Joëlle Fustec

Nitrogen rhizodeposition of legumes. A review

Because nitrogen is one of the major elements limiting growth of plants in agrosystems, large amounts of N fertilisers have been used in the second half of the twentieth century. Chemical fertilisers have contributed to increasing crop yields and food supply, but they have induced environmental damage such as nitrate pollution and wasting fossil fuel. The use of legumes grown in rotations or intercropping is now regarded as an alternative and sustainable way of introducing N into lower input agrosystems. Here we review agricultural practices, measurement methods and biological pathways involved in N cycling. We show that plant roots interact intimately with soil microflora to convert the most abundant but relatively inert form of N, atmospheric N2, into biological substrates available for growth of other plants, through two consecutive processes; namely, N2 fixation and N rhizodeposition. In intercropping, companion plants benefit from biological fixation by legumes and subsequent transfer of N from legumes to non-legumes. This transfer from legumes to the release of N compounds by legume roots, a process named rhizodeposition, then the uptake by the companion crop. The two main rhizodeposition pathways are (i) decomposition and decay of nodules and root cells, and (ii) exudation of soluble N compounds by plant roots. The contribution of root N and rhizodeposited N to the soil-N pool is difficult to measure, particularly in the field. Firstly, root N is often underestimated because root recovery is problematic. Second, assessment of N rhizodeposition is challenging. Several 15N labelling methods have been performed for different legume species. Rhizodeposition of N, as a percentage of total plant N, varied from 4 to 71%. The high variability of the results illustrates the need for more studies of the environmental and genetic factors influencing the amount of N rhizodeposits released by legumes under field conditions.

Interspecific Competition for Soil N and its Interaction with N2 Fixation, Leaf Expansion and Crop Growth in Pea–Barley Intercrops

Field experiments were carried out during three successive years to study through a dynamic approach the competition for soil N and its interaction with N2 fixation, leaf expansion and crop growth in pea–barley intercrops. The intensity of competition for soil N varied between experiments according to soil N supply and plant densities. This study demonstrates the key role of competition for soil N which occurs early in the crop cycle and greatly influences the subsequent growth and final performance of both species. Relative yield values for grain yield and N accumulation increased with the intensity of competition for soil N. Barley competed strongly for soil N in the intercrop. Its competitive ability increased steadily during the vegetative phase and remained constant after the beginning of pea flowering. The period of strong competition for soil N (500–800 degree-days after sowing) also corresponded to the period of rapid growth in leaf area for both species and therefore an increasing N demand. For each species, the leaf area per plant at the beginning of pea flowering was well correlated with crop nitrogen status. Barley may meet its N needs more easily in intercrops (IC) and has greater leaf area per plant than in sole crops (SC). Barley having a greater soil N supply results in an even higher crop N status and greater competitive ability relative to pea in intercrop. Competition by barley for soil N increased the proportion of pea N derived from fixation. The nitrogen nutrition index (NNI) values of pea were close to 1 whatever the soil N availability in contrast to barley. However N2 fixation started later than soil N uptake of pea and barley and was low when barley was very competitive for soil N. Due to the time necessary for the progressive development and activity of nodules, N2 fixation could not completely satisfy N demand at the beginning of the crop cycle. The amount of N2 fixed per plant in intercrops was not only a response to soil N availability but was largely determined by pea growth and was greatly affected when barley was too competitive.

Nitrogen Rhizodeposition of Legumes

Because nitrogen is one of the major elements limiting growth of plants in agrosystems, large amounts of N fertilisers have been used in the second half of the twentieth century. Chemical fertilisers have contributed to increasing crop yields and food supply, but they have induced environmental damage such as nitrate pollution and wasting fossil fuel. The use of legumes grown in rotations or intercropping is now regarded as an alternative and sustainable way of introducing N into lower input agrosystems. Here we review agricultural practices, measurement methods and biological pathways involved in N cycling. We show that plant roots interact intimately with soil microflora to convert the most abundant but relatively inert form of N, atmospheric N2, into biological substrates available for growth of other plants, through two consecutive processes; namely, N2 fixation and N rhizodeposition. In intercropping, companion plants benefit from biological fixation by legumes and subsequent transfer of N from legumes to non-legumes. This transfer from legumes to the release of N compounds by legume roots, a process named rhizodeposition, then the uptake by the companion crop. The two main rhizodeposition pathways are (i) decomposition and decay of nodules and root cells, and (ii) exudation of soluble N compounds by plant roots. The contribution of root N and rhizodeposited N to the soil-N pool is difficult to measure, particularly in the field. Firstly, root N is often underestimated because root recovery is problematic. Second, assessment of N rhizodeposition is challenging. Several 15N labelling methods have been performed for different legume species. Rhizodeposition of N, as a percentage of total plant N, varied from 4 to 71%. The high variability of the results illustrates the need for more studies of the environmental and genetic factors influencing the amount of N rhizodeposits released by legumes under field conditions.

Beaver lodge location on the upstream Loire River

In the part of the Loire River recently colonized by Eurasian beavers, we compared habitat characteristics among sites with lodges, sites with cut trees and sites without beaver. The absence of sandbank and canopy cover (by 10-15-m tall trees, by tall Salicaceae, and by bushy Salicaceae) appeared as good predictors for lodge settling. Based on this model, the number of proper lodge sites was estimated for the next downstream 36 kilometers stretch. The number of favourable sites decreases as anthropization increases.

Traits affecting early season nitrogen uptake in nine legume species

Legume crops are known to have low soil N uptake early in their life cycle, which can weaken their ability to compete with other species, such as weeds or other crops in intercropping systems. However, there is limited knowledge on the main traits involved in soil N uptake during early growth and for a range of species. The objective of this research was to identify the main traits explaining the variability among legume species in soil N uptake and to study the effect of the soil mineral N supply on the legume strategy for the use of available N sources during early growth. Nine legume species were grown in rhizotrons with or without N supply. Root expansion, shoot and root biomass, nodule establishment, N2 fixation and mineral soil N uptake were measured. A large interspecific variability was observed for all traits affecting soil N uptake. Root lateral expansion and early biomass in relation to seed mass were the major traits influencing soil N uptake regardless of the level of soil N availability. Fenugreek, lentil, alfalfa, and common vetch could be considered weak competitors for soil N due to their low plant biomass and low lateral root expansion. Conversely, peanut, pea, chickpea and soybean had a greater soil N uptake. Faba bean was separated from other species having a higher nodule biomass, a higher N2 fixation and a lower seed reserve depletion. Faba bean was able to simultaneously fix N2 and take up soil N. This work has identified traits of seed mass, shoot and root biomass, root lateral expansion, N2 fixation and seed reserve depletion that allowing classification of legume species regarding their soil N uptake ability during early growth.

Selection of nesting habitat by Montagu's Harriers Circus pygargus and Hen Harriers Circus cyaneus in managed heaths

Capsule Montagus Harriers avoided tree plantations and selected plots with dense heath of intermediate height, while sympatric Hen Harriers preferred afforested or taller plots of heath. Aims To investigate relationships between vegetation structure and nest selection by Montagus and Hen Harriers. Methods We defined three vegetation strata: upper, intermediate and lower. Height and cover of plant species in each stratum were recorded in a 9-m2 area centred on nests or unused random points. Statistical analysis was performed to reveal predictors for classifying plots (i) with a Montagus Harrier nest, (ii) with a Hen Harrier nest and (iii) without a nest. Results Montagus Harriers selected places where tree cover was less than 25%, the maximum height of the upper stratum was 1.87 m, and the cover of the intermediate stratum was greater than 75%. Hen Harriers nested either in plots where the upper stratum was higher than 1.87 m, or where tree coverage was greater than 25%. Conclusions To encourage the breeding of Montagus Harriers, the planting of pine should be avoided and the heath should be rejuvenated before it reaches 1.80 m in height. For Hen Harriers, certain plots of heath higher than 2 m should be kept. In agricultural landscapes, patches of managed shrubland or heathland could help conservation of both species.

Utilisation of woody plants for lodge construction by European beaver (Castor fiber) in the Loire valley, France

Abstract Selection and use of woody plants in beaver (Castor fiber) lodge construction on the Loire River are poorly understood. We investigated woody species used by beaver for construction and analysed the efficiency of the tree-cutting technique used. We observed that beavers used branches of Salicaceae of large diameter (approx. 4.5 cm) to build the frames of their lodges, even in sites where these were scarce. They then cut small branches of the more abundant species in any given site (Salix fragilis, S. alba, Fraxinus angustifolia, Ulmus minor) to cover the lodge. The shapes of cut tree sections depended on diameter at the cut, and orientation was related to bank slope. Most cut trees (55%) fell into the water, where they were more easily exploited than those that fell in other directions. We conclude that beavers of the Loire River need Salicaceae for construction, even where they are scarce. The tree-cutting technique is efficient for exploiting fallen trees, but leads to changes in plant morphology. Repeated use by beaver creates shrubby re-growth that is too small for use in lodge construction.