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The rhizosphere priming effect (RPE) is the change in decomposition of soil organic matter caused by root activity, and it can be affected by plant species, nutrient availability, and by many other factors. Although plant and soil nitrogen (N) are likely to greatly influence RPE and vice-versa, not much is known about these relationships. There is a need for more research, particularly in N-fertilized cropping systems. We examined the RPE on soil N mineralization in a 42-day greenhouse experiment. The experiment included three vegetable crops (broccoli, lettuce, and spinach) and two grain crops (sunflower and maize) and an unplanted control. These plantings were crossed with two fertilization treatments: fertilized (100 kg ha−1 of N) and unfertilized. We evaluated NO3−, NH4+, microbial N, and mobilized N, plant biomass, plant tissue N, and %RPE by planting treatment. We found that vegetables almost completely depleted NO3− in both fertilized and unfertilized soils, whereas grain crops did not. Further, vegetables took up more than twice as much N in their aboveground tissue compared to grain crops under N fertilization. Both vegetables and crops produced RPEs on N mineralization. Nitrogen fertilization significantly enhanced the RPE for vegetable crops but not for grain crops. This result corroborates the assertion that plants with higher N concentration in their tissues tend to produce a higher RPE than plants with lower N concentration. The RPE of vegetable crops were significantly enhanced with N fertilization. Our data strongly rejects the “N-mining” hypothesis which would predict a much reduced RPE with N fertilization, and suggest that this hypothesis is not a universal explanation for the observed priming phenomena.

The rhizosphere priming effect (RPE) is increasingly recognized as an important factor in mediating soil organic matter (SOM) decomposition, which influences the CO2 release from terrestrial systems to the atmosphere. However, little is known about the long-term RPE of non-woody perennial species and the physical mechanisms underlying the RPE. Here the RPEs of three non-woody perennials (Stipa grandis, Leymus chinensis, Medicago sativa) differing in root traits and exudation were quantified using a natural 13C tracer method (C3 plant - C4 soil system) during a 476-day experiment. The results indicated that all plant species showed positive RPEs, with M. sativa showing the largest RPE and S. grandis the smallest RPE. Differences in the RPEs between species are likely associated with differences in root biomass, length, and exudates. Furthermore, the positive RPEs of the three species increased continuously with sampling time, indicating the long-term nature of the RPE. Results from structural equation modeling indicated that plant roots accelerated SOM decomposition via three mechanisms. The first mechanism (biological) was through the positive effect on dissolved organic C and microbial biomass C by the presence of the rhizosphere, consistent with the microbial activation hypothesis. The second mechanism (biological) was where plant roots significantly reduced soil mineral N and in turn promoted SOM decomposition, providing support for the microbial N-mining hypothesis. The third mechanism (physical) was where plant roots caused a net positive effect on SOM decomposition through net destruction of macroaggregates possibly as a result of high root length density and the associated drying-rewetting cycles. Overall, this study provides the first experimental evidence for the aggregate destruction hypothesis as a physical mechanism causing the RPE and highlights the importance of multiple mechanisms of the RPE on long-term SOM decomposition.

Aim: To conduct the clinical, genetic, and molecular characterization of 494 Han Chinese subjects with tic disorders (TD). Methods: In the present study, we performed the mutational analysis of 22 mitochondrial tRNA genes in a large cohort of 494 Han Chinese subjects with TD via Sanger sequencing. These variants were then assessed for their pathogenic potential via phylogenetic, functional, and structural analyses. Results: A total of 73 tRNA gene variants (49 known and 24 novel) on 22 tRNA genes were identified. Among these, 18 tRNA variants that were absent or present in Limitations: The phenotypic variability and incomplete penetrance of tic disorders in pedigrees carrying these tRNA mutations suggested the involvement of modifier factors, such as nuclear encoded genes associated mitochondrion, mitochondrial haplotypes, epigenetic, and environmental factors. Conclusion: Our data provide the evidence that mitochondrial tRNA mutations are the important causes of tic disorders among Chinese population. These findings also advance current understanding regarding the clinical relevance of tRNA mutations, and will guide future studies aimed at elucidating the pathophysiology of maternal tic disorders.

Coastal wetlands are increasingly recognized as important ecosystems for long-term carbon (C) storage. However, how soil aggregation mediates C accumulation and sequestration in these ecosystems remains unclear. Using the 13C isotope tracer from the invasion of a C4 plant, Spartina alterniflora, into the native ecosystem originally covered by C3 plants across Eastern Chinese coastal wetlands, we investigated a potential C stabilization process via soil structural protection. We quantified changes in soil aggregates, soil organic carbon (SOC), soil total nitrogen (STN), and natural 13C isotope abundance within aggregate fractions across a chronosequence of 0-, 4-, 8-, and 12-year S. alterniflora invasion. Our results showed that soil aggregate stability increased significantly along the chronosequence. Meanwhile, SOC and STN concentrations increased with invasion time in the whole soil and aggregate fractions, which were linked to increasing soil aggregate stability. The contribution of S. alterniflora-derived SOC increased from 18.96 to 40.24% in the 0–20 cm layer and from 4.66 to 32.04% in the 20–40 cm layer across the chronosequence from 4 to 12 years with the highest proportion observed in macro-aggregates. Our results indicate that invasion of S. alterniflora to coastal wetlands can sequester more C largely due to formation and stabilization of soil aggregates by soil structural protection.

The rhizosphere priming effect (RPE) is the stimulation or suppression of soil organic matter decomposition by living roots and associated rhizosphere organisms. The RPE is pivotal in regulating biogeochemical cycles in terrestrial ecosystems. However, biological mechanisms, especially soil micro-food web interactions, behind the RPE remain largely unknown. We quantified the RPE of soybean and cottonwood at three growth stages using a natural 13C tracer method, measured soil microbial and nematode community composition, and investigated their relations with the RPE. The magnitude of the RPE varied widely at different growth stages. Soybean produced a greater cumulative RPE than cottonwood. The plant species effect was also observed in the bacterial PLFA with higher values found in the soybean treatment. Mantel test analysis suggested that the variations in microbial community were closely related with the RPE, soil and plant characteristics. The nematode community affected the RPE indirectly through altering the structure of the microbial community. We demonstrated that the RPE was connected with interactions of soil micro-food webs. This connection indicates that soil micro-food web interactions in the rhizosphere may either regulate microbial turnover and/or microbial community composition, subsequently modulating the RPE.

Rhizosphere priming effects (RPEs) play a central role in modifying soil organic matter mineralization. However, effects of tree species and intraspecific competition on RPEs are poorly understood. We investigated RPEs of three tree species (larch, ash and Chinese fir) and the impact of intraspecific competition of these species on the RPE by growing them at two planting densities for 140 d. We determined the RPE on soil organic carbon (C) decomposition, gross and net nitrogen (N) mineralization and net plant N acquisition. Differences in the RPE among species were associated with differences in plant biomass. Gross N mineralization and net plant N acquisition increased, but net N mineralization decreased, as the RPE on soil organic C decomposition increased. Intraspecific competition reduced the RPE on soil organic C decomposition, gross and net N mineralization, and net plant N acquisition, especially for ash and Chinese fir. Microbial N mining may explain the overall positive RPEs across species, whereas intensified plant-microbe competition for N may have reduced the RPE with intraspecific competition. Overall, the species-specific effects of tree species play an important role in modulating the magnitude and mechanisms of RPEs and the intraspecific competition on soil C and N dynamics.

Plant litter inputs can influence soil organic carbon (SOC) decomposition via the priming effect. However, our understanding of the priming effect and underlying mechanisms is primarily from studies with leaf litter addition, while little is known about root litter effects, particularly of woody plants. Here, using a13C natural tracer approach, we conducted a 12-week incubation experiment to investigate litter decomposition and priming effect of mature-tree root orders (1st to 5th) of Cunninghamia lanceolata (Lamb.) Hook. We explored how litter decomposition and the priming effect were related to microbial biomass of main groups, enzyme activities, and root tissue chemistry. Root litter decomposition rates increased with increasing root order, especially during the first 4 weeks, which was likely due to higher non-structural C and lower tannin concentrations for higher order roots. A negative priming effect occurred at this initial intensive stage when microbes may have preferred utilizing litter-derived labile C. Subsequently, the priming effect switched to positive, and showed larger priming effects for the higher order roots than the lower order ones. Higher order roots also showed higher fungi to bacteria ratios and enzyme activities than the lower order roots. These patterns of fungi to bacteria ratios and enzyme activities and thus the priming effect could be attributed to the difference in carbon:nitrogen ratio among root orders. Overall, we for the first time provide strong evidence for the effect of root order on the priming effect, and thus highlight that separating root litter based on root order is necessary for accurately evaluating its influence on SOC decomposition.

Tree roots and their associated microbes can significantly influence soil organic matter (SOM) decomposition, i.e., the rhizosphere priming effect. This effect is expected to be greater in trees associated with arbuscular mycorrhizal (AM) fungi, which produce higher extracellular enzymes especially surrounding hyphae, than in trees associated with ectomycorrhizal (ECM) fungi. Here, we selected five tree species associated with AM (Juglans mandshurica Maxim. and Cunninghamia lanceolata (Lamb.) Hook.) or ECM (Picea koraiensis Nakai, Quercus mongolica Fischer ex Turcz. and Larix kaempferi (Lamb.) Carriere). We grew tree seedlings inside of cores lined with different mesh sizes to investigate how roots, hyphae and exudates influence soil carbon (C) and nitrogen (N) mineralization via the rhizosphere priming effect, using a13C natural abundance approach and a15N pool dilution method, concurrently. We found that tree seedlings significantly accelerated soil C decomposition by on average 78%, i.e., positive priming, compared to unplanted control pots. AM-associated trees induced 2.1 times greater soil C decomposition than ECM-associated trees across all mesh sizes. In contrast, gross N mineralization did not differ between tree-mycorrhizal associations. Compared to ECM counterparts, AM-associated trees had higher C- and lower N-degrading enzyme activities. Consequently, AM-associated trees induced a significantly higher C:N ratio of SOM decomposition than their ECM counterparts, which could be associated with the differences in soil enzyme activities for C and N degradation. Further, for both AM- and ECM-associated trees, we found no significant influences of mesh size on soil C decomposition, suggesting that the rhizosphere priming effect of mycorrhizal symbiosis was predominantly driven by root exudates. We conclude that SOM decomposition caused by AM-associated trees may have a higher C:N ratio than that by ECM-associated trees mainly due to differences in microbial enzyme investment. Our findings imply that tree-mycorrhizal associations are capable of modulating soil biogeochemical cycling via the rhizosphere priming effect.

Evidence from growth chamber and greenhouse experiments indicates that the rhizosphere priming effect (RPE, stimulation or suppression of soil organic matter (SOM) decomposition by live roots and their rhizospheric biota) may be sensitive to environmental factors such as temperature and level of sunlight. However, little is known about potential variations of the RPE under different latitudinal regions that vary in temperature and sunlight levels. We explored the RPE of soybean (Glycine max) and cottonwood (Populus × euramericana cv. ‘74/76’) at two different latitudinal locations under variable conditions of light and temperature for two growing seasons using a natural 13C tracer method. Different plant species at different time of season produced significantly different RPEs. The RPE of both soybean and cottonwood was small at the initial stage when the plants were small. The RPE of soybean ranged from 0.4% to135%, reached the highest level at the flowering stage and declined afterward. The RPE of cottonwood ranged from −17% to 121% and tended to increase in both growing seasons. The cumulative RPE of soybean was significantly higher than that of cottonwood at both locations and during the two growing seasons. Overall the cumulative RPE values were larger at low latitude than at high latitude. The cumulative RPE of soybean at the low latitudinal location was 1.7 times higher than at the high latitudinal location. The RPE of both plant species was significantly related to specific rhizosphere respiration, but negatively related to soil dissolved total nitrogen content. The RPE of both plant species varied at both locations during the two growing seasons. Particularly for soybean, the cumulative RPE was higher at the low latitudinal location than at the high latitudinal location, possibly because of higher light levels. Our results further indicate that plant species, time of season, specific rhizosphere respiration, and soil dissolved total nitrogen are important variables in controlling the RPE. Overall, these results provide the needed support for using RPE data in a broader context.

Rhizosphere priming is crucial for regulating soil carbon and nitrogen biogeochemical cycles. An appreciable number of studies have been conducted to quantify the rhizosphere priming effect (RPE), and have shown that the RPE is sensitive to changes of plant and soil conditions. These diverse results across individual studies offer us an opportunity to explore for potential general patterns and variability. In this study, we conducted a meta-analysis of RPE values taken from 31 publications. Our results showed that, on average, the RPE enhanced soil organic carbon mineralization rate by 59% across all studies. The magnitudes of the RPE significantly varied among plant types and soil texture. Within plant types, woody species showed the highest RPE followed by grasses while crops had the lowest level of the RPE, indicating that plant traits and physiology may exert important controls on the RPE. Soils with finer texture tended to produce stronger RPEs than soils with coarser texture, suggesting that interactions between the rhizosphere and the soil matrix may modulate the RPE. Furthermore, the level of the RPE is positively correlated with aboveground plant biomass, but surprisingly not with root biomass which is the commonly believed key variable for influencing the RPE. In addition, the RPE increased with the length of experimental duration, which implies that the RPE may persist much longer than previously believed because it impacts stabilized soil carbon more than labile carbon as the length of experimental duration increases. Overall, the results from this meta-analysis further illustrate several complex features of the RPE and call for future attentions to decipher this complexity.

Root turnover is a key process contributing to soil carbon storage, nutrient cycling, and other ecosystem functions. However, quantifying root turnover rates remains highly uncertain and methodologically challenging. Field rhizotrons were employed to quantify root turnover times using median longevities of five branching orders in a Larix gmelinii plantation. Root images were recorded by scanning the rhizotron windows at a monthly interval during four growing seasons. Root demographic data and branching orders were obtained by analyzing these images using Rootfly software coupled with manual mouse‐tracing of individual roots. Root longevities and turnover estimates were calculated using these data. Roots of different branching orders showed significantly different turnover times. The mean turnover times of the first‐order roots and second‐order roots were 284 and 994 d, respectively. Roots of higher branching orders (third to fifth orders) remained alive at the end of the 4‐yr experimental period, indicating much longer turnover times than the duration of the experiment. Root turnover times increased exponentially with branching orders. Further analysis of these data suggested that root branching orders combined with sampling biases, timing of root cohorts, and longevity distribution patterns crucially influenced root turnover times. The method of combining field glass rhizotrons with electronic scanning permits quantification of root turnover for five branching orders in a temperate forest. The overall result empirically demonstrates the crucial role of branching orders for accurately quantifying root turnover times.

The influence of plant roots and the associated rhizosphere activities on decomposition of soil organic matter (SOM), the rhizosphere priming effect, has emerged as a crucial mechanism regulating global carbon (C) and nitrogen (N) cycles. However, the role of root morphology in controlling the rhizosphere priming effect remains largely unknown. To investigate the link between root hairs, a critical part of the entire root morphology, and the rhizosphere priming effect, we grew a barley wild type and a barley mutant without root hairs in a greenhouse and continuously labeled them with 13 C-depleted CO 2 . Soil CO 2 efflux was measured during tillering and head-emergence stages of plant growth. Based on its δ 13 C signature, total CO 2 was partitioned for root-derived and SOM-derived CO 2, and the SOM decomposition primed in the rhizosphere was calculated. Soil microbial biomass C and N, and the activities of six extracellular enzymes (β-cellobiohydrolase, β-glucosidase, acid phosphatase, β-xylosidase, leucin-aminopeptidase, and N-acetyl-β-glucosaminidase) were measured to test the effects of root hairs. During the early stage of development (tillering), when plants were sufficiently supplied with nutrients, the barley mutant without root hairs produced more shoot biomass. In contrast, high C costs for root-hair formation likely reduced the growth of the barley wild type. At this stage, the wild type with regular root hairs produced a positive rhizosphere priming effect (69% increase), but the mutant without root hairs produced a negative priming effect on SOM decomposition (28% decline). At the head-emergence stage, when nutrients were scarce, plant biomass production of the mutant was reduced, probably due to inefficient nutrient uptake in the absence of root hairs. At this stage, both barley types produced positive rhizosphere priming effects (72% and 209% increase for the wild type and the mutant, respectively) and the microbial biomass was higher for both planted soils compared to the tillering stage. Extracellular enzymes responsible for the decomposition of stable SOM had higher activities in cases of positive priming effects. Overall, root hairs played an important role in regulating rhizosphere priming.

Loss of plant diversity with increased anthropogenic nitrogen (N) deposition in grasslands has occurred globally. In most cases, competitive exclusion driven by preemption of light or space is invoked as a key mechanism. Here, we provide evidence from a 9-yr N-addition experiment for an alternative mechanism: differential sensitivity of forbs and grasses to increased soil manganese (Mn) levels. In Inner Mongolia steppes, increasing the N supply shifted plant community composition from grass-forb codominance (primarily Stipa krylovii and Artemisia frigida, respectively) to exclusive dominance by grass, with associated declines in overall species richness. Reduced abundance of forbs was linked to soil acidification that increased mobilization of soil Mn, with a 10-fold greater accumulation of Mn in forbs than in grasses. The enhanced accumulation of Mn in forbs was correlated with reduced photosynthetic rates and growth, and is consistent with the loss of forb species. Differential accumulation of Mn between forbs and grasses can be linked to fundamental differences between dicots and monocots in the biochemical pathways regulating metal transport. These findings provide a mechanistic explanation for N-induced species loss in temperate grasslands by linking metal mobilization in soil to differential metal acquisition and impacts on key functional groups in these ecosystems.

Woody plant roots can be classified into absorptive roots and transport roots based on root functions, order and traits. While there is an emerging view that living roots actively affect soil organic carbon (SOC) decomposition via the rhizosphere priming effect (RPE), the linkages of the RPE with C allocation to absorptive roots (relative to total roots) and their functional traits across soils are virtually unknown. Here, we investigated the RPE by growing a tree species (Chinese fir, Cunninghamia lanceolata) in three isotopically-distinct C4 soil types with different soil properties such as C/nitrogen (N) ratio and texture, and by growing three tree species (Chinese fir, larch (Larix kaempferi) and ash (Fraxinus mandshurica)) with wide variations in root functional traits in one of the C4 soils. We classified living roots into absorptive roots (first and second orders) and transport roots (third and higher orders) and then quantified their C allocation (relative to total roots) and morphological and chemical traits associated with economic construction, rhizodeposition and resource acquisition. We found that the RPE of Chinese fir across the three soils decreased with an increase in soil C/N ratio. This result conflicted with the N mining hypothesis and suggests that soil C stabilization mechanisms associated with clay minerals may play an important role. Further, significant differences in the RPE among tree species were largely accounted for by the C allocation to absorptive roots. Moreover, there was a significantly negative relationship between specific surface area of absorptive roots and the specific RPE (per unit biomass of absorptive roots) among tree species, suggesting that absorptive root traits shaping the extent of the rhizosphere may regulate the RPE. Taken together, our results provide evidence that absorptive roots play a predominant role in causing the RPE. These findings present an important step toward improving our capability to predict plant effects on SOC decomposition through linking the RPE to absorptive root functional traits.

Live roots can stimulate microbial soil organic matter (SOM) decomposition and nutrient cycling, which is termed as the rhizosphere priming effect (RPE). Compared to nitrogen (N) availability, fewer studies have focused on the effect of phosphorus (P) availability on the RPE. Here we investigated the RPEs of ryegrass (Lolium perenne) and clover (Trifolium repens) with and without P fertilization (4 g P m−2) at three sampling times (Day 30, Day 44, and Day 58 after planting). A continuous 13C–CO2 labeling method was used to separate soil-derived CO2 from root-derived CO2. A nutrient budget method was applied to evaluate the rhizosphere effect on net soil N and P release for plant uptake. We found that ryegrass and clover induced positive RPEs in most plant-soil combinations, ranging from −1% to 134%. Ryegrass exhibited a larger RPE than clover by Day 30, but clover exhibited a larger RPE than ryegrass by Day 44 and Day 58, possibly due to larger shoot biomass regrowth rates, root activity, and rhizodeposition during the later stages. P fertilization significantly decreased the RPE of ryegrass by Day 44 and Day 58, but did not change the RPE of ryegrass by Day 30 and clover at all three sampling times. The reduced RPE of ryegrass with P fertilization was associated with increased microbial biomass N, more root-derived microbial C, and less shoot biomass and root-derived CO2. These findings suggest that P fertilization coupled with C supply from root exudates induced more microbial N immobilization, which reduced the RPE of ryegrass during later stages when soil N limitation negatively impacted plant growth. However, P-induced microbial N immobilization did not affect clover as much because its biological N fixation, on average 37% of total plant N, may have alleviated soil N limitation. We further observed significant positive relationships between excess net soil N and P release and the RPE by Day 58 across all planted treatments, indicating that soil N and P release by plants can be directly linked to rhizosphere C mineralization. Overall, our results demonstrate the importance of C–N–P interactions for understanding the RPE, which have significant implications for P cycling in plant-soil systems.

The root-driven soil aggregate turnover dynamics and rhizosphere priming effect (RPE, changes in soil organic carbon (SOC) decomposition caused by living roots) are central to the understanding of SOC cycling. However, the association between aggregate turnover and the RPE has not been illuminated in plant-soil systems because of methodological difficulties. Using rare earth oxides to trace the transformations among different aggregates and 13C natural abundance labeling, we for the first time simultaneously investigated aggregate turnover and the RPE at two phenological stages of two grass species (Agropyron cristatum and Koeleria cristata): tillering (40 days after planting, DAP40) and jointing-heading (DAP63). We found that aggregate turnover rates varied widely, with a range between 0.006 day−1 and 0.024 day−1, i.e., turnover times (the reciprocal of turnover rates) ranged from 41 to 168 days, and were significantly influenced by plant species, sampling date and their interaction. Particularly, greater aggregate turnover rates (2% ~ 68%) and transformations in breakdown and formation pathways were found for K. cristata than for A. cristatum at DAP63. The RPEs increased with plant growth and ranged from −29% to +163%. Especially, the RPE and microbial biomass C were significantly greater for K. cristata than for A. cristatum at DAP63. Root-driven aggregate turnover was tightly associated with the RPE, possibly because of the release of aggregate-protected C for microbial decomposition. There was no net C loss mainly because increased aggregate formation could have sequestrated root-derived C in macroagrgegates and thus counteracted the C loss by the positive RPE. We therefore propose a new framework of root-driven aggregate turnover for considering how plant roots influence SOC dynamics via aggregate turnover. Root-accelerated aggregate turnover acts as a “key”: enhancing SOC decomposition (i.e. RPE), while simultaneously accelerating the occlusion of root-derived C and thus facilitating new C sequestration. This framework highlights that living root-driven aggregate turnover alters the physical protection of SOC and regulates the RPE, which aligns well with the emerging perspective of SOC stabilization.

Enhanced deposition of atmospheric nitrogen (N) has profound impacts on ecosystem processes such as above-ground productivity and community structure in grasslands across the globe. But how N deposition affects below-ground processes of grasslands is less well known. Here, we evaluated the effects of chronic N amendment at a relatively low rate (20kgha(-1)year(-1)) on root traits (root productivity, root biomass, root/shoot ratio) in Inner Mongolia steppes by rhizotron and ingrowth core and soil monolith techniques at levels of individual species, functional groups and ecosystem. For 8years, N amendment suppressed above-ground net primary production (ANPP), photosynthetic rates and root biomass of forbs, but enhanced ANPP and root biomass of grasses. This led to an overall reduction in below-ground productivity of the grassland by 24-33%, while ANPP remained unchanged. Nitrogen amendment acidified soil and subsequently increased extractable soil manganese (Mn) concentration. Nitrogen amendment increased foliar Mn concentrations in forb, but not grass species, leading to a significant inhibition of photosynthetic rates in forb species.Synthesis. These findings highlight the importance of the differentiating responses of plant functional groups to long-term N deposition and the important consequences of these responses for below-ground productivity and long-term soil C sequestration.

Living roots and their rhizodeposits affect microbial activity and soil carbon (C) and nitrogen (N) mineralization. This so-called rhizosphere priming effect (RPE) has been increasingly recognized recently. However, the magnitude of the RPE and its driving mechanisms remain elusive. Here we investigated the RPE of two plant species (soybean and sunflower) grown in two soil types (a farm or a prairie soil) and sampled at two phenological stages (vegetative and mature stages) over an 88-day period in a greenhouse experiment. We measured soil C mineralization using a continuous 13C-labeling method, and quantified gross N mineralization with a 15N-pool dilution technique. We found that living roots significantly enhanced soil C mineralization, by 27–245%. This positive RPE on soil C mineralization did not vary between the two soils or the two phenological stages, but was significantly greater in sunflower compared to soybean. The magnitude of the RPE was positively correlated with rhizosphere respiration rate across all treatments, suggesting the variation of RPE among treatments was likely caused by variations in root activity and rhizodeposit quantity. Moreover, living roots stimulated gross N mineralization rate by 36–62% in five treatments, while they had no significant impact in the other three treatments. We also quantified soil microbial biomass and extracellular enzyme activity when plants were at the vegetative stage. Generally, living roots increased microbial biomass carbon by 0–28%, β-glucosidase activity by 19–56%, and oxidative enzyme activity by 0–46%. These results are consistent with the positive rhizosphere effect on soil C (45–79%) and N (10–52%) mineralization measured at the same period. We also found significant positive relationships between β-glucosidase activity and soil C mineralization rates and between oxidative enzyme activity and gross N mineralization rates across treatments. These relationships provide clear evidence for the microbial activation hypothesis of RPE. Our results demonstrate that root–soil–microbial interactions can stimulate soil C and N mineralization through rhizosphere effects. The relationships between the RPE and rhizosphere respiration rate and soil enzyme activity can be used for explicit representations of RPE in soil organic matter models.

Living roots and their rhizodeposits can stimulate microbial activity and soil organic matter (SOM) decomposition up to several folds. This so-called rhizosphere priming effect (RPE) varies widely among plant species possibly due to species-specific differences in the quality and quantity of rhizodeposits and other root functions. However, whether the RPE is influenced by plant inter-species interactions remains largely unexplored, even though these interactions can fundamentally shape plant functions such as carbon allocation and nutrient uptake. In a 60-day greenhouse experiment, we continuously labeled monocultures and mixtures of sunflower, soybean and wheat with 13C-depleted CO2 and partitioned total CO2 efflux released from soil at two stages of plant development for SOM- and root-derived CO2. The RPE was calculated as the difference in SOM-derived CO2 between the planted and the unplanted soil, and was compared among the monocultures and mixtures. We found that the RPE was positive under all plants, ranging from 43% to 136% increase above the unplanted control. There were no significant differences in RPE at the vegetative stage. At the flowering stage however, the RPE in the soybean–wheat mixture was significantly higher than those in the sunflower monoculture, the sunflower–wheat mixture, and the sunflower–soybean mixture. These results indicated that the influence of plant inter-specific interactions on the RPE is case-specific and phenology-dependent. To evaluate the intensity of inter-specific effects on priming, we calculated an expected RPE for the mixtures based on the RPE of the monocultures weighted by their root biomass and compared it to the measured RPE under mixtures. At flowering, the measured RPE was significantly lower for the sunflower–wheat mixture than what can be expected from their monocultures, suggesting that RPE was significantly reduced by the inter-species effects of sunflower and wheat. In summary, our results clearly demonstrated that inter-species interactions can significantly modify rhizosphere priming on SOM decomposition.

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.

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. Published by Elsevier Ltd.

The effects of two plant species—soybean (Glycine max), and sunflower (Helianthus annuus) on nutrient availability in two soils (an organically farmed soil, OF, and a native grassland soil, GS, both Alfisols)—were measured with anion exchange membranes (Plant Root Simulator, PRS TM probes) in a greenhouse study. Vegetation (especially sunflower) in the OF soil caused significant reductions in soil N and K availability (which was interpreted as due to uptake), and significant increases in P, S, Cu, Fe, Mn, and Zn availability. The increases in the latter case were consistent with the results of a previous study showing rhizosphere-enhanced mineralization of native soil organic matter in this soil. Vegetation had no significant effects on Ca, Mg, B, or Al availability in the OF soil and no significant effects on any measured nutrient in the GS soil. Collectively, these results show that the presence of plants can have either a negative or a positive effect on soil nutrient availability, and that plant uptake and soil nutrient availability are interdependent. r 2007 Elsevier Ltd. All rights reserved.

Nitrogen and water management are essential factors for achieving adequate crop growth and development in organic production systems. A three-year field study examined effects of different forms of organic N fertilizers applied at side-dress and with different irrigation application rates on leaf, stem, and floret yields, volumetric soil water content (P v), and crop water use efficiency (WUE) in organically-grown broccoli (Brassica oleracea L.) in two regions of California; Santa Cruz (UCSC farm) and Five Points (Harris farm). At preplant, ‘compost only’ treatment (CO) was applied at 140 kg·ha−1 of N with an additional 112 kg·ha−1 of N applied as side-dress in one of the following forms: (1) fish powder (FP); (2) Phytamin [bloodmeal and feathermeal mix (BF)]; (3) BF mixed with NaNO3 (SN), or (4) seabird guano (SG). Leaf, stem, and floret yields collected from the UCSC farm had a greater response to additional N from side-dressing treatments irrespective of the form than plants at the Harris farm...

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.

Summary •T ree root respiration utilizes a major portion of the primary production in forests and is an important process in the global carbon cycle. Because of the lack of ecologically relevant methods, tree root respiration in situ is much less studied compared with above-ground processes such as photosynthesis and leaf respiration. • This study introduces a new 13 C natural tracer method for measuring tree root respiration in situ . The method partitions tree root respiration from soil respiration in buried root chambers. • Rooting media substantially influenced root respiration rates. Measured in three media, the fine root respiration rates of longleaf pine were 0.78, 0.27 and 0.18 mg CO 2 carbon mg − 1 root nitrogen d − 1 at 25 ° C in the native soil, tallgrass prairie soil, and sand‐vermiculite mixture, respectively. Compared with the root excision method, the root respiration rate of longleaf pine measured by the field chamber method was 18% higher when using the native soil as rooting medium, was similar in the prairie soil, but was 42% lower if in the sand‐vermiculite medium. • This natural tracer method allows the use of an appropriate rooting medium and is capable of measuring root respiration nondestructively in natural forest conditions.

In the present study, ‘natural 13 C tracer method’ was used to partition the belowground respiration into rhizosphere respiration and soil microbial respiration to test the hypothesis that defoliation affects rhizosphere respiration and rhizosphere priming effect on decomposition of soil organic matter (SOM). A C3 plant species, Helianthus annuus (sunflower), was grown in ‘C4’ soil in microcosms so that the CO2 evolved from plant-soil system can be partitioned. Four levels of defoliation intensities were established by manual clipping. CO2 evolved from plant-soil system was trapped during 0–4 h after defoliation (HAD), 5–22 HAD and 23–46 HAD using a closed circulating system, respectively. We found that both rhizosphere respiration and soil microbial respiration of the clipped plants were either unchanged or significantly enhanced compared to unclipped plants at 45% defoliation level during all sampling intervals. Soil microbial respiration increased significantly at all defoliation levels during 0–4 HAD, however, both rhizosphere respiration and soil microbial respiration decreased significantly during 5–22 HAD or 23–46 HAD when 20% or 74 clearly demonstrated that the defoliation treatments modified the rhizosphere priming effect on SOM decomposition. The total cumulative rhizosphere priming effects on SOM decomposition during 0–46 HAD were 146%, 241%, 204% and 205% when 0%, 20%, 45% and

Plant species and soil fertility presumably control rhizosphere effects on soil organic matter (SOM) decomposition, but qualitative and quantitative descriptions of such controls are still sparse. In this study, rhizosphere effects of soybean [Glycine max (L.) Merr.] and spring wheat (Triticum aestivum L.) on SOM decomposition were investigated at four phenological stages under three levels of fertilization in a greenhouse experiment using natural 13 C tracers. The magnitude of the rhizosphere effect ranged from 0% to as high as 383% above the decomposition rate in the no-plant control, indicating that the rhizosphere priming can substantially intensify decomposition. The rhizosphere priming effect was responsible for a major portion of the total soil C efflux. Cumulative soil C loss caused by rhizosphere effects during the whole growing season equated to the amount of root biomass C for the soybean treatment, and 71% of root biomass C for the wheat treatment. Different plant species produced significantly different rhizosphere priming effects. The overall rhizosphere priming effect of soybean plants was significantly higher than for wheat plants. Plant phenology significantly influenced the rhizosphere priming effect. Little rhizosphere effect occurred in both wheat and soybean treatments initially. The priming effect of the wheat rhizosphere reached 287% above the no-plant control at the flowering stage and declined significantly afterward. The priming effect of the soybean rhizosphere peaked at 383% above the no-plant control during the late vegetative stage and remained at high levels onward. Contrary to many published reports, NPK fertilization did not significantly modify the rhizosphere priming effect.

Using a natural abundance 13 C method, soil organic matter (SOM) decomposition was studied in a C3 plant – ‘C4 soil’ (C3 plant grown in a soil obtained from a grassland dominated by C4 grasses) system and a C4 plant – ‘C3 soil’ (C4 plant grown in a soil taken from a pasture dominated by C3 grasses) system. In C3 plant – ‘C4 soil’ system, cumulative soil-derived CO2–C were higher in the soils planted with soybean (5499 mg pot −1 ) and sunflower (4484 mg pot −1 ) than that in ‘C4 soil’ control (3237 mg pot −1 ) without plants. In other words, the decomposition of SOM in soils planted with soybean and sunflower were 69.9% and 38.5% faster than ‘C4 soil’ control. In C4 plant – ‘C3 soil’ system, there was an overall negative priming effect of live roots on the decomposition of SOM. The cumulative soil-derived CO2–C were lower in the soils planted with sorghum (2308 mg pot −1 )a nd amaranthus (2413 mg pot −1 ) than that in ‘C3 soil’ control (2541 mg pot −1 ). The decomposition of SOM in soils planted with sorghum and amaranthus were 9.2% and 5.1% slower than ‘C3 soil’ control. Our results also showed that rhizosphere priming effects on SOM decomposition were positive at all developmental stages in C3 plant – ‘C4 soil’ system, but the direction of the rhizosphere priming effect changed at different developmental stages in the C4 plant – ‘C3 soil’ system. Implications of rhizosphere priming effects on SOM decomposition were discussed.

Two plant–soil systems, C3 plant - `C4 soil' (obtained from a grassland dominated by C4 grasses) and C4 plant - `C3 soil' (obtained from a pasture dominated by C3 grasses), were used in this study to monitor the dynamics of rhizosphere respiration (root-derived CO2-C) at different plant developmental stages. In C3 plant - `C4 soil' system, CO2-C derived from soybean roots increased significantly from vegetative stage (55.69 mg C d−1 pot−1) to flowering stage (132.18 mg C d−1 pot−1), and it declined thereafter (83.37-111.63 mg C d−1 pot−1). However, no significant change of CO2-C derived from sunflower roots was observed at different plant developmental stages (67.05–77.66 mg C d−1 pot−1). CO2-C derived from soybean (Glycine max (L) Merr) roots was significantly higher than that derived from sunflower (Helianthus annuus) roots except for that at vegetative stage. In C4 plant - `C3 soil' system, CO2-C derived from sorghum roots was significantly higher at flowering stage (169.51 mg C d−1 pot−1) than at other stages (75.89–113.26 mg C d−1 pot−1). CO2-C derived from amaranthus roots was the highest at vegetative stage (88.88 mg C d−1 pot−1) and it declined significantly thereafter (23.42–53.47 mg C d−1 pot−1). CO2-C derived from sorghum (Sorghum bicolor) roots was significantly higher than that from amaranthus (Amaranthus hypochondriacus) roots except for that at vegetative stage. In conclusion, rhizosphere respiration varied not only with plant species but also with plant phenology. With the soil volume used in our pots, the overall percentages of cumulative root-derived CO2-C to total soil respiration were 61.22, 61.14, 81.84 and 67.37% for soybean, sunflower, sorghum and amaranthus, respectively. Specific rhizosphere respiration was also discussed and was used as an index of root activity and vitality.

This study investigated the effects of a gradual versus step increases in carbon dioxide (CO2) on plant photosynthesis and growth at two nitrogen (N) levels. Plantago lanceolata were grown for 80 days and then treated with the ambient CO2 (as the control), gradual CO2 increase and step CO2 increase as well as low and high N additions for 70 days. While [CO2] were kept at constant 350 and 700 mol mol − 1 for the ambient and step CO2 treatments, respectively, [CO2] in the gradual CO2 treatment was raised by 5 mol mol − 1 day − 1 , beginning at 350 mol mol −1 and reaching 700 mol mol − 1 by the end of experiment. The step CO2 treatment immediately resulted in an approximate 50% increase in leaf photosynthetic carbon fixation at both the low and high N additions, leading to a 20–24% decrease in leaf N concentration. The CO2-induced nitrogen stress, in return, resulted in partial photosynthetic downregulation since the third week at the low N level and the fourth week at the high N level after treatments. In comparison, the gradual CO2 treatment induced a gradual increase in photosynthetic carbon fixation, leading to less reduction in leaf N concentration. In comparison to the ambient CO2, both the gradual and step CO2 increases resulted in decreases in specific leaf area, leaf N concentration but an increase in plant biomass. Responses of plant shoot:root ratio to CO2 treatments varied with N supply. It decreased with low N supply and increased with high N supply under the gradual and step CO2 treatments relative to that under the ambient CO2. Degrees of those changes in physiological and growth parameters were usually larger under the step than the gradual CO2 treatments, largely due to different photosynthetic C influxes under the two CO2 treatments. © 2002 Elsevier Science B.V. All rights reserved.

The effect of elevated CO2 on photosynthesis, respiration, and growth efficiency of sunflower plants at the whole-stand level was investigated using a whole-system gas exchange facility (the EcoCELLs at the Desert Research Institute) and a 13 C natural tracer method. Total daily photosynthesis (GPP), net primary production (NPP), and respiration under the elevated CO2 treatment were consistently higher than under the ambient CO2 treatment. The overall level of enhancement due to elevated CO2 was consistent with published results for a typical C3 plant species. The patterns of daily GPP and NPP through time approximated logistic curves under both CO2 treatments. Regression analysis indicated that both the rate of increase (the parameter ‘r’) and the maximum value (the parameter ‘k’) of daily GPP and NPP under the elevated CO2 treatment were significantly higher than under the ambient CO2 treatment. The percentage increase in daily GPP due to elevated CO2 varied systematically through time according to the logistic equations used for the two treatments. The GPP increase due to elevated CO2 ranged from approximately 10% initially to 73% at the peak, while declining to about 33%, as predicted by the ratio of the two maximum values. Different values of percentage increase in GPP and NPP were obtained at different sampling times. This result demonstrated that one-time measurements of percentage increases due to elevated CO2 could be misleading, thereby making interpretation difficult. Although rhizosphere respiration was substantially enhanced by elevated CO2, no effect of elevated CO2 on R:P (respiration:photosynthesis) was found, suggesting an invariant NPP:GPP ratio during the entire experiment. Further validation of the notion of an invariant NPP:GPP ratio may significantly simplify the process of quantifying terrestrial carbon sequestration by directly relating total photosynthesis to net primary production.

Nutrient acquisition by plants occurs in an environment characterized by complex interactions between roots, micro-organisms and animals, termed the rhizosphere. Competition for mineral elements in this sphere is high. The rhizosphere processes are driven by photosynthetically fixed carbon released by roots either directly to mycorrhizal fungal symbionts or as exudates fuelling a wider spectrum of organisms, mainly bacteria. In particular, the role of the soil fauna interacting with rhizosphere micro-organisms and plant roots has so far found little attention. We present evidence that the interaction between plant roots, root exudates and micro-organisms can only be understood in relation to soil faunal activity, indicating that the soil fauna has an important function in regulating rhizosphere microbial processes and therefore significantly affects plant growth.

Sunflower canopies were grown in mesocosom gas exchange chambers at ambient and elevated CO 2 concentrations (360 and 700 ppm) and leaf photosynthetic capacities measured at several depths within each canopy. Elevated [CO 2 ] had little effect on whole-canopy photosynthetic capacity and total leaf area, but had marked effects on the distribution of photosynthetic capacity and leaf area within the canopy. Elevated [CO 2 ] did not significantly reduce the photosynthetic capacities per unit leaf area of young leaves at the top of the canopy, but it did reduce the photosynthetic capacities of older leaves by as much as 40%. This effect was not dependent on the canopy light environment since elevated [CO 2 ] also reduced the photosynthetic capacities of older leaves exposed to full sun on the south edge of the canopy. In addition to the effects on leaf photosynthetic capacity, elevated [CO 2 ] shifted the distribution of leaf area within the canopy so that more leaf area was concentrated near the top of the canopy. This change resulted in as much as a 50% reduction in photon flux density in the upper portions of the elevated [CO 2 ] canopy relative to the ambient [CO 2 ] canopy, even though there was no significant difference in the total canopy leaf area. This reduction in PFD appeared to account for leaf carbohydrate contents that were actually lower for many of the shaded leaves in the elevated as opposed to the ambient [CO 2 ] canopy. Photosynthetic capacities were not significantly correlated with any of the individual leaf carbohydrate contents. However, there was a strong negative correlation between photosynthetic capacity and the ratio of hexose sugars to sucrose, consistent with the hypothesis that sucrose cycling is a component of the biochemical signalling pathway controlling photosynthetic acclimation to elevated [CO 2 ].

To investigate the interactions among mineral N, C availability, microbial biomass, and respiration in arctic soils, we sampled soils five times during a growing season from a toposequence on a slope in northern Alaska. The toposequence consisted of six vegetative types from the ridge top to the stream bank: lichen heath, dry cassiope, moist carex (Carex spp.), water track, tussock tundra (intertussock), and riparian. The spatial distribution and temporal variation of soil mineral N, microbial biomass, soil C availability, and C turnover were soil type dependent. During the growing season, the concentration of soil NH 4+-N decreased in tussock tundra soils but increased in lichen heath soils. Soil C availability at all locations was the highest at the beginning of the growing season and declined thereafter. The C availability index (CAI) and the potential C turnover rate increased as soils became wetter. Tussock-forming tundra soil was generally colder than other sites and had high C/N ratios, low amounts of mineral N, and a low potential C turnover index, and therefore, was the least biologically active type. In contrast, water track was the most biologically active site in the sequence and had the highest C and N availability, the highest potential C turnover index, and the highest microbial biomass C and N. The mosaic of diverse plant communities and soil types that comprise arctic landscapes necessitates that accurate estimates of large-scale C or N budget can only be made by integration of all types of plant communities and soils.

It is widely known that the carbon availability in the rhizosphere is much higher than in the bulk soil. However, studies have failed to show whether microbial respiration in the rhizosphere is carbon-limited. Precise and timely measurements are lacking. We measured carbon availability index (basal respiration divided by substrate-induced respiration), and water soluble carbon in soils sampled at four spatial points (rhizoplane, rhizosphere, bulk soil and root-free soil) in the rhizosphere continuum in greenhouse and field experiments. Carbon availability index and water soluble carbon were inversely related to the relative distance from the root surface, with several times higher concentrations in the rhizoplane soils. Microbial respiration was not limited by available carbon in the rhizoplane and in the rhizosphere.

Many difficulties exist in studying rhizosphere carbon flows in situ because of limited methodologies. Current photosynthate allocation to root respiration, rhizosphere microbial respiration and soluble root exudates in the rhizospheres of winter wheat (Triticum aestivum L.), tall fescue (Festuca arundinacea Schreb.), and buffalo gourd (Cucurbita foetidissima HBK) were investigated using an isotopic trapping procedure. Plants were grown in containers with 165 g of sandy clay loam soil for 3 wk before the time of 14 C pulse labeling. Most root exudates were utilized by the microbes in the immediate rhizosphere and released as 14 CO 2 . Only a small amount of exudates was found in the bulk soil (2.7, 2.5, and 3.7%) in the control treatments of wheat, tall fescue, and buffalo gourd respectively)

We conducted field experiments to test the general hypothesis that the com- position of decomposer communities and their trophic interactions can influence patterns of plant litter decomposition and nitrogen dynamics in ecosystems. Conventional (CT) and no-tillage (NT) agroecosystems were used to test this idea because of their structural sim- plicity and known differences in their functional properties. Biocides were applied to ex- perimentally exclude bacteria, saprophytic fungi, and microarthropods in field exclosures. Abundances of decomposer organisms (bacteria, fungi, protozoa, nematodes, microar- thropods), decomposition rates, and nitrogen fluxes were quantified in surface and buried litterbags (Secale cereale litter) placed in both NT and CT systems. Measurements of in situ soil respiration rates were made concurrently. The abundance and biomass of all microbial and faunal groups were greater on buried than surface litter. The mesofauna contributed more to the total heterotrophic C in buried litter from CT (6-22%) than in surface litter from NT (0.4-1/1%). Buried litter decay rates (1.4-1.7%/d) were -2.5 times faster than rates for surface litter (0.5-O.7%/d). Ratios of fungal to bacterial biomass and fungivore to bacterivore biomass on NT surface litter generally increased over the study period resulting in ratios that were 2.7 and 2.2 times greater, respectively, than those of CT buried litter by the end of the summer. The exclusion experiments showed that fungi had a somewhat greater influence on the decomposition of surface litter from NT while bacteria were more important in the de- composition of buried litter from CT. The fungicide and bactericide reduced decomposition rates of NT surface litter by 36 and 25% of controls, respectively, while in CT buried litter they were reduced by 21 and 35% of controls, respectively. Microarthropods were more important in mobilizing surface litter nitrogen by grazing on fungi than in contributing to litter mass loss. Where fungivorous microarthropods were experimentally excluded, there was less than a 5% reduction in mass loss from litter of both NT and CT, but fungi- fungivore interactions were important in regulating litter N dynamics in NT surface litter. As fungal densities increased following the exclusion of microarthropods on NT surface litter, there was 25% greater N retention as compared to the control after 56 d of decay. Saprophytic fungi were responsible for as much as 86% of the net N immobilized (1.81 g /m2) in surface litter by the end of the study when densities of fungivorous microarthropods were low. Although bacteria were important in regulating buried litter decomposition rates and the population dynamics of bacterivorous fauna, their influence on buried litter N dynamics remains less clear. The larger microbial biomass and greater contribution of a bacterivorous fauna on buried litter is consistent with the greater carbon losses and lower carbon assimilation in CT than NT agroecosystems. In summary, our results suggest that litter placement can strongly influence the com- position of decomposer communities and that the resulting trophic relationships are im- portant to determining the rates and timing of plant litter decomposition and N dynamics. Furthermore, cross placement studies suggest that the decomposer communities within each tillage system, while not discrete, are adapted to the native litter placements in each.

Carbon dioxide efflux and soil microenvironmental factors were measured diurnally in Carex aquatilus-and Eriophorum angustifolium-dominated riparian tundra communities to determine the relative importance of soil environmental factors controlling ecosystem carbon dioxide exchange with the atmosphere. Measurements were made weekly between 18 June and 24 July 1990. Diurnal patterns in carbon dioxide efflux were best explained by changes in soil temperature, while seasonal changes in efflux were correlated with changes in depth to water table, depth to frozen soil and soil moisture. Carbon dioxide efflux rates were lowest early in the growing season when high water tables and low soil temperatures limited microbial and root activity. Individual rainfall events that raised the water table were found to strongly reduce carbon dioxide efflux. As the growing season progressed, rainfall was low and depth to water table and soil temperatures increased. In response, carbon dioxide efflux increased strongly, attaining rates late in the season of approximately 10 g CO2 m−2 day−1. These rates are as high as maxima recorded for other arctic sites. A mathematical model is developed which demonstrates that soil temperature and depth to water table may be used as efficient predictors of ecosystem CO2 efflux in this habitat. In parallel with the field measurements of CO2 efflux, microbial respiration was studied in the laboratory as a function of temperature and water content. Estimates of microbial respiration per square meter under field conditions were made by adjusting for potential respiring soil volume as water table changed and using measured soil temperatures. The results indicate that the effect of these factors on microbial respiration may explain a large part of the diurnal and seasonal variation observed in CO2 efflux. As in coastal tundra sites, environmental changes that alter water table depth in riparian tundra communities will have large effects on ecosystem CO2 efflux and carbon balance.

Summary. Earthworm and enchytraeid densities and biomass were sampled over an 18-month period in conventional and no-tillage agroecosystems. Overall, earthworm densities and biomass in the no-till system were 70% greater than under conventional tilling, and enchytraeid densities and biomass in the no-till system were 50%-6007o greater. To assess the role of annelids in the breakdown of soil organic matter, carbofuran was applied to field enclosures and target (earthworm and enchytraeid biomass, standing stocks of organic matter) and non-target effects (bacteria, fungi, protozoa, nematode and microarthropod densities, litter decay rates, plant biomass) were determined in two 10-month studies. In the winter-fall study, carbofuran reduced the annelid biomass, and total soil organic matter standing stocks were 47% greater under no-till with carbofuran compared to control enclosures. Twelve percent of the difference could have been due to non-target effects of carbofuran, as determined from litterbag decay rates. In the summer-spring study, carbofuran again significantly reduced the annelid biomass, and treated pens in the no-till area had significantly greater standing stocks of fine organic matter (43 % -45 07o). Although the densities of bacteria and nematodes were reduced in carbofuran-treated litterbags under a no-till system, the rates of decay were not reduced and estimates of the amount of organic matter processed could not be adjusted for non-target effects. A 7607o difference in the standing stock of coarse organic matter between control and carbofuran-treated pens in the conventional-till system indicated further non-target effects. We concluded that our estimates of the amount of organic matter processed by annelids in no-till and conventionally tilled agroecosystems represented a maxi

Publisher Summary This chapter discusses carbon fluxes in the rhizosphere. Terrestrial ecosystems are intimately connected to atmospheric CO2 levels through photosynthetic fixation of CO2, sequestration of C in biomass and soils, and the subsequent release of CO2 through respiration and decomposition of organic matter. Rhizodeposition is defined as all material lost from plant roots, including water-soluble exudates, secretions of insoluble materials, lysates, dead fine roots, and gases, such as CO2 and ethylene. Plants grown under elevated CO2 conditions often exhibit increased growth and a disproportional increase in C allocation to roots, total rhizosphere respiration, and rhizodeposition. Large quantities of rhizodeposits apparently represent a significant portion of the plant carbon balance and an important source of substrates for soil organisms. Microbial carbon assimilation efficiency is commonly defined as microbial biomass produced as a proportion of total carbon utilized. The timing of root exudation determines how closely rhizosphere processes are linked with plant photosynthesis and aboveground physiology. Many researchers may agree that method development has been, and remains, a key prerequisite for advancing rhizosphere science. In the past few decades, research on carbon fluxes in the rhizosphere have been mostly restricted to cereal crops. If higher plants invest a significant amount of fixed carbon into the rhizosphere to support a portion of the soil biota, then the coevolution between plants and rhizosphere biota must shape the quantity and the quality of rhizosphere C fluxes through selection and adaptation.

Effects of above-ground herbivory on short-term plant carbon allocation were studied using maize (Zea mays) and a generalist lubber grasshopper (Romalea guttata). We hypothesized that above-ground herbivory stimulates current net carbon assimilate allocation to below-ground components, such as roots, root exudation and root and soil respiration. Maize plants 24 days old were grazed (c. 25–50% leaf area removed) by caging grasshoppers around individual plants and 18 h later pulse-labelled with14CO2. During the next 8 h,14C assimilates were traced to shoots, roots, root plus soil respiration, root exudates, rhizosphere soil, and bulk soil using carbon-14 techniques. Significant positive relationships were observed between herbivory and carbon allocated to roots, root exudates, and root and soil respiration, and a significant negative relationship between herbivory and carbon allocated to shoots. No relationship was observed between herbivory and14C recovered from soil. While herbivory increased root and soil respiration, the peak time for14CO2 evolved as respiration was not altered, thereby suggesting that herbivory only increases the magnitude of respiration, not patterns of translocation through time. Although there was a trend for lower photosynthetic rates of grazed plants than photosynthetic rates of ungrazed plants, no significant differences were observed among grazed and ungrazed plants. We conclude that above-ground herbivory can increase plant carbon fluxes below ground (roots, root exudates, and rhizosphere respiration), thus increasing resources (e.g., root exudates) available to soil organisms, especially microbial populations.

Global nitrogen (N) deposition and climate change have been identified as two of the most important causes of current plant diversity loss. However, temporal patterns of species turnover underlying diversity changes in response to changing precipitation regimes and atmospheric N deposition have received inadequate attention. We carried out a manipulation experiment in a steppe and an old-field in North China from 2005 to 2009, to test the hypothesis that water addition enhances plant species richness through increase in the rate of species gain and decrease in the rate of species loss, while N addition has opposite effects on species changes. Our results showed that water addition increased the rate of species gain in both the steppe and the old field but decreased the rates of species loss and turnover in the old field. In contrast, N addition increased the rates of species loss and turnover in the steppe but decreased the rate of species gain in the old field. The rate of species change was greater in the old field than in the steppe. Water interacted with N to affect species richness and species turnover, indicating that the impacts of N on semi-arid grasslands were largely mediated by water availability. The temporal stability of communities was negatively correlated with rates of species loss and turnover, suggesting that water addition might enhance, but N addition would reduce the compositional stability of grasslands. Experimental results support our initial hypothesis and demonstrate that water and N availabilities differed in the effects on rate of species change in the temperate grasslands, and these effects also depend on grassland types and/or land-use history. Species gain and loss together contribute to the dynamic change of species richness in semi-arid grasslands under future climate change.

Carbon dioxide efflux and soil microenvironment were measured in three upland tundra communities in the foothills of the Brooks Range in arctic Alaska to determine the magnitude of CO{sub 2} efflux rates and the relative importance of the belowground factors that influence them. Gas exchange and soil microenvironment measurements were made weekly between 14 June and 31 July 1990. The study communities included lichen-heath, a sparse community vegetated by lichens and dwarf ericaceous shrubs on rocky soils, moist Cassiope dwarf-shrub heath tundra, dominated by Carex and evergreen and deciduous shrubs on relatively deep organic soils, and dry Cassiope dwarf-shrub heath of stone-stripe areas, which was of intermediate character. Rates of CO{sub 2} efflux were similar for the three communities until mid-season when they peaked at rates between 4.9 and 5.9 g m{sup {minus}2} d{sup {minus}1}. Following the mid-season peak, the rates in all three communities declined, particularly in the lichen-heath. Seasonal patterns of CO{sub 2} efflux, soil temperature, and soil moisture suggest changing limitations to CO{sub 2} efflux, soil temperature, and soil moisture suggest changing limitations to CO{sub 2} efflux over the course of the season. Rates of carbon dioxide efflux followed changes in soil temperature early in the seasonmore » when soil moisture was highest. Mid-season efflux appeared to be limited by soil, moss, and lichen hydration until the end of July, when temperature again limited efflux. Differences between the communities were related to microenvironmental differences and probable differences in carbon quality. The presence of peat-forming mosses is suggested to play an important role in differences in efflux and micro-environment among the communities. 32 refs., 3 figs., 4 tab.« less

Growth rates, death rates and distribution of grain sorghum and weed roots in conventional tillage (CT) and no-tillage (NT) plots were measured using the minirhizotron technique. Total root production in the two systems was estimated by combining minirhizotron and soil coring methods (...)

Stable carbon isotopes are used extensively to partition total soil CO2 efflux into root-derived rhizosphere respiration or autotrophic respiration and soil-derived heterotrophic respiration. However, it remains unclear whether CO2 from rhizosphere respiration has the same δ13C value as root biomass. Here we investigated the magnitude of 13C isotope fractionation during rhizosphere respiration relative to root biomass in six plant species. Plants were grown in a carbon-free sand-perlite medium inoculated with microorganisms from a farm soil for 62 days inside a greenhouse. We measured the δ13C value of rhizosphere respiration using a closed-circulation 48-hour CO2 trapping method during 40~42 and 60~62 days after sowing. We found a consistent depletion in 13C (0.9~1.7‰) of CO2 from rhizosphere respiration relative to root biomass in three C3 species (Glycine max L. Merr., Helianthus annuus L. and Triticum aestivum L.), but a relatively large depletion in 13C (3.7~7.0‰) in three C4 species (Amaranthus tricolor L., Sorghum bicolor (L.) Moench and Zea mays L. ssp. mays). Overall, our results indicate that CO2 from rhizosphere respiration is more 13C-depleted than root biomass. Therefore, accounting for this 13C fractionation is required for accurately partitioning total soil CO2 efflux into root-derived and soil-derived components using natural abundance stable carbon isotope methods.