Sylvain Diquélou

Leaf mineral nutrient remobilization during leaf senescence and modulation by nutrient deficiency

Higher plants have to cope with fluctuating mineral resource availability. However, strategies such as stimulation of root growth, increased transporter activities, and nutrient storage and remobilization have been mostly studied for only a few macronutrients. Leaves of cultivated crops (Zea mays, Brassica napus, Pisum sativum, Triticum aestivum, Hordeum vulgare) and tree species (Quercus robur, Populus nigra, Alnus glutinosa) grown under field conditions were harvested regularly during their life span and analyzed to evaluate the net mobilization of 13 nutrients during leaf senescence. While N was remobilized in all plant species with different efficiencies ranging from 40% (maize) to 90% (wheat), other macronutrients (K–P–S–Mg) were mobilized in most species. Ca and Mn, usually considered as having low phloem mobility were remobilized from leaves in wheat and barley. Leaf content of Cu–Mo–Ni–B–Fe–Zn decreased in some species, as a result of remobilization. Overall, wheat, barley and oak appeared to be the most efficient at remobilization while poplar and maize were the least efficient. Further experiments were performed with rapeseed plants subjected to individual nutrient deficiencies. Compared to field conditions, remobilization from leaves was similar (N–S–Cu) or increased by nutrient deficiency (K–P–Mg) while nutrient deficiency had no effect on Mo–Zn–B–Ca–Mn, which seemed to be non-mobile during leaf senescence under field conditions. However, Ca and Mn were largely mobilized from roots (-97 and -86% of their initial root contents, respectively) to shoots. Differences in remobilization between species and between nutrients are then discussed in relation to a range of putative mechanisms.

A study of 15N transfer between legumes and grasses

The overuse of classical N fertilisers contributes substantially to environmental degradation by pollution of groundwater by nitrates. This leaching of N in waters is also an economic flaw for farmers because only a part of the fertiliser is used by the plants. Here, systems involving mixtures of legumes and grasses represent a sustainable alternative because legumes can fix atmospheric N2 using symbiotic microbes. N transfer in those mixtures has been thoroughly investigated but little is known concerning the effect of N fertiliser on N transfer between N-fixing legumes and companion grasses. In white clover (Trifolium repens L.) — perennial ryegrass (Lolium perenne L.) associations, N is transferred mostly through rhizodeposition into the soil by clover followed by re-uptake by ryegrass. Rhizodeposition of N occurs through senescence and decomposition of legume tissue or through exudation of N compounds by living cells. Ammonium and amino acids are the main compounds exuded and their exudation is thought to occur by passive diffusion attributed to a concentration gradient from root to soil. In this study, we test the hypothesis that greater N transfer from clover to grass, as seen in N-rich soils or nutrient solutions, is due to greater N rhizodeposition brought about by higher ammonium and amino acid content of roots. The relations between N input, root N content, N net exudation and N transfer between legumes and grasses were investigated using 15N by growing white clover and perennial ryegrass with increasing N application in axenic microlysimeters or in pots. Ammonium and amino acid concentrations were measured in root tissues, in root bathing solutions and in soils. We found that mineral N application strongly reduced atmospheric N fixation by clover, from 3.0 to 0.9 mg per plant, and root amino acid content, from 164 to 49 nmoles per g dry weight, but had no effect on ammonium and amino acid concentrations in sterile exudates, showing for the first time that amino acid net exudation is independent of root content. In contrast, ammonium and amino acid concentrations in clover soils increased with N fixation, showing the link between N fixation and N rhizodeposition in soils. Nitrate application increased ryegrass root growth by 7–8 times, and transfer of N between clover and ryegrass (by 3 times). It is concluded that N fertiliser does not modify N exudation but decreases N fixation and ammonium rhizodeposition in soil by clover. N fertiliser increases N transfer between clover and ryegrass by increasing soil exploration by ryegrass and giving a better access to different available N sources, including the N compounds exuded from clover.

Using plant traits to explain plant-microbe relationships involved in nitrogen acquisition.

It has long been recognized that plant species and soil microorganisms. are tightly linked, but understanding how different species vary in their effects on soil is currently limited. In this study, we identified those. plant characteristics (identity, specific functional traits, or resource acquisition strategy) that were the best predictors of nitrification and denitrification processes. Ten plant populations representing eight species collected from three European grassland sites were chosen for their contrasting plant trait values and resource acquisition strategies. For each individual plant, leaf and root traits and the associated potential microbial activities (i.e., potential denitrification rate [DEA], maximal nitrification rate [NEA], and NH4+ affinity of the microbial community [NHScom]) were measured at two fertilization levels under controlled growth conditions. Plant traits were powerful predictors of plant-microbe interactions, but relevant plant traits differed in relation to the microbial function studied. Whereas denitrification was linked to the relative growth rate of plants, nitrification was strongly correlated to root trait characteristics (specific root length, root nitrogen concentration, and plant affinity for NH4+) linked to plant N cycling. The leaf economics spectrum (LES) that commonly serves as an indicator of resource acquisition strategies was not correlated to microbial activity. These results suggest that the LES alone is not a good predictor of microbial activity, whereas root traits appeared critical in understanding plant-microbe interactions.

Vegetation ecology meets ecosystem science: Permanent grasslands as a functional biogeography case study

The effect of biodiversity on ecosystem functioning has been widely acknowledged, and the importance of the functional roles of species, as well as their diversity, in the control of ecosystem processes has been emphasised recently. However, bridging biodiversity and ecosystem science to address issues at a biogeographic scale is still in its infancy. Bridging this gap is the primary goal of the emerging field of functional biogeography. While the rise of Big Data has catalysed functional biogeography studies in recent years, comprehensive evidence remains scarce. Here, we present the rationale and the first results of a country-wide initiative focused on the C3 permanent grasslands. We aimed to collate, integrate and process large databases of vegetation relevés, plant traits and environmental layers to provide a country-wide assessment of ecosystem properties and services which can be used to improve regional models of climate and land use changes. We outline the theoretical background, data availability, and ecoinformatics challenges associated with the approach and its feasibility. We provide a case study of upscaling of leaf dry matter content averaged at ecosystem level and country-wide predictions of forage digestibility. Our framework sets milestones for further hypothesis testing in functional biogeography and earth system modelling.

Assessment of sulphur deficiency in commercial oilseed rape crops from plant analysis

Sulphur (S) is one of the six main macroelements required to sustain the growth of plants. Sources include soil, fertilizer and atmospheric deposition, which has been reduced by 85% over the last three decades. Risks of S deficiencies are now recognized in high S-demanding species such as Brassica napus L. With the aims of evaluating the risk of excessive or insufficient fertilization and identifying robust relationships that may be used as plant S status indicators, 57 commercial crops of oilseed rape were selected among contrasting soils and along a rainfall gradient that may affect soil S availability. Cultivation practices were investigated and the S and nitrogen (N) concentrations of soil, senescing leaves, stems and seeds were analysed. Despite an excessive organic N supply and large variation in S supply (from 0 to 112 kg S/ha), principal component analysis using 43 parameters indicated that seed yield was poorly related to N and S fertilization rates. While the N and protein-N concentrations in seeds were inversely related to oil and glucosinolate concentrations, they were linked to S and sulphate (SO42-) accumulation in the seeds. Sulphate concentrations in senescing leaves, stems or seeds could be deduced from total S concentrations, as they were positively and highly correlated. Sulphate accounted for on average 0 69 of total S in senescing leaves with minimum and maximumvalues of 0 007 and 0 94, which revealed conditions of limited and excess supply of S, respectively. This high variation of SO42- concentration in leaves can be interpreted as the result of its mobilization triggered by S deficiency, but cannot be used alone as an indicator of plant S status. A comparison with plants grown in controlled conditions under different S supplies suggests that the intensity of S starvation affects N metabolism, leading to NO3-(nitrate) accumulation. It is further suggested that dual evaluation of SO42- and NO3- concentrations in senescing leaves could be used at the vegetative stage as a field indicator to adjust S fertilization.

Functional plasticity of Trifolium repens L. in response to sulphur and nitrogen availability

Recent control of atmospheric SO2 pollution is leading to important soil sulphur impoverishment. Plasticity could be a mechanism allowing species to adapt to this rapid global change. Trifolium repens L. is a key grassland species whose performances in community are strongly linked to nitrogen availability. Plasticity of three white clover lines contrasting in their ability to use atmospheric N2 or soil N was assessed by analysing a set of functional traits along a gradient of nitrogen and sulphur fertilisation applied on a poor soil. White clover traits showed high morphological and physiological plasticity. Nitrogen appeared to be the most limiting factor for the VLF (Very Low Fixation) line. S was the element that modulated the most traits for the nitrogen fixing lines NNU (Normal Nitrate Uptake) and LNU (Low Nitrate Uptake). As expected, N fertilisation inhibited white clover fixation, but we also observed that N2 fixation was enhanced when S was added. S fertilisation increased nodule length as well as the proportion of nodules containing leghaemoglobin. S fertilisation, with a direct effect and an indirect effect through N2 fixation, increases white clover performances particularly with regards to photosynthesis and potential vegetative reproduction. The important plasticity in response to S availability should allow it to adapt to a large range of abiotic conditions, but its sensitivity to S nutrition would be a disadvantage for competition in a situation of soil sulphur impoverishment. In contrast, S fertilisation could help maintain this species when nitrogen status is against it.

Effects of nitrogen and sulphur gradients on plant competition, N and S use efficiencies and species abundance in a grassland plant mixture

Sulphur (S) depletion of grassland soils has occurred in Europe for many decades. This is known to promote a decrease in ecosystem productivity and is suspected to alter plant community structure. Considering the strong links between nitrogen (N) and S metabolism in plants, these effects should depend on N availability. We tested this hypothesis in a pot experiment, considering a four grassland species plant mixture (three Poaceæs: Lolium perenne, Agrostis capillaris and Poa pratensis and one Fabaceæ: Trifolium repens), and submitted it to a double N and S gradient. We used labelled 15N-fertilizer and 34S-fertilizer in order to determine both nutrient use efficiencies by each species and to analyze the influence of competition for these nutrients on plant mixture dynamics. We compared species relative physiological performance (RPP) in the monoculture and their relative ecological performance (REP) in the mixture of the four species. We analysed gradient effects at establishment and at regrowth after cutting. At establishment, grass production and S use efficiency increased along the N gradient. The S gradient slightly favoured the dominance of L. perenne, increased A. capillaris production and enhanced N use efficiency of both species. At regrowth, increased S promoted more significant effects, enhancing T. repens performance in increasing its N2 fixation ability and maintaining this at high N. It also induced a change in grass species relative performance (dry matter production and N use efficiency) at high N, enhancing that of L. perenne and decreasing that of A. capillaris. At both establishment and regrowth, RPP did not reflect REP, meaning that species behave differently along the gradient when grown in mixture. Finally, the S gradient and the N gradient modulated relative plant species abundance. It appears that modulation of S availability could be used as a tool to drive grassland community structure.

How to improve the hygienic quality of forages for horse feeding

BACKGROUND Improving the hygienic quality of forages for horse nutrition seems to be a reasonable target for decreasing the prevalence of pulmonary diseases. The aim of the experiment was to study the effects of different agricultural practices on the main aero-allergens contained in forages, including breathable dust, fungi, mycotoxins and pollens. RESULTS Results showed that the late harvest of hay, a second crop or a haylage production provides a good alternative to increase hygienic quality by reducing fungi contamination and breathable dust content. Barn drying of hay, while having no effect on breathable dust, similarly reduced fungi contamination. In contrast, when hay was harvested at a lower dry mass content (750 g DM kg⁻¹ versus 850 g DM kg⁻¹), both breathable dust and fungi contaminations were increased, which could at least be reversed by adding propionic acid just before baling. Zearalenone was detected in different hays, and even in one case, in breathable dust. CONCLUSION Overall, our data suggest that different approaches can be used to increase forage hygienic quality for horse feeding and thus reduce their exposure to factors involved in equine pulmonary disease.

Rapeseed-legume intercrops: plant growth and nitrogen balance in early stages of growth and development

In this study we tested whether legumes can improve the growth and N and S nutrition of rapeseed in an intercropping system and compared the effect of mixtures on legume N-fixation and soil N-resources. Rapeseed was cultivated in low N conditions in monocrops using one (R) or two plants (RR) per pot and in mixtures with lupine, clover or vetch. The R monocrop was the most relevant control, intraspecific competition inducing a significant growth delay resulting in a significantly lower leaf number, in RR monocrop compared to R and the three mixtures considered. Plant biomass, and the N and S contents of rapeseed grown in mixtures were the same than those measured in R monocrop. Compared to the monocrop, the proportion of N derived from the atmosphere was increased by 34, 140 and 290% in lupine, clover and vetch, respectively when intercropped with rapeseed. In mixture with clover and lupine, the soil N pool at harvest was higher than in other treatments, while N export by crop was constant. Legumes suffered from competition for soil S resulting in a decrease of 40% in their S content compared to the monocrop. Compared to rapeseeds grown in R monocrop and in mixture with lupine and vetch, rapeseed mixed with clover showed significantly higher SPAD values in old leaves. In our conditions, mixing legumes with rapeseed is relevant to reduce N fertilization and improve nutrition and growth of rapeseed.

Nitrogen transfer from Lupinus albus L., Trifolium incarnatum L. and Vicia sativa L. contribute differently to rapeseed (Brassica napus L.) nitrogen nutrition

Nitrogen (N) transfer is well documented in legume-cereal intercropping but this is less often reported for legume-Brassica intercrops even though Brassica crops require higher levels of N fertilizers. The present study was carried out to quantify N transfer from legumes (Lupinus albus L., Trifolium incarnatum L. or Vicia sativa L.) to rapeseed (Brassica napus L.) using the split-root 15N-labelling method. After three months we observed that legumes did not alter the growth of rapeseed. Vetch showed the lowest growth and demonstrated low 15N shoot to root translocation and no significant N transfer to rapeseed. In contrast, significant 15N enrichment was found in lupine and clover and 15N was transferred to the associated rapeseed plants (around 6 and 4 mg N plant−1, respectively), which contributed 2 to 3% of the rapeseed total N. Additionally, the data revealed that N2 fixation dominated the N nutrition in lupine despite the high N level provided in the donor compartment, suggesting a greater niche segregation between companion plants. Based on the results of this study we suggest that intercropping can be a relevant contributor to rapeseed N nutrition. Among the three legumes tested, clover and lupine seemed to be the best intercropping candidates.