玉米施加氮肥（M-N）,观测玉米田土壤呼吸各组分的日变化规律,同时观测土壤温度、气温等环境因子。结果表明,不种植玉米处理（CK和CK-N）土壤呼吸速率（土壤基础呼吸）为0.57～1.23μmol·m-2·s-1,施加氮肥对土壤基础呼吸没有显著影响;种植玉米条件下,施氮处理（M-N）的季节平均土壤呼吸速率为3.14μmol·m-2·s-1,显著高于不施氮处理（M）,增幅达31.9%。CK和CK-N处理的土壤基础呼吸温度敏感系数Q10分别为1.20、1.25,而不施氮和施氮条件下玉米根际呼吸的Q10值则分别为1.27、1.49。施加氮肥导致玉米根际呼吸温度敏感性明显增强（Q10值增大）,而土壤基础呼吸的温度敏感性则无明显变化,两种效应的叠加使得种植玉米土壤的总呼吸速率温度敏感性明显增加。     

Decomposition of mulched versus incorporated crop residues: Modelling with PASTIS clarifies interactions between residue quality and location

Crop residue management has been shown to significantly affect the decomposition process of plant debris in soil. In previous studies examining this influence, the extrapolation of laboratory data of carbon and/or nitrogen mineralization to field conditions was often limited by a number of interactions that could not be taken into account by a mere experimental approach. Therefore, we demonstrated the interactive effect between crop residue location in soil (mulch vs. incorporation) and its biochemical and physical quality, in repacked soil columns under artificial rain. Decomposition of ¹³C and ¹⁵N labelled rape and rye residues, with associated C and N fluxes, was analysed using the mechanistic model PASTIS, which turned out to be necessary to understand the interacting factors on the C and N fluxes. The influence of soil and residue water content on decomposition and nitrification was evaluated by the moisture limitation factor of PASTIS. This factor strongly depended on residue location and to a smaller extent on physical residue properties, resulting in a lower decomposition rate of about 35% for surface placed compared to incorporated residues. Irrespective of its placement, the biochemical residue quality (e.g. N availability for decomposition, amount of soluble compounds and lignin) was responsible for a faster and more advanced decomposition of about 15% in favour of rye compared to rape, suggesting only a limited interaction between residue quality and its location. Net N mineralization after nine weeks was larger for rye than for rape, equivalent to 59 and 10 kg NO₃⁻-N ha⁻¹ with incorporation, and 71 and 34 kg NO₃⁻-N ha⁻¹ with mulch, respectively. This net N mineralization in soil resulted from the interaction between soil water content, depending on residue placement, and N availability, which was determined by the biochemical residue quality. Moisture limitation appeared more important than N limitation in the decomposition of mulched residues. Modelling of gross N mineralization and immobilization also revealed that leaving crop residues at the soil surface may increase the risk of nitrate leaching compared to residue incorporation, if (i) soil water content under mulch is larger than with residue incorporation (more gross N mineralization), and (ii) availability to the applied C-source is limited (less gross N immobilization). Scenario analyses with PASTIS confirmed the importance of moisture conditions on the decomposition of mulched residues and the small interaction between biochemical crop residue quality and its location in soil.