Claire Jouany

Leaf traits capture the effects of land use changes and climate on litter decomposability of grasslands across Europe

Land use and climate changes induce shifts in plant functional diversity and community structure, thereby modifying ecosystem processes. This is particularly true for litter decomposition, an essential process in the biogeochemical cycles of carbon and nutrients. In this study, we asked whether changes in functional traits of living leaves in response to changes in land use and climate were related to rates of litter potential decomposition, hereafter denoted litter decomposability, across a range of 10 contrasting sites. To disentangle the different control factors on litter decomposition, we conducted a microcosm experiment to determine the decomposability under standard conditions of litters collected in herbaceous communities from Europe and Israel. We tested how environmental factors (disturbance and climate) affected functional traits of living leaves and how these traits then modified litter quality and subsequent litter decomposability. Litter decomposability appeared proximately linked to initial litter quality, with particularly clear negative correlations with lignin-dependent indices (litter lignin concentr tion, lignin:nitrogen ratio, and fiber component). Litter quality was directly related to community-weighted mean traits. Lignin-dependent indices of litter quality were positively correlated with community-weighted mean leaf dry matter content (LDMC), and negatively correlated with community-weighted mean leaf nitrogen concentration (LNC). Consequently, litter decomposability was correlated negatively with community-weighted mean LDMC, and positively with community-weighted mean LNC. Environmental factors (disturbance and climate) influenced community-weighted mean traits. Plant communities experiencing less frequent or less intense disturbance exhibited higher community-weighted mean LDMC, and therefore higher litter lignin content and slower litter decomposability. LDMC therefore appears as a powerful marker of both changes in land use and of the pace of nutrient cycling across 10 contrasting sites.

CHARACTERIZATION OF THE WETTING PROPERTIES OF AIR-DRIED PEATS AND COMPOSTS

The hydric behavior of various samples of peats and composts which are commonly used in horticulture were studied. The wettability was assessed from contact angle measurements performed on air-dried pellets. Void index and water index curves were given in the process of first dehydration and then after air drying and rewetting, in a second dehydration process. Concerning the solid-liquid contact angle on air-dried material, peats turned out to be hydrophobic; this character was more obvious for woody (122.1°) and herbaceous (116.8°) peats than sphagnum peat (110.9°). These results were correlated with the humification degree (von Post index and pyrophosphate index). In comparison, fresh sphagnum is quite hydrophilic. The contact angles measured on composts are lower than 90°, whereas the woody materials used as litter are hydrophobic. Contact angle decreased during the composting process. The hydrophobic character of herbaceous and woody peat, together with their lack of rigidity, explains the irreversibility of the shrinkage. The opposite sphagnum peat, due to its hydrophilic character and elasticity, presented a reversible behavior after air drying. After air drying, the hydric behavior of composts, which are made of rigid and hydrophilic materials, was not greatly modified.

Leaf Traits as Functional Descriptors of the Intensity of Continuous Grazing in Native Grasslands in the South of Brazil

Abstract Plant functional types (PFT) have been used to describe the response of native vegetation to environmental factors (i.e., fertility) and to livestock disturbance, but rarely under conditions of continuous grazing. In this work we investigate whether the long-term response of grassland communities submitted to a gradient of continuous grazing pressure can be described with such an approach. After 15 yr of differentiation of the grazing pressure applied to native grasslands we measured leaf dry-matter content (LDMC) and specific leaf area (SLA) of Poaceae populations of the communities. A grazing pressure gradient was created by levels of daily forage allowance: 4, 8, 12, and 16 kg of dry matter per day per 100 kg of animal live weight, monitored monthly. PFTs were defined by numerical analysis, where an algorithm finds the optimal trait subset based on the agreement between matrices of species × traits, paddocks × grass biomass, and environmental variables (levels of forage allowance and soil characteristics). The results show that it is possible to describe a gradient of grazing pressure by means of LDMC and/or SLA measured only on the Poacea contributing at least 80% of the total Poaceae biomass. Four PFTs were differentiated by these leaf traits. PFTs having low LDMC and high SLA are characteristic of high intensity of use and are made up largely of stoloniferous C4 species typical of rapid resource capture strategies. Conversely, PFTs characterized by high LDMC and low SLA include species that are representative of low grazing pressure. Variations in the aggregate value of traits are due to changes in the species proportions and not to leaf-size adaptation as hypothesized. We conclude than in the absence of a gradient of fertility, plants with strategies of resource capture tend to be more represented under high grazing pressures. This situation results in a loss of functional diversity, but in particular a reduction in forage availability, which is incompatible with high animal production.

Using Leaf Traits to Rank Native Grasses According to Their Nutritive Value

Abstract Leaf traits (leaf dry matter content [LDMC], specific leaf area [SLA] and leaf life span [LLS]) previously proposed to predict plant strategies for resource use, were studied to test if they can be used to rank grasses for digestible organic matter (DOM). On 14 native grass species from natural meadows in the French Pyrenees, leaf blade chemical components (fiber, cellulose, hemi-cellulose and lignin) and DOM were estimated for two growing periods using two different methods (chemical-enzymatic and Near Infrared Reflectance Spectroscopy). The ranking of species based on LDMC, SLA and LLS was conserved. Fiber content and DOM were significantly correlated even though the data were obtained in different years (2001 and 2002), on different organs (youngest adult blades in 2001 and all the green blades of tillers in 2002) and by different analytical methods. LDMC seems to be the most suitable trait to rank native grasses according to their nutritive value because it ranks species as well as leaf traits and it is the easiest to measure. We suggest using LDMC as an indicator to rank grassland communities for herbage nutritive values.

Comparison of methods for assessing the impact of different disturbances and nutrient conditions upon functional characteristics of grassland communities

BACKGROUND AND AIMS Predicting the response of plant communities to variation in resources and disturbance is still a challenge, because findings depend on how ecological gradients are characterized and how grassland functional composition is assessed. Focusing on leaf dry matter content (LDMC), the efficacy of different methods for evaluating the best response of plant communities to either environmental or disturbance change is examined. METHODS Data were collected on 69 grasslands located at four sites in the Pyrenees and Massif Central. N-Ellenberg indices and plant nutrient content (Ni) were compared to assess fertility, and either LDMC (meas) measured or calculated from a trait database for which traits were measured under the same environmental conditions (db). Management regime (MR) was characterized in terms of categories (grazing, cutting) and plant height. KEY RESULTS LDMCdb was positively correlated to LDMCmeas, but depended significantly on site temperature. N-Ellenberg and Ni were significantly correlated, and there was a significant effect of MR and temperature. LDMC responded to fertility, MR and temperature. Replacing MR by plant height in an REML analysis reduced the uncertainty of the LDMC prediction. LDMC was correlated to plant height at community level, whereas the correlation was weak at species level. Differences in LDMC between plant communities under any of the management regimes were significantly correlated to the standing herbage mass. CONCLUSION The N-Ellenberg index is a better indicator of fertility than Ni which is short-term and environment-dependent. LDMC taken from a database allows plant trait variation due to species abundance (excluding variation due to trait plasticity in response to management) to be captured. So the former is better suited for assessing agricultural services that mainly depend on plant phenology and tissue composition. LDMC responded to defoliation regime in addition to fertility because plant height is roughly correlated with LDMC at plant community level.

Modeling of phosphorus dynamics in contrasting agroecosystems using long-term field experiments

Morel, C., Ziadi, N., Messiga, A., Bélanger, G., Denoroy, P., Jeangros, B., Jouany, C., Fardeau, J. C., Mollier, A., Parent, L. E., Proix, N., Rabeharisoa, L. and Sinaj, S. 2014. Modeling of phosphorus dynamics in contrasting agroecosystems using long-term field experiments. Can. J. Soil Sci. 94: 377-387. Long-term field experiments on phosphorus (P) fertilization were originally designed to study crop needs in different soil types by analyzing the effects of several rates of P fertilization on yields, their P concentrations and dynamics of plant-available soil P. The objective of this study was to test a computer-based model to simulate the P dynamics at the field scale using plant database and analyzing for plant-available P by a hierarchical process-based approach. It predicts both the concentration (CP) of phosphate ions (Pi) in soil solution and the associated Pi amounts that in time equilibrate with Pi in solution. Five experiments, representative of contrasting soil types, land-use, and climates were selected. Our model equilibrates the change in plant-available P in the upper soil layer to the P budget between annual P inputs and outputs. Rates of P fertilization affected simulations following the same expected pattern across sites. Field-observed and simulated values are in good agreements in all sites. The field-observed variations of CP per unit of P budget ranged from 0.007 to 2.49 (µg P L-1) (kg P ha-1)-1. The predictions are of the same order of magnitude. Predictions were compared with empirical long-term data and mismatches were discussed. This investigation highlights the scientific interest of long-term field P experiments to test and validate models describing P dynamics at the scale of the agricultural fields under different agricultural management practices.

Biological Phosphorus Cycling in Grasslands: Interactions with Nitrogen

The complexity of soil–plant–animal interactions in grassland ecosystems highlights the importance of studying biological phosphorus (P) cycling in relation with that of nitrogen (N). Several case studies employing original approaches developed by both agronomists and ecologists are presented. The nutrition index approach, based on nutrient dilution in the process of biomass accumulation, is used in order to evaluate the relative response of grassland to P in relation to changes in nutritional status (N and P). In parallel, the functional characterization of grassland vegetation from plant functional type (PFT) definition is presented. It relies on the fact that grassland communities may contain a wide diversity of species that influence nutrient biological cycling and regulate the biogeochemical cycle of nutrients. Finally, the effect of grazing herbivores on the biogeochemical cycle of major elements is examined to give a better insight into the complexity of the processes associated with N and P cycles in grazed grasslands.

Effect of pooling soil samples on the diffusive dynamics of phosphate ionic species

Limited information is available on the effect of pooling subsamples of soils on the dynamics of diffusive phosphorus (P), the phosphate ions in the soil solid phase that can equilibrate with the solution under a gradient of ionic P concentration. We examined the change in the parameters of the kinetic Freundlich equation, which accounts for the interactive effects of time (t, min) and soil solution P on the gross amounts of diffusive soil P. A common procedure was used to prepare a single pooled sample from four individual soil samples taken from each block of the same agronomic treatment of a grassland experiment. The pooling procedure consisted of combining four individual soil subsamples and mixing them on orbital shaker (100 r.p.m.) with agate balls. One of the individual samples was mixed without pooling. The kinetic Freundlich parameters were estimated for all individual and pooled samples by coupling a sorption experiment with a subsequent 32P labeling and dilution analysis in soil suspensions at steady state. The pooling procedure of soils, more specifically, the mixing step, was found to significantly change the kinetic Freundlich parameters that characterize both the immediate and the slow reactions. The immediate sorption capacity increased by 50%, whereas the slow process decreased by about 10%. This pooling procedure should, therefore, be avoided because it leads to a large and variable effect on the dynamics of diffusive soil P. We recommend determining the kinetic Freundlich parameterization on individual soil samples.

Calibration of Rapid Test and Sampling for Measurement of Phosphorus Nutrition Status in Maize

Fertilization management of intensive crops requires control methods to adjust the mineral fertilizer input to the actual needs of the crop. This paper outlines a method to estimate phosphorus (P) nutrient status of maize (Zea mays hybrid c.v. Volga) by analyzing the plant sap extracted from vascular tissue of the stem base. Phosphate concentration is analyzed with phosphomolybdic complex reaction, using test strip with a small autonomous pocket reflectometer. The experimental conditions for maize diagnosis P nutrition are: stem base harvested between 10 and 12 a.m., phosphate concentration measured in sap squeezed by a hand hydraulic press. The critical level of P nutrition should be between 20 and 25 ppm. The validation of this method by comparison of colorimetry and chromatography measurement on different plant P status indicates a reverse correlation between weight production and P lower stem sap concentration.