Nitrogen response efficiency of a managed and phytodiverse temperate grassland

Aims

Our goal was to assess how management and sward functional diversity affect nitrogen response efficiency (NRE), the ratio of plant biomass production to supply of available nitrogen (N) in temperate grassland.

Methods

A three-factorial design was employed: three sward compositions, two mowing frequencies, and two fertilization treatments.

Results

NRE was largely influenced by fertilization followed by mowing frequency and sward composition. NRE was larger in unfertilized than fertilized plots, in plots cut thrice than plots cut once per year, and in control swards than in monocot- or dicot-enhanced swards. Fertilization decreased NRE through decreases in both N uptake efficiency (plant N uptake per supply of available N) and N use efficiency (NUE, biomass produced per plant N uptake) whereas mowing frequency and sward composition affected NRE through N uptake efficiency rather than NUE. The largest NRE in the control sward with 70 % monocots and 30 % dicots attests that these proportions of functional groups were best adapted in this grassland ecosystem.

Conclusions

Optimum NRE may not be a target of most farmers, but it is an appropriate tool to evaluate the consequences of grassland management practices, which farmers may employ to maximize profit, on environmental quality.     

Grassland cutting regimes affect soil properties, and consequently vegetation composition and belowground plant traits

Background and aims

Machine mowing, mimicking the traditional hand mowing, is often used as a successful management tool to maintain grassland biodiversity, but few studies have investigated the long-term effects of traditional versus mechanical mowing of plant communities. Machine mowing as opposed to hand mowing causes soil compaction and reduction of soil aeration. In response, we expected strong effects on below-ground plant traits: root aerenchyma formation and superficial root growth, and no specific effects on aboveground traits. Effects were expected to be more pronounced in soils vulnerable to compaction.

Methods

We evaluated the changes in above- and belowground plant traits in a long-term (38-year) experiment with annual hand-mowing and machine-mowing management regimes on two different soil types: a coarse structured sandy soil and a finer structured sandy-organic soil

Results

Only on the organic soil, long-term machine mowing leads to lower soil aeration (more compacted soil) and a marked change in the belowground trait distribution of the plant community. Here we find a higher cover of superficially rooting species and marginally significant lower cover of species without morphological adaptations to soil hypoxia, but no effect on species with a high capacity of forming aerenchyma.

Conclusion

Mowing with heavy machines on soils vulnerable to compaction affect the vegetation according to changes in soil physical conditions. This is reflected in a shift towards communities with greater proportion of superficially rooting species. Our results illustrate the sensitivity of grasslands to slight changes in the management regime.     

Drought changes the structure and elemental composition of very fine roots in seedlings of ten woody tree species. Implications for a drier climate

Background and aims

Water availability is often one of the most limiting factors for plants. Climate change predictions for many areas suggest an intensification of water limitation. The ability of a plant to modify its root characteristics can be an important mechanism for preventing drought stress.

Methods

We studied the drought response of seedlings of 10 woody species and compared the biomass allocation, vertical root distribution across different root diameters, and the key traits of very fine roots (root diameter <0.5 mm) under two water regimes (no water limitation and severe drought).

Results

Under drought conditions, the very fine roots had a higher specific root length (SRL, root length: biomass ratio), smaller root diameter and higher root tissue mass density, as well as a lower nitrogen concentration. A higher value of the mean root plasticity index was related to higher drought resistance. A quantitative literature review showed that there was a wide variation in the effect of the drought on SRL, thus there was not a clear effect of drought on SRL.

Conclusions

Certain species have the necessary root traits and plasticity to survive drought. We have identified plasticity in root characteristics as a whole-plant trait which plays a significant role in separating out species into those which are vulnerable and those which are resistant to drought.     

Root plasticity maintains growth of temperate grassland species under pulsed water supply

Background and aims

The frequency of rain is predicted to change in high latitude areas with more precipitation in heavy, intense events interspersed by longer dry periods. These changes will modify soil drying cycles with unknown consequences for plant performance of temperate species.

Methods

We studied plant growth and root traits of juveniles of four grasses and four dicots growing in a greenhouse, when supplying the same total amount of water given either regular every other day or pulsed once a week.

Results

Pulsed water supply replenished soil moisture immediately after watering, but caused substantial drought stress at the end of the watering cycle, whereas regular watering caused more moderate but consistent drought. Grasses had lower water use efficiency in the pulsed watering compared to regular watering, whereas dicots showed no difference. Both grasses and dicots developed thinner roots, thus higher specific root length, and greater root length in the pulsed watering. Growth of dicots was slightly increased under pulsed watering.

Conclusions

Temperate species coped with pulsed water supply by eliciting two responses: i) persistent shoot growth, most likely by maximizing growth at peaks of soil moisture, thus compensating for slower growth during drought periods; ii) plasticity of root traits related to increased resource uptake. Both responses likely account for subtle improvement of growth under changed water supply conditions.     

Effect of species, root branching order and season on the root traits of 13 perennial grass species

Aims

Inter-specific comparisons of plant traits may vary depending on intra-specific variation. Here we examine the impact of root branching order and season on key functional root traits for grass species. We also compare root traits among co-existing grass species as a step towards defining root trait syndromes.

Methods

Monocultures of 13 grass species, grown under field conditions and subjected to intensive management, were used to record root trait values for coarse roots (1st order, >0.3?mm), fine roots (2nd and 3rd orders, <0.2?mm) and mixed root samples over three growing seasons.

Results

Branching order and species had a significant effect on root trait values, whereas season showed a marginal effect. The diameter of coarse roots was more variable than that of fine roots and, as expected, coarse roots had higher tissue density and lower specific root length values than fine roots. Principal component analysis run on eight root traits provided evidence for two trait syndromes related to resource acquisition and conservation strategies across grass species.

Conclusions

Our data show that root branching order is the main determinant of root trait variation among species. This highlights the necessity to include the proportion of fine vs coarse roots when measuring traits of mixed root samples.     

Impacts of long-term plant biomass management on soil phosphorus under temperate grassland

Aims

We assessed and quantified the cumulative impact of 20 years of biomass management on the nature and bioavailability of soil phosphorus (P) accumulated from antecedent fertiliser inputs.

Methods

Soil (0–2.5, 2.5–5, 5–10 cm) and plant samples were taken from replicate plots in a grassland field experiment maintained for 20 years under contrasting plant biomass regimen- biomass retained or removed after mowing. Analyses included dry matter production and P uptake, root biomass, total soil carbon (C), total nitrogen (N), total P, soil P fractionation, and ³¹P NMR spectroscopy.

Results

Contemporary plant production and P uptake were over 2-fold higher for the biomass retained compared with the biomass removed regimes. Soil C, total P, soluble and labile forms of inorganic and organic soil P were significantly higher under biomass retention than removal.

Conclusions

Reserves of soluble and labile inorganic P in soil were significantly depleted in response to continued long-term removal of P in plant biomass compared to retention. However, this was only sufficient to sustain plant production at half the level observed for the biomass retention after 20 years, which was partly attributed to limited mobilisation of organic P in response to P removal.

     

Root traits and soil properties in harvested perennial grassland,annual wheat,and never-tilled annual wheat

Background and aims

Root functional traits are determinants of soil carbon storage; plant productivity; and ecosystem properties. However, few studies look at both annual and perennial roots, soil properties, and productivity in the context of field scale agricultural systems.

Methods

In Long Term and Conversion studies in North Central Kansas, USA; root biomass and length, soil carbon and nitrogen, microbial biomass, nematode and micro-arthropod communities were measured to a depth of one meter in paired perennial grassland and cropland wheat sites as well as a grassland site that had been converted to cropland using no tillage five years prior.

Results

In the Long Term Study root biomass was three to seven times greater (9.4 Mg ha^?1 and 2.5 Mg ha^?1 in May), and root length two times greater (52.5 km m^?2 and 24.0 km m^?2 in May) in perennial grassland than in cropland. Soil organic carbon and microbial biomass carbon were larger, numbers of Orbatid mites greater (2084 vs 730 mites m^?2), and nematode communities more structured (Structure Index 67 vs 59) in perennial grassland versus annual cropland. Improved soil physical and biological properties in perennial grasslands were significantly correlated with larger, deeper root systems. In the Conversion Study root length and biomass, microbial biomass carbon, mite abundance and nematode community structure differed at some but not all dates and depths. Isotope analysis showed that five years after no-till conversion old perennial roots remained in soils of annual wheat fields and that all soil fractions except coarse particulate organic matter were derived from C₄ plants.

Conclusions

Significant correlation between larger, longer roots in grasslands compared to annual croplands and improved soil biological, physical and chemical properties suggest that perennial roots are an important factor allowing perennial grasslands to maintain productivity and soil quality with few inputs. Perennial roots may persist and continue to influence soil properties long after conversion to annual systems.     

Response of soil microbial activity to grazing, nitrogen deposition, and exotic cover in a serpentine grassland

Background and aims

Exotic species, nitrogen (N) deposition, and grazing are major drivers of change in grasslands. However little is known about the interactive effects of these factors on below-ground microbial communities.

Methods

We simulated realistic N deposition increases with low-level fertilization and manipulated grazing with fencing in a split-plot experiment in California’s largest serpentine grassland. We also monitored grazing intensity using camera traps and measured total available N to assess grazing and nutrient enrichment effects on microbial extracellular enzyme activity (EEA), microbial N mineralization, and respiration rates in soil.

Results

Continuous measures of grazing intensity and N availability showed that increased grazing and N were correlated with increased microbial activity and were stronger predictors than the categorical grazing and fertilization measures. Exotic cover was also generally correlated with increased microbial activity resulting from exotic-driven nutrient cycling alterations. Seasonal effects, on abiotic factors and plant phenology, were also an important factor in EEA with lower activity occurring at peak plant biomass.

Conclusions

In combination with previous studies from this serpentine grassland, our results suggest that grazing intensity and soil N availability may affect the soil microbial community indirectly via effects on exotic cover and associated changes in nutrient cycling while grazing directly impacts soil community function.     

Grassland species show similar strategies for sulphur and nitrogen acquisition

Backgrounds and aims

Plant nutrition strategies play a crucial role in community structure and ecosystem functioning. However, these strategies have been established only for nitrogen (N) acquisition, and it is not known whether similar strategies hold for other macronutrients such as sulphur (S). The aim of our study was to determine whether strategies for S acquisition of some grassland species were similar to those observed for N acquisition, and to analyse the relationships between these plant strategies and the soil microbial activity involved in soil organic S mineralisation.

Methods

We used three exploitative and three conservative grass species grown with and without S fertilisation. We measured a set of plant traits, namely root and shoot biomass, leaf area, root length, N and S content, leaf nutrient use efficiency, and sulphate uptake rates in plants, and one microbial trait linked to S mineralisation, namely soil arylsulphatase activity.

Results

The set of plant traits differentiated exploitative from conservative species. Close relationships were found between traits associated with strategies for N acquisition, namely total N content and Leaf N Use Efficiency (LNUE), and traits associated with strategies for S acquisition, namely total S content and Leaf S Use Efficiency (LSUE). Exploitative species exhibited similar or lower sulphate uptake capacities per unit of biomass than conservative species, but acquired more S through their larger root systems. Greater arylsulphatase activity was observed in the rhizosphere of the most exploitative species.

Conclusion

Overall, our results show that nutrient strategies defined in grassland species for N acquisition can be extended to S.     

Hierarchical responses of plant stoichiometry to nitrogen deposition and mowing in a temperate steppe

Background and aims

Stoichiometric relations drive powerful constraints on many ecosystem processes. However, our understanding of the hierarchical responses of plant C:N:P stoichiometry at different levels of biological organization to global change factors remains limited.

Methods

we examined the plant C:N:P stoichiometric responses to N deposition and mowing (hay making) at both species- and community-level by carrying out a 4-year field experiment in the temperate steppe of northern China.

Results

Our results showed that N addition and mowing resulted in higher plant N concentrations, lower C:N, and higher N:P at both species- and community-level. Mowing had a limited negative influence on the effects of N addition. We observed divergent responses of both plant P concentrations and C:P to N addition at species-level and community-level: N addition led to higher plant P and lower C:P at species-level, but this effect was not observed at the community-level.

Conclusions

Our results indicate that stoichiometric responses at community-level to N addition and mowing diverge from more traditionally examined species-specific responses. Our results suggest that the hierarchical responses of plant stoichiometry to anthropogenic disturbance deserves more attention when we model the interactions of terrestrial ecosystem C, N, and P cycling under scenarios of increasing N availability concomitantly occurring with active land management.     

Multivariate control of root biomass in a semi-arid grassland on the Loess Plateau,China

Aim

Root biomass has long been under-represented in biodiversity–ecosystem functioning studies, despite its dominance in biomass in many arid and semi-arid ecosystems. We aimed to explore the multivariate control over root biomass by plant diversity, together with other biotic and abiotic factors and to evaluate the relative importance of these factors.

Methods

Above- and below-ground traits of 13 communities and soil properties were measured in semi-arid grasslands on the Loess Plateau, China. Structural equation modeling (SEM) was used to evaluate the relative importance of the community and soil characteristics, emphasizing the direct and indirect effects of plant diversity on root biomass.

Results

Significant indirect effects of plant species richness on root biomass were found, although no direct correlation was detected between them. In the indirect pathways, plant species richness showed a positive effect on soil total nitrogen, but a significant negative influence on soil total carbon. Soil total nitrogen and plant diversity had the largest and smallest total effect respectively on root biomass in the model.

Conclusions

Plant species richness was not the strongest determinant of root biomass but had a significant indirect effect, mediated through soil total carbon and nitrogen. This study suggests that greater plant species richness, through a positive influence on soil total nitrogen, may indirectly promote root carbon stock.     

Physiological properties of a drought-resistant wild soybean genotype: Transpiration control with soil drying and expression of root morphology

Aims

Wild soybean accession PI 468917 [Glycine soja (Sieb. and Zucc.)] was examined for traits that could potentially be beneficial for development of drought resistant soybean cultivars.

Methods

Water use was examined in controlled environment chambers at three temperatures (25, 30, and 35 °C). Root morphology of plants grown in hydroponics was analyzed using digital imaging software.

Results

Wild soybean had lower transpiration efficiency in producing mass than the domesticated soybean cultivar Hutcheson at all temperatures. As soil dried, wild soybean decreased transpiration earlier (at a higher soil water content) than domesticated soybean, but only at 25 °C. Wild soybean had much greater root length than the modern soybean when grown at 25 or 30 °C in hydroponics, with the increase observed in the 0.25 to 0.50 mm diameter class. Wild soybean’s advantages dissipated at higher growth temperatures.

Conclusions

Wild soybean populations, potentially, can offer useful traits for improving drought resistance of modern soybean. Sensitive transpiration control in response to soil drying would contribute to ‘slow-wilting’ strategies known to be advantageous for drought resistance, and greater root length would enhance water acquisition from the soil profile. Use of the traits in breeding programs will require extending the temperature range for trait expression.     

Co-occurring grass species differ in their associated microbial community composition in a temperate native grassland

Background and aims

Specific associations exist between plant species and the soil microbial community and these associations vary between habitat types and different plant groups. However, there is evidence that the associations are highly specific. Hence, we aimed to determine the specificity of plant-microbe relationships amongst co-occurring grass species in a temperate grassland.

Methods and results

We examined the broad microbial groups of bacteria and fungi as well as a specific fungal group, the arbuscular mycorrhizal community amongst two dominant C₃ and C₄ species and one sub-dominant C₃ species using terminal restriction fragment length polymorphism (T-RFLP) analysis. We found that the two dominant species were more similar to each other in their bacterial and arbuscular mycorrhizal community composition than either was to the sub-dominant species, but not in their fungal community composition. We also found no clear evidence that those differences were directly linked to soil chemical properties.

Conclusions

Our results demonstrate that co-occurring grass species have a distinct soil microbial community and T-RFLP analysis is able to detect plant species effect on the microbial community composition on an extremely local scale, providing an insight into the differences in the response of bacterial, fungal and arbuscular mycorrhizal communities to different, but similar and co-occurring, plant species.     

Temporal variation in microbial and plant biomass during summer in a Mediterranean high-mountain dry grassland

Aims

We assessed the temporal changes on microbial biomass in relation to changes in soil moisture, dissolved organic carbon and plant biomass during the summer season in a Mediterranean high-mountain grassland.

Methods

Temporal variations were tested by two-way ANOVA. The relationships among microbial biomass, plant biomass, soil water content, soil organic carbon, dissolved organic carbon and total soil nitrogen during the summer season were assessed by means of structural equation modeling.

Results

Microbial biomass did not show variation, while dissolved organic carbon and root biomass decreased throughout the summer. Aboveground plant biomass peaked in the middle of the summer, when soil water content was at its minimum. Soil water content directly and negatively affected soil microbial biomass, and positively affected dissolved organic carbon. Moreover soil microbial biomass and dissolved organic carbon were negatively related. Plant biomass effects on soil microbial biomass were driven by root biomass, which indirectly affected soil microbial biomass through effects on soil organic carbon and soil nitrogen.

Conclusions

The temporal dynamic of microbial biomass during the summer season appeared to differ from previous observations in temperate alpine communities, and indicated the drought resistance of the microbial community during the summer in Mediterranean high-mountain grasslands. During the dry period, microbial biomass may play an alternative role in soil carbon conservation.     

Grassland root demography responses to multiple climate change drivers depend on root morphology

Aims

We examine how root system demography and morphology are affected by air warming and multiple, simultaneous climate change drivers.

Methods

Using minirhizotrons, we studied root growth, morphology, median longevity, risk of mortality and standing root pool in the upper soil horizon of a temperate grassland ecosystem for 3 years. Grassland monoliths were subjected to four climate treatments in a replicated additive design: control (C); elevated temperature (T); combined T and summer precipitation reduction (TD); combined TD and elevated atmospheric CO₂ (TDCO₂).

Results

Air warming (C vs T) and the combined climate change treatment (C vs TDCO₂) had a positive effect on root growth rate and standing root pool. However, root responses to climate treatment varied depending on diameter size class. For fine roots (≤ 0.1 mm), new root length and mortality increased under warming but decreased in response to elevated CO₂ (TD vs TDCO₂); for coarse roots (> 0.2 mm), length and mortality increased under both elevated CO₂ and combined climate change drivers.

Conclusions

Our data suggest that the standing roots pool in our grassland system may increase under future climatic conditions. Contrasted behaviour of fine and coarse roots may correspond to differential root activity of these extreme diameter classes in future climate.     

Threshold effect of the groundwater depth on the photosynthetic efficiency of <Emphasis Type="Italic">Tamarix chinensis</Emphasis> in the Yellow River Delta

Background and aims

Functional traits may underlie differences in niches, which promote plant species co-existence, but also differences in competitive ability, which drive competitive exclusion. Empirical evidence concerning the contribution of different traits to niche differentiation and the ability to supress and tolerate competitors is very limited, particularly when considering belowground interactions.

Methods

We grew 26 temperate grassland species along a density gradient of interspecific competitors to determine which belowground traits a) explain species’ ability to suppress and tolerate neighbours and b) contribute to niche differentiation, such that species with dissimilar trait values experience reduced competition.

Results

We found that having larger root systems with extensive horizontal spread and lower root tissue density enabled efficient suppression of neighbours but did not significantly contribute to the ability to tolerate competition. Species with deeper root systems, lower specific root length and less branched roots were better at tolerating competition, but these traits did not significantly affect the ability to suppress neighbours. None of the measured traits contributed significantly to niche differentiation, either individually or in combination.

Conclusions

This study provides little support for belowground traits contributing to species co-existence through niche differentiation. Instead, different sets of weakly correlated traits enable plants to either suppress or tolerate their competitors.

     

Spatial distribution and phenotypic variation in root cortical aerenchyma of maize (Zea mays L.)

Background and aims

Previous research has suggested that root cortical aerenchyma (RCA) can enhance soil exploration and crop performance by decreasing root respiration. This trait is a potential breeding target for adaptation to abiotic stresses such as drought and low nutrient availability. However, little is known of phenotypic variation in aerenchyma or its distribution among root classes.

Methods

The spatial distribution of RCA was evaluated in freehand sections from 13 sites in the root systems of 11 recombinant inbred and commercial lines of maize (Zea mays). RCA variation was evaluated in 583 recombinant inbred lines of maize at one sampling position.

Results

RCA varied significantly among root classes and axial positions. Genotypic differences were observed for the amount of RCA at corresponding sampling locations and for the mean amount of RCA across all sampling locations, but genotypes did not differ in the proportional distribution of RCA within the whole root system. The amount of RCA in a cross-section was independent of several other anatomical traits.

Conclusions

There is substantial genetic variation for RCA, and this variation is independent of other anatomical traits. RCA can be phenotyped in greenhouse-grown plants by sampling the middle parts of second- or third-whorl crown roots.     

Linking Above- and Belowground Responses to 16 Years of Fertilization,Mowing, and Removal of the Dominant Species in a Temperate Grassland

Species-rich oligotrophic meadows are affected by a wide range of management interventions that influence their functioning and capacity to deliver ecosystem services, but long-term studies on the above- and belowground adaptations to different management tools are still scarce. We focused on the interactive effects of NPK fertilization, mowing, and removal of the initially dominant species (Molinia caerulea) on plant, soil, and microbial responses in wet oligotrophic grassland in a 16-year full-factorial manipulative experiment. Changes in vegetation composition, soil pH, and nutrient availability were accompanied by altered microbial phospholipid fatty acid (PLFA) composition, whereas treatment effects on soil microbial biomass and carbon (C) mineralization were mainly related to changes in soil organic matter (SOM) content and nutrient availability. Fertilization decreased plant species richness aboveground and lowered SOM storage and microbial activity belowground. Mowing preserved high plant diversity and led to more efficient recycling of N within the grassland, whereas Molinia removal significantly affected only plant community composition. Mowing combined with fertilization maintained high species richness only in the short term. Belowground, mowing reduced N leaching from the fertilized system but did not prevent SOM depletion, soil acidification, and concomitant adverse effects on soil microbes. We conclude that annual mowing is the appropriate type of extensive management for oligotrophic species-rich meadows, but the concomitant nutrient depletion should not be compensated for by regular NPK fertilization due to its adverse effects on soil quality.     

Natural variation in the regulation of leaf senescence and relation to N and root traits in wheat

Objectives

To identify parameters that can be used for the analysis of natural variation in leaf senescence of wheat; and to understand the association between the onset and progression of leaf senescence with N uptake and root traits.

Methods

Chlorophyll content and the proportion of yellow leaves were used as senescence indicators and their relation with other morphological and physiological traits were measured in contrasting early senescing (ES) and late senescing (LS) wheat lines.

Results

There were significant genotype effects on the onset and progress of senescence. The ES lines in which leaf senescence commenced early had significantly lower root biomass and N uptake than LS lines. The strong negative association between the extent of leaf senescence with root biomass and N uptake indicated that the poor root growth induced N limitation caused the early senescence of ES lines.

Conclusions

The leaf senescence development in ES lines was precocious and constitutive as the trait expressed even under optimal growth conditions suggesting they could be useful in understanding the genetic regulation of senescence under different abiotic stress situations. Accelerated leaf senescence in wheat could be a mechanism to compensate for limitations in the root system that tend to restrict nutrient uptake.     

Response of dominant grassland species in the temperate steppe of Inner Mongolia to different land uses at leaf and ecosystem levels

In order to study the responses of dominant species to different land uses in the semiarid temperate grassland of Inner Mongolia, we tested the physiological responses of Stipa grandis, Leymus chinensis, and Artemisia frigida to mowing, grazing exclusion, and grazing land uses at the leaf and ecosystem levels. The grazing-exclusion and mowing sites released CO₂, but the grazing site was a net carbon sink. L. chinensis and S. grandis contributed more to the ecosystem CO₂ exchange than A. frigida. At the grazing-exclusion and mowing sites, Leymus chinensis and Stipa grandis both exhibited a higher light-saturation point and higher maximum photosynthetic rate than that at the grazing site, which increased photosynthesis and growth compared to those at the grazing site. In contrast, A. frigida possessed a higher nitrogen content than the other species, and more of the light energy used for photosynthesis, particularly at the grazing site.