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Increased atmospheric N deposition and altered precipitation regimes are two important global-change factors influencing grassland communities by changing soil nutrient availability and acquisition by plants. These findings highlight that mowing cannot mitigate the negative effects of N enrichment on soil MBC, and that soil, plant and microbial properties play important roles in the response of soil MBC to mowing in N-enriched soil. Liu et al. ([22]) reported that water supply significantly affected plant C:N and N:P ratios, and these effects differed among plant species of Inner Mongolian grasslands. Given the importance of plant-soil interactions in grassland ecosystems, it is necessary to evaluate the roles of plant-soil interactions in response of grasslands in MP to global change and human activities. [Extracted from the article]

Aims: Mycorrhiza and rhizodeposition are important for nutrient and water uptake but their role under variable water conditions remains unclear. We investigated how mycorrhiza and rhizodeposition contributed to plant uptake of water, nitrogen (N) and phosphorus (P) in response to water availability and variability. Methods: Two tomato genotypes (a mycorrhiza-defective tomato mutant, rmc, and its mycorrhizal wild-type progenitor, Solanum lycopersicum cv. Rio Grande 76R were grown in a greenhouse under different watering conditions (wet, medium, and dry conditions, and dry-rewet conditions). Results: We found that non-mycorrhizal plants were as successful as mycorrhizal plants in terms of N and water uptake under both reduced and variable water availability. However, we observed lower water use efficiency and shoot N recovery in mycorrhizal plants that could be driven by mycorrhizal requirements for water and N. Mycorrhizal plants were more extensively colonised and were more successful in taking up P under dry conditions compared to non-mycorrhizal plants. On the other hand, greater specific root exudation in non-mycorrhizal plants across all watering conditions resulted in relatively greater uptake of N than of P. Conclusions: A flexible carbon allocation towards mycorrhiza and root exudation may help plants in maintaining a balanced uptake of N and P under variable water conditions. [ABSTRACT FROM AUTHOR]

Purpose: Belowground carbon (C) allocation for nitrogen (N) acquisition plays a crucial role in determining primary productivity and plant competitiveness in legume-grass mixtures, but beyond modeling and qualitative assessments, this remains poorly understood, especially with regard to drought stress and interspecific interactions. Methods: We grew a legume (Trifolium repens) and a grass (Lolium perenne) in monocultures and as a 50:50 mixture (with same plant density), at 70% and 50% soil water holding capacity representing non-drought and drought conditions, for 104 days in a growth chamber experiment. By using continuous 13CO2 labelling and 15N pulse soil-labelling, we analyzed how drought and interspecific interaction affected belowground C allocation (including root biomass, root respiration and rhizodeposition) and N acquisition through soil N uptake and biological N fixation. Results: Drought increased belowground C allocation per unit of N acquisition in the legume, but not in the grass. Drought significantly reduced biological N fixation in the legume, so that the legume allocated relatively more C to take up soil N. Interspecific competition increased belowground C allocation per unit of N acquisition, which could be attributed to a reduction in biological N fixation by the legume and an increased abundance of the grass. Conclusions: We highlight that drought and interspecific competition for N strongly alter C allocation towards biological N fixation and soil N uptake. Our measurements provide important process-based information to improve modeling drought effects on productivity and composition in legume-grass mixtures. [ABSTRACT FROM AUTHOR]

Aims: We investigated whether plant resistance to soil impoverishment would depend on their flexibility in taking up nutrients and on maintaining elemental stoichiometry. Methods: We mixed sand with grassland soil in mass proportions of 0, 10, 30, 50 and 70% to simulate soil impoverishment as caused by a gradient of desertification intensity and examined how plant nitrogen (N) uptake (15NH4NO3 and NH415NO3 labelling) and nutrient stoichiometry were associated with plant growth responses of a sedge (Carex duriuscula) and a forb (Potentilla chinensis). Results: With increasing intensity of soil impoverishment, plant biomass, cover and density decreased for C. duriuscula, but not for P. chinensis. Interestingly, uptake of both 15NO3− and 15NH4+ increased for the sedge but not for the forb, with the former species showing stronger preference of NO3−. However, the sedge showed decreasing nutrient contents and concentrations of phosphorus (P) and magnesium (Mg), and thus increasing stoichiometric ratios of N:P and calcium (Ca):Mg. In contrast, the forb maintained its biomass by showing strong stability in nutrient content, concentration and stoichiometry. Conclusions: Stability of N uptake and nutrient stoichiometry were associated with higher resistance to soil impoverishment of the forb than the sedge. Our study highlights the importance of stability in nutrient uptake and stoichiometry for determining plant-species resistance to nutrient-poor conditions. [ABSTRACT FROM AUTHOR]

Aims: Soil aggregates constitute spatially-separated microbial habitats and architectural units for biogeochemical reactions. However, little is known about how aggregates varying in size can affect soil carbon (C) storage and mediate the direction of plant-soil interactions. Methods: In a meadow steppe, we assessed soil aggregate C storage affected by nitrogen (N) addition and mowing and to what degree plant-soil interactions differed among soil aggregate-size classes. Results: Nitrogen addition increased plant biomass, soil C concentration in macroaggregates and relative abundance of bacteria, but decreased fungal relative abundance and the activity of oxidative enzymes. However, mowing counteracted N effects on plant productivity and enzyme activity. In macroaggregates, close linkages with plant biomass and species richness were found for soil available nutrients and ratio of fungi to bacteria (F:B ratio). In contrast, strong physical protection and restricted fungal dominance and activity contributed to C stabilization in microaggregates, where plant community and soil properties were less linked with each other. Conclusions: Our study clearly demonstrated that stronger plant-soil linkages occurred in macro- vs. microaggregates. This linkage was associated with a rise in plant biomass and soil C accumulation but a drop in F:B ratio in macroaggregates under N addition. [ABSTRACT FROM AUTHOR]

Background and aims: Through agriculture and industry, humans are increasing the deposition and availability of nitrogen (N) in ecosystems worldwide. Carbon (C) isotope tracers provide useful insights into soil C dynamics, as they allow to study soil C pools of different ages. We evaluated to what extent N enrichment affects soil C dynamics in experiments that applied C isotope tracers. Methods: Using meta-analysis, we synthesized data from 35 published papers. We made a distinction between "new C" and "old C" stocks, i.e., soil C derived from plant C input since the start of the isotopic enrichment, or unlabeled, pre-existing soil C. Results: Averaged across studies, N addition increased new soil C stocks (+30.3%), total soil C stocks (+6.1%) and soil C input proxies (+30.7%). Although N addition had no overall, average, effect on old soil C stocks and old soil C respiration, old soil C stocks increased with the amount of N added and respiration of old soil C declined. Nitrogen-induced effects on new soil C and soil C input both decreased with the amount of extraneous N added in control treatments. Conclusion: Although our findings require additional confirmation from long-term field experiments, our analysis provides isotopic evidence that N addition stimulates soil C storage both by increasing soil C input and (at high N rates) by decreasing decomposition of old soil C. Furthermore, we demonstrate that the widely reported saturating response of plant growth to N enrichment also applies to new soil C storage. [ABSTRACT FROM AUTHOR]

Aims: Here we assessed N and P uptake of four grassland species grown together in response to a short-term drought event along a soil P gradient. Methods: We used 15N and 32P tracers to examine uptake of N and P by the grasses Bothriochloa macra, Themeda triandra, Lolium perenne and Microlaena stipoides grown together in pots with initial available P levels of 3, 8, 12, 20 mg P kg−1 soil. Soil moisture in half the pots was reduced from 60 to 30% water holding capacity during a 7-day period to simulate drought. Results: Plant P uptake was strongly reduced by drought in all species across all P levels, much more so than N uptake, indicating decoupling in N and P uptake. Soil available P (Bray method) was not affected by drought, suggesting that the reduced P uptake with drought was due to reduced soil P mobility. Plant competition for N and P changed with drought and soil P levels, where relatively more N and P was taken up with drought by M. stipoides at the lowest soil P level. Conclusions: We showed that greater reductions in P than in N uptake and shifts in N and P uptake among species caused by a short-term drought have strong consequences for plant growth. [ABSTRACT FROM AUTHOR]

Aims: Determining base cation and trace element dynamics in plant-soil systems is important for sustainable utilization of grasslands affected by global ecosystem nitrogen (N) enrichment. Methods: A 4-year field experiment was conducted with urea addition rates of 0, 2.5, 5, 7.5, 10, 15, 20, and 30 g N m−2 yr.−1 to investigate the dynamics of base cations (Ca, Mg, K) and trace elements (Fe, Mn, Cu and Zn) in soil and a dominant species Leymus chinensis in a semi-arid meadow grassland of northeast China. Results: Soil showed a significant decrease in exchangeable Ca and increases in extractable Fe, Mn and Zn concentrations, while plant Ca and Zn remained unchanged and plant Fe decreased with N addition. For both plant and soil compartments, stoichiometric ratios of trace elements were more variable than base cations, mainly driven by plant physiological requirements, plant nutrient uptake intensity, and elemental interactions. Excessive soil Mn mobilization and plant uptake affected concentrations of Fe, Cu and Zn in the plant and even inhibited Fe absorption (antagonistic interactions), resulting in a plant Fe:Mn ratio lower than 1.5 with high N addition rates, indicating Mn phytotoxicity or Fe deficiency. As a result, plant elemental stoichiometry did not remain homeostatic and was even more variable than in soil. Conclusions: Our results provided direct evidence of decoupled plant-soil elemental responses, stricter homeostatic control of macronutrients than trace elements in plants, and altered stoichiometric variability in the plant-soil system, and suggest that N addition enhances metal nutrient imbalances in the meadow steppe grassland. [ABSTRACT FROM AUTHOR]

Background and aims: Variations in plant nitrogen (N) and phosphorus (P) concentrations and ratios have great implications for primary productivity and nutrient cycling. Here, we reported their changes at functional group and community levels along an aridity gradient. Methods: We carried out a field investigation and set up 26 sampling sites along a transect in the drylands of northern China to examine the responses of plant nutrient status to increasing aridity and the relationships between plant and soil nutrients. Results: The responses of N and P concentrations to increasing aridity differed between woody (sub-) shrubs and herbaceous species (including annual/biennial forbs and grasses, perennial forbs and perennial grasses). Nitrogen and P concentrations of shrubs decreased while those of herbs generally increased with increasing aridity. Plant N:P ratios increased significantly with increasing aridity for the plant functional groups and at the community level. At both organizational levels, nutrient concentrations in plants were negatively related to those in soils. Conclusion: Our results indicated that the responses of plant nutrition to aridity were dependent on plant functional group. Plant growth appeared increasingly limited by P, and plant and soil nutrients become increasingly decoupled at large spatial scale with increasing aridity. [ABSTRACT FROM AUTHOR]

Background and aims: The rhizosphere priming effect (RPE) plays an important role in the decomposition of soil organic matter (SOM) and the global carbon cycle. Although plant, soil, and environmental factors affecting the RPE have been examined, it is still unclear how soil water and nitrogen (N) status together alter the RPE.Methods: In order to examine plant, water, and N interacting effects on the RPE, we conducted a semi-natural experiment with two plant species (Leymus chinnensis and Medicago sativa) grown at two soil moisture levels (45 and 85% of water holding capacity) with or without N fertilization (0 and 10 g N m−2 y−1). We used a natural 13C abundance tracer method for measuring the RPE, and an N budgeting method for measuring net N mineralization or immobilization.Results: Both positive (up to 253%) and negative (down to −21%) RPEs were observed in different treatment combinations. L. chinensis exhibited greater RPE than M. sativa under low soil moisture, possibly due to specific root architecture and root activity of L. chinensis. High soil moisture significantly reduced the RPE, while N fertilization significantly increased the RPE. To reconcile our current results and previous observations with existing individual hypotheses related to the RPE, we put forward a new hypothesis: when N mineralization dominates, the rhizosphere input enhances the RPE; when N immobilization dominates, the rhizosphere input reduces the RPE.Conclusion: Overall, this study demonstrates that the RPE on soil organic carbon decomposition is intimately linked with N processes and soil water status. [ABSTRACT FROM AUTHOR]

Background and aims: The nitrogen isotope composition (δ15N) of plants in arid and semiarid grasslands is affected by environmental factors, especially water availability. Nevertheless, it is unclear whether the response of δ15N to water availability differs between C3 and C4 photosynthetic pathways.Methods: We investigated plant δ15N of coexisting C3 and C4 species as a function of aridity along a 3200 km aridity gradient across the arid and semi-arid grasslands of northern China.Results: Aridity was positively correlated with plant δ15N values in both C3 and C4 plants and also in the entire plant community, whereas soil bulk δ15N values increased first and then decreased along the aridity gradient. The N uptake by C4 plants appeared to be more affected by competition pressure of neighboring plants and soil microbes than for C3 plants along the transect.Conclusions: The decoupled relationship between plant and soil δ15N values indicated that variations in vegetation and soil δ15N values were driven by differential biogeochemical processes, while different soil N sources were used for plant growth along the climatic gradient. The advantage of C3 plants in the use of N may counteract the competitive advantage that C4 plants have over C3 plants due to their higher water use efficiency under drier conditions. These findings can help understand why C4 plants do not completely replace C3 plants in drier environments despite their higher water use efficiency. [ABSTRACT FROM AUTHOR]

Background: Soil amendment with biochar can increase biological nitrogen fixation (BNF) and improve growing conditions for legumes, while such effects may alter when biochar properties change with time (biochar aging).Methods: We examined BNF and competitiveness of legumes in a mixed pasture (dominated by clover, Trifolium repens) after 26 months of biochar amendment (field aged biochar) and in a pot study with grass (Lolium perenne) and clover mixture using fresh and chemically aged biochars.Results: In both studies, the fraction of N derived from symbiotic fixation (%Ndfa) was not affected by aged biochar treatments, while it reduced from 73% in the control to 68% with fresh biochar addition in the pot study. However, this reduction in %Ndfa was compensated with greater biomass production resulting in the largest amount of N fixed with fresh biochar application. The amount of N fixed was reduced by field aged biochar, although it was not affected by chemically aged biochars. Moreover, in the field study grasses became more competitive than legumes with biochar, while such an advantage by grasses was not observed in the pot study.Conclusions: Aging of biochar can reduce BNF and competitiveness of legumes in mixed pastures with significant implications for pasture management. [ABSTRACT FROM AUTHOR]

Background and aims: 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. Methods: 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 C tracer method. Results: 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. Conclusions: 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. [ABSTRACT FROM AUTHOR]

Background and aims: Aridity has increased in the past decades and will probably continue to increase in arid and semiarid regions. Here we decipher the plant and soil capacity to retain metal cations when climate evolves to more arid conditions. Methods: We analyzed K, Na, Ca, Mg, Fe, Mn, Zn and Cu concentrations in 580 soil samples and 666 plant (shoot and root) samples along a 3600 km aridity gradient in northern China. Results: The concentrations of soil exchangeable K, Mg, Mn, Fe and Cu clearly decreased with increasing aridity due to the relationships of aridity with soil clay content and soil pH. Increases in exchangeable Na and Ca concentrations at mid- and high-aridity levels are probably due to the soil salinization, whereas increased exchangeable Fe concentrations at extreme levels of aridity may be more related to a reduced pH. Element concentrations in both plant shoots and roots were unrelated to soil exchangeable element concentrations; instead they increased monotonously with increasing aridity, corresponding with decreases in plant size and shoot/root ratios. The shoot/root mineralomass ratios in general increased with increasing aridity. The proportional higher element contents in shoots than in roots with increasing aridity are related to increased water uptake and/or use efficiency. Conclusions: The extractability of soil elements in response to changing climate varied with the nature of specific elements that are controlled by biological and geochemical processes, i.e., some decreased linearly with increasing aridity, whereas others first decreased and then increased with different thresholds. These contrasting effects of aridity on nutrient availability could further constrain plant growth and should be incorporated into biogeochemical models. The prevailing paradigm of a positive relationship between concentrations of plant and soil elements needs to be reconsidered under changing climatic conditions. [ABSTRACT FROM AUTHOR]

Background and aims: Water and nitrogen are essential for plant growth and yield. Plants depend on surface roots for nutrient uptake, but frequently rely on deep root systems for water uptake, especially in semi-arid, rain fed crop production systems. Methods: We used H O and NH tracers in pots to determine water and NH uptake at depth by two wheat genotypes watered from the surface or from the bottom. Root traits and transpiration rates were related to water and NH uptake. Results: We observed a significant positive relationship between transpiration rate and water uptake measured by H O tracer (R = 0.91), confirming that the H O tracer method was successful. Surface soil drying (bottom watering) decreased both water and NH uptake from the top soil. However, increased water availability with bottom watering increased water uptake from the bottom soil layer, but not NH uptake from the bottom soil layer. Water uptake was positively related to total root length, while NH uptake was positively related to root biomass. Conclusions: With surface soil drying, plants grew more and longer roots in the bottom soil layer, shifting water uptake from the top to the bottom soil, while N uptake was reduced in the top soil because of a decrease in root biomass. Different root traits need to be considered when optimizing water and NH uptake by wheat in water deficient seasons. [ABSTRACT FROM AUTHOR]

Background and Aims: The commonly observed trade-off between plant water use efficiency (WUE) and nitrogen use efficiency (NUE) has been attributed to physiological constraints in the leaf. We examined if a similar trade-off can occur between WUE and phosphorus use efficiency (PUE) and if changes in NUE and PUE in response to water and nutrient supply can be related to microbial N and P immobilisation. Methods: We studied water and nutrient use efficiencies in leaves of four tree species ( Eucalyptus sideroxylon, Eucalyptus tereticornis, Casuarina cunninghamiana, and Pinus radiata) that were grown under rainout shelters for 16 months at low and high levels of water and nutrient supply. Results: Across all four species, WUE increased when water supply was low and nutrient supply was high, while NUE increased when water supply was high and nutrient supply was low. As a consequence, a trade-off was found between WUE and NUE for all four species. In contrast, no trade-off was found between WUE and PUE, likely because PUE and microbial P immobilisation in the soil unexpectedly increased with high nutrient supply. Conclusions: With variable water and nutrient supply, physiological constraints generate a trade-off between WUE and NUE, but not between WUE and PUE; the latter may have been obscured by microbial control over P supply to plants. [ABSTRACT FROM AUTHOR]

Background and aims: Understanding the effect of wheat genotype on soil properties will be crucial in breeding towards more sustainable wheat production and increased yield. We examined the relationships between root traits, soil properties and grain yield for 24 wheat genotypes grown under field conditions. Methods: Wheat genotypes ( Triticum spp.) with a range of different genetic backgrounds (varieties, breeding and synthetic lines) were planted in the field in northern NSW, Australia. Specific root length (SRL), root diameter (RD) and the proportion of root length in different diameter classes were characterized at the stem elongation growth stage. Soil microbial biomass carbon (MBC) and nitrogen (MBN), total dissolved nitrogen (TDN), organic nitrogen (DON), inorganic nitrogen (DIN), and soil bacterial community structure were measured. Results: A significant wheat genotype effect was found for SRL, proportion of coarse roots (1-2 mm), MBN, MBC/MBN, TDN, and DIN. Genotypes with more fine roots generally had lower TDN, DIN and DON, higher MBN, and lower grain yield. We observed no wheat genotype effect on the structure and diversity of the soil bacterial community. Conclusions: Long, thin wheat roots may promote microbial growth and N immobilisation, negatively affecting grain yield. We suggest that SRL, MBN and inorganic N should be considered by plant breeding schemes. [ABSTRACT FROM AUTHOR]

Aims: A lacking of understanding about cooling effects on methane (CH) fluxes and potential asymmetrical responses to warming and cooling causes uncertainty about climate change effects on the atmospheric CH concentration. We investigated CH fluxes in an alpine meadow on the Tibetan Plateau in response to climate warming and cooling. Methods: A 2-year reciprocal translocation experiment was implemented to simulate climate warming (i.e. downward translocation) and cooling (i.e. upward translocation) along an elevation gradient with four different vegetation types (at 3200, 3400, 3600 and 3800 m elevation) during the growing season (May to October) in 2008 and 2009. Results: Although the effects of warming and cooling varied depending on vegetation type, elevation and timescale (i.e., daily and seasonally), warming increased average seasonal CH uptake by 60 %, while cooling reduced it by 19 % across all vegetation types, based on a 1.3-5.1 °C difference in soil temperature at 20 cm depth. Soil temperature over the range of 4-10 °C explained 11-25 % of the variation in average seasonal CH fluxes, while there was no relationship with soil moisture over the range of 13-39 % and soil NH-N and NON content. Methane uptake was more sensitive to warming than to cooling. Conclusions: Because of warming and cooling spells in the alpine region, warming effects on CH uptake would be over-estimated by 64 % if cooling effects on it are not considered. Our findings suggest that asymmetrical responses of CH fluxes to warming and cooling should be taken into account when evaluating the effects of climate change on CH uptake in the alpine meadow on the Tibetan plateau. [ABSTRACT FROM AUTHOR]

Aim: Desert herbs, a crucial component of desert ecosystems, are sensitive to water and nutrient availability and therefore to environmental change. We aimed to determine element concentrations in desert herbs and their relationships with life form, taxonomy, climate, and soil environment. Methods: We measured concentrations of 11 elements in shoots and roots of 26 dominant desert herb species from 45 sites in a temperate desert. Results: Shoots of desert herbs had greater concentrations of elements related to photosynthesis and water use efficiency (N, P, Mg, K) than roots. Concentrations of these elements (except N and P) were also greater in annual herbs than in perennial herbs. Greater Mg, K, and Na concentrations were observed in shoots of Chenopodiaceae (mostly C species) than in Poaceae (mostly C species). Soil properties and taxonomy explained 3.6-26 % and 2.8-24 % of the variation in shoot element concentrations, respectively, whereas climate factors explained only 0.05-6.5 % of the variation. Conclusions: Water and nutrient availability, which are affected by environmental change, influence concentrations of mineral elements in desert plants and their biogeochemical cycles in desert ecosystems. [ABSTRACT FROM AUTHOR]

Warming and elevated atmospheric CO (eCO) can elicit contrasting responses on different SOM pools, thus to understand the effects of combined factors it is necessary to evaluate individual pools. Over two years, we assessed responses to eCO and warming of SOM pools, their susceptibility to decomposition, and whether these responses were mediated by plant inputs in a semi-arid grassland at the PHACE ( Prairie Heating and CO Enrichment) experiment. We used long-term soil incubations and assessed relationships between plant inputs and the responses of the labile and resistant pools. We found strong and contrasting effects of eCO and warming on the labile C pool. In 2008 labile C was increased by eCO and was positively related to plant biomass. In contrast, in 2007 eCO and warming had interactive effects on the labile C, and the pool size was not related to plant biomass. Effects of warming and eCO in this year were consistent withtreatment effects on soil moisture and temperature and their effects on labile C decomposition. The decomposition rate of the resistant C was positively related to indicators of plant C inputs. Our approach demonstrated that SOM pools in this grassland can have early and contrasting responses to climate change factors. The labile C pool in the mixed-grass prairie was highly responsive to eCO and warming but the factors behind such responses were highly dynamic across years. Results suggest that in this grassland the resistant C pool could be negatively affected by increases in plant-production driven available soil C. [ABSTRACT FROM AUTHOR]

Elevated CO2 affects plant productivity, but also water availability and plant species composition in semi-arid grasslands, thereby potentially causing complex effects on CH4 consumption and N2O emission. We studied the effects of atmospheric CO2 concentration (400 vs 780 μL L−1), water content (15 vs 20% gravimetric soil moisture), and composition of semi-arid grassland species (perennial grasses Bouteloua gracilis, Hesperostipa comata, and Pascopyrum smithii; sub-shrub Artemisia frigida; invasive forb Linaria dalmatica grown in monoculture and all five species together) on CH4 consumption and N2O emission in a full factorial greenhouse experiment. We used a unique method where we measured microbial effects on CH4 consumption and N2O emission in isolation from effects of gas diffusivity. Microbially mediated CH4 consumption was significantly higher under elevated CO2 (by 20%), but was not affected by soil water content or plant species composition. Microbially mediated N2O emission was not significantly affected by elevated CO2, but was significantly higher with high water content (by 67%) and differed significantly among species. Treatment effects on CH4 consumption and N2O emission often could not be explained simply by differences in soil moisture, suggesting that treatment-induced changes in other soil and microbial properties played a role in causing these effects. [ABSTRACT FROM AUTHOR]

The natural abundance of 15N in plant biomass has been used to infer how N dynamics change with elevated atmospheric CO2 and changing water availability. However, it remains unclear if atmospheric CO2 effects on plant biomass 15N are driven by CO2-induced changes in soil moisture. We tested whether 15N abundance (expressed as δ15N) in plant biomass would increase with increasing soil moisture content at two atmospheric CO2 levels. In a greenhouse experiment we grew sunflower ( Helianthus annuus) at ambient and elevated CO2 (760 ppm) with three soil moisture levels maintained at 45, 65, and 85% of field capacity, thereby eliminating potential CO2-induced soil moisture effects. The δ15N value of total plant biomass increased significantly with increased soil moisture content at both CO2 levels, possibly due to increased uptake of 15N-rich organic N. Although not adequately replicated, plant biomass δ15N was lower under elevated than under ambient CO2 after adjusting for plant N uptake effects. Thus, increases in soil moisture can increase plant biomass δ15N, while elevated CO2 can decrease plant biomass δ15N other than by modifying soil moisture. [ABSTRACT FROM AUTHOR]

While increased atmospheric CO2 concentrations, increased N deposition, and changes in plant diversity have all been shown to significantly alter soil carbon (C) and nitrogen (N) dynamics, the effects of these factors have never been studied simultaneously and in combination. We studied the response of soil C and N dynamics to changes in atmospheric CO2 (ambient, 560?ppm), N fertilization (0, 4?g?N?m-2?yr-1), plant species number (1, 4 species), and plant functional group number (1, 4 groups; all with 4 species) in a grassland field experiment in Minnesota, USA. During the fourth season of treatments, we used laboratory incubations to assess soil C pool sizes and dynamics and net N mineralization, and determined microbial C and N and total soil C and N. Elevated CO2 increased labile C and microbial biomass, but had no effect on net N mineralization, respiration of more recalcitrant C, or total soil C and N. Nitrogen fertilization increased net N mineralization, because of faster decomposition or less immobilization by litter with higher N concentrations. In the four species plots, N fertilization also increased total soil C and N, likely because greater litter production more than offset any increases in decomposition. Increasing the species number from one to four increased C respiration that could largely be attributed to greater soil C inputs from increased biomass accumulation, but reduced net N mineralization, likely because of greater immobilization in the more productive four-species plots. An increase in functional group number did not affect any of the soil parameters measured. While elevated CO2, N fertilization, and increased species number all increased plant biomass accumulation, they had divergent effects on soil C and N dynamics. [ABSTRACT FROM AUTHOR]

Background and aims: Rhizodeposition is organic matter released by living plant roots that can be transformed by microbes into particulate organic matter (POM), but that can also become more stable through the adsorption of organic matter onto soil minerals (mineral-associated organic matter, MAOM), thereby playing an important role in mitigating climate change. We examined how root-derived carbon (C) as a proxy for rhizodeposition contributed to POM and MAOM formation in a grassland affected by nitrogen (N) fertilisation and defoliation frequency, and to what degree rhizodeposition was incorporated into microbial biomass.We applied N fertiliser (0 vs. 40 kg N ha−1 yr−1) and defoliation frequencies (3–4 vs. 6–8 clipping events year−1, simulating low and high grazing intensity) for three years, then used a 13CO2 pulse labelling technique to examine the incorporation of rhizodeposition into microbial biomass, POM and MAOM fractions.With N fertilisation, rhizodeposition contributed less to the formation of MAOM compared to the formation of POM, while defoliation frequency decreased the contribution of rhizodeposition into both POM and MAOM, particularly with N fertilisation. Although the MAOM fraction was relatively rich in N (C: N ratio of 10.5 vs. 13.5 for POM), our results suggest that adding inorganic N promoted the formation of POM more than of MAOM from rhizodeposition.A large proportion of rhizodeposition was taken up by microbes that eventually could contribute to POM and MAOM formation. Our results provide insightful information regarding the stabilisation of rhizodeposition into different soil organic matter pools.Methods: Rhizodeposition is organic matter released by living plant roots that can be transformed by microbes into particulate organic matter (POM), but that can also become more stable through the adsorption of organic matter onto soil minerals (mineral-associated organic matter, MAOM), thereby playing an important role in mitigating climate change. We examined how root-derived carbon (C) as a proxy for rhizodeposition contributed to POM and MAOM formation in a grassland affected by nitrogen (N) fertilisation and defoliation frequency, and to what degree rhizodeposition was incorporated into microbial biomass.We applied N fertiliser (0 vs. 40 kg N ha−1 yr−1) and defoliation frequencies (3–4 vs. 6–8 clipping events year−1, simulating low and high grazing intensity) for three years, then used a 13CO2 pulse labelling technique to examine the incorporation of rhizodeposition into microbial biomass, POM and MAOM fractions.With N fertilisation, rhizodeposition contributed less to the formation of MAOM compared to the formation of POM, while defoliation frequency decreased the contribution of rhizodeposition into both POM and MAOM, particularly with N fertilisation. Although the MAOM fraction was relatively rich in N (C: N ratio of 10.5 vs. 13.5 for POM), our results suggest that adding inorganic N promoted the formation of POM more than of MAOM from rhizodeposition.A large proportion of rhizodeposition was taken up by microbes that eventually could contribute to POM and MAOM formation. Our results provide insightful information regarding the stabilisation of rhizodeposition into different soil organic matter pools.Results: Rhizodeposition is organic matter released by living plant roots that can be transformed by microbes into particulate organic matter (POM), but that can also become more stable through the adsorption of organic matter onto soil minerals (mineral-associated organic matter, MAOM), thereby playing an important role in mitigating climate change. We examined how root-derived carbon (C) as a proxy for rhizodeposition contributed to POM and MAOM formation in a grassland affected by nitrogen (N) fertilisation and defoliation frequency, and to what degree rhizodeposition was incorporated into microbial biomass.We applied N fertiliser (0 vs. 40 kg N ha−1 yr−1) and defoliation frequencies (3–4 vs. 6–8 clipping events year−1, simulating low and high grazing intensity) for three years, then used a 13CO2 pulse labelling technique to examine the incorporation of rhizodeposition into microbial biomass, POM and MAOM fractions.With N fertilisation, rhizodeposition contributed less to the formation of MAOM compared to the formation of POM, while defoliation frequency decreased the contribution of rhizodeposition into both POM and MAOM, particularly with N fertilisation. Although the MAOM fraction was relatively rich in N (C: N ratio of 10.5 vs. 13.5 for POM), our results suggest that adding inorganic N promoted the formation of POM more than of MAOM from rhizodeposition.A large proportion of rhizodeposition was taken up by microbes that eventually could contribute to POM and MAOM formation. Our results provide insightful information regarding the stabilisation of rhizodeposition into different soil organic matter pools.Conclusion: Rhizodeposition is organic matter released by living plant roots that can be transformed by microbes into particulate organic matter (POM), but that can also become more stable through the adsorption of organic matter onto soil minerals (mineral-associated organic matter, MAOM), thereby playing an important role in mitigating climate change. We examined how root-derived carbon (C) as a proxy for rhizodeposition contributed to POM and MAOM formation in a grassland affected by nitrogen (N) fertilisation and defoliation frequency, and to what degree rhizodeposition was incorporated into microbial biomass.We applied N fertiliser (0 vs. 40 kg N ha−1 yr−1) and defoliation frequencies (3–4 vs. 6–8 clipping events year−1, simulating low and high grazing intensity) for three years, then used a 13CO2 pulse labelling technique to examine the incorporation of rhizodeposition into microbial biomass, POM and MAOM fractions.With N fertilisation, rhizodeposition contributed less to the formation of MAOM compared to the formation of POM, while defoliation frequency decreased the contribution of rhizodeposition into both POM and MAOM, particularly with N fertilisation. Although the MAOM fraction was relatively rich in N (C: N ratio of 10.5 vs. 13.5 for POM), our results suggest that adding inorganic N promoted the formation of POM more than of MAOM from rhizodeposition.A large proportion of rhizodeposition was taken up by microbes that eventually could contribute to POM and MAOM formation. Our results provide insightful information regarding the stabilisation of rhizodeposition into different soil organic matter pools. [ABSTRACT FROM AUTHOR]

Background: In a recent framework, Raven et al. New Phytologist 217: 1420-1427. (2018) considered carbon cost of acquiring phosphorus by mycorrhizal and non-mycorrhizal plants.We broaden their conceptual framework by incorporating belowground carbon allocation for both nitrogen and phosphorus acquisition in conditions of nutrient co-limitation and shifts in nutrient limitation, symbiotic associations with nitrogen-fixing bacteria, and nutrient mining via rhizosphere priming. We introduce a new parameter: carbon efficiency for nutrient acquisition (CENA) defined as the amount of nutrient acquisition per unit carbon allocated belowground.We explain how CENA increases with increased nutrient availability, and how it reaches a plateau when the increased availability of one limiting nutrient leads to the emergence of limitation by another nutrient. We describe how the relationship between CENA and mycorrhizal plants may be less steep compared to non-mycorrhizal plants so that CENA may be higher for mycorrhizal plants when nutrient availability is low and vice versa. In contrast, the CENA of nitrogen-fixing plants would be independent of soil nitrogen availability as long as biological nitrogen fixation meets plant nitrogen demand, but it would increase with increased soil phosphorus availability. The CENA would be more affected by soil nitrogen and phosphorus locked in organic matter or insoluble forms if plants perform nutrient mining strategies, but would be more sensitive to soil nitrogen and phosphorus availabilities if plants rely on nutrient scavenging strategies.The updated conceptual frameworks would provide better understanding of how plants optimize belowground carbon allocation for nutrient acquisition that is affected by perturbations in nutrient availability.Scope: In a recent framework, Raven et al. New Phytologist 217: 1420-1427. (2018) considered carbon cost of acquiring phosphorus by mycorrhizal and non-mycorrhizal plants.We broaden their conceptual framework by incorporating belowground carbon allocation for both nitrogen and phosphorus acquisition in conditions of nutrient co-limitation and shifts in nutrient limitation, symbiotic associations with nitrogen-fixing bacteria, and nutrient mining via rhizosphere priming. We introduce a new parameter: carbon efficiency for nutrient acquisition (CENA) defined as the amount of nutrient acquisition per unit carbon allocated belowground.We explain how CENA increases with increased nutrient availability, and how it reaches a plateau when the increased availability of one limiting nutrient leads to the emergence of limitation by another nutrient. We describe how the relationship between CENA and mycorrhizal plants may be less steep compared to non-mycorrhizal plants so that CENA may be higher for mycorrhizal plants when nutrient availability is low and vice versa. In contrast, the CENA of nitrogen-fixing plants would be independent of soil nitrogen availability as long as biological nitrogen fixation meets plant nitrogen demand, but it would increase with increased soil phosphorus availability. The CENA would be more affected by soil nitrogen and phosphorus locked in organic matter or insoluble forms if plants perform nutrient mining strategies, but would be more sensitive to soil nitrogen and phosphorus availabilities if plants rely on nutrient scavenging strategies.The updated conceptual frameworks would provide better understanding of how plants optimize belowground carbon allocation for nutrient acquisition that is affected by perturbations in nutrient availability.Results: In a recent framework, Raven et al. New Phytologist 217: 1420-1427. (2018) considered carbon cost of acquiring phosphorus by mycorrhizal and non-mycorrhizal plants.We broaden their conceptual framework by incorporating belowground carbon allocation for both nitrogen and phosphorus acquisition in conditions of nutrient co-limitation and shifts in nutrient limitation, symbiotic associations with nitrogen-fixing bacteria, and nutrient mining via rhizosphere priming. We introduce a new parameter: carbon efficiency for nutrient acquisition (CENA) defined as the amount of nutrient acquisition per unit carbon allocated belowground.We explain how CENA increases with increased nutrient availability, and how it reaches a plateau when the increased availability of one limiting nutrient leads to the emergence of limitation by another nutrient. We describe how the relationship between CENA and mycorrhizal plants may be less steep compared to non-mycorrhizal plants so that CENA may be higher for mycorrhizal plants when nutrient availability is low and vice versa. In contrast, the CENA of nitrogen-fixing plants would be independent of soil nitrogen availability as long as biological nitrogen fixation meets plant nitrogen demand, but it would increase with increased soil phosphorus availability. The CENA would be more affected by soil nitrogen and phosphorus locked in organic matter or insoluble forms if plants perform nutrient mining strategies, but would be more sensitive to soil nitrogen and phosphorus availabilities if plants rely on nutrient scavenging strategies.The updated conceptual frameworks would provide better understanding of how plants optimize belowground carbon allocation for nutrient acquisition that is affected by perturbations in nutrient availability.Conclusions: In a recent framework, Raven et al. New Phytologist 217: 1420-1427. (2018) considered carbon cost of acquiring phosphorus by mycorrhizal and non-mycorrhizal plants.We broaden their conceptual framework by incorporating belowground carbon allocation for both nitrogen and phosphorus acquisition in conditions of nutrient co-limitation and shifts in nutrient limitation, symbiotic associations with nitrogen-fixing bacteria, and nutrient mining via rhizosphere priming. We introduce a new parameter: carbon efficiency for nutrient acquisition (CENA) defined as the amount of nutrient acquisition per unit carbon allocated belowground.We explain how CENA increases with increased nutrient availability, and how it reaches a plateau when the increased availability of one limiting nutrient leads to the emergence of limitation by another nutrient. We describe how the relationship between CENA and mycorrhizal plants may be less steep compared to non-mycorrhizal plants so that CENA may be higher for mycorrhizal plants when nutrient availability is low and vice versa. In contrast, the CENA of nitrogen-fixing plants would be independent of soil nitrogen availability as long as biological nitrogen fixation meets plant nitrogen demand, but it would increase with increased soil phosphorus availability. The CENA would be more affected by soil nitrogen and phosphorus locked in organic matter or insoluble forms if plants perform nutrient mining strategies, but would be more sensitive to soil nitrogen and phosphorus availabilities if plants rely on nutrient scavenging strategies.The updated conceptual frameworks would provide better understanding of how plants optimize belowground carbon allocation for nutrient acquisition that is affected by perturbations in nutrient availability. [ABSTRACT FROM AUTHOR]