Guénaëlle Corre-Hellou

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.

Intercropping with pulses to concentrate nitrogen and sulphur in wheat

SUMMARY The effects of intercropping wheat with faba bean (Denmark, Germany, Italy and UK) and wheat with pea (France), in additive and replacement designs on grain nitrogen and sulphur concentrations were studied in field experiments in the 2002/03, 2003/04 and 2004/05 growing seasons. Inter- cropping wheat with grain legumes regularly increased the nitrogen concentration of the cereal grain, irrespective of design or location. Sulphur concentration of the cereal was also increased by inter- cropping, but less regularly and to a lesser extent compared with effects on nitrogen concentration. Nitrogen concentration (g/kg) in wheat additively intercropped with faba bean was increased by 8 % across all sites (weighted for inverse of variance), but sulphur concentration was only increased by 4 %, so N :S ratio was also increased by 4 %. Intercropping wheat with grain legumes increased sodium dodecyl sulphate (SDS)-sedimentation volume. The effect of intercropping on wheat nitrogen concentration was greatest when intercropping had the most deleterious effect on wheat yield and the least deleterious effect on pulse yield. Over all sites and seasons, and irrespective of whether the design was additive or replacement, increases in crude protein concentration in the wheat of 10 g/kg by intercropping with faba bean were associated with 25-30 % yield reduction of the wheat, compared with sole-cropped wheat. It was concluded that the increase in protein concentration of wheat grain in intercrops could be of economic benefit when selling wheat for breadmaking, but only if the bean crop was also marketed effectively.

Assessment of Root System Dynamics of Species Grown in Mixtures under Field Conditions using Herbicide Injection and 15N Natural Abundance Methods: A Case Study with Pea, Barley and Mustard

Two methods were developed and used to study the root system dynamics of two species grown together or separately under field conditions. The first method, based on herbicide injection at different soil depths, was used to determine the rooting depth penetration rate of each species in pea–barley and pea–mustard mixtures. The roots absorbed the herbicide when they reached the treated zone leading to visible symptoms on the leaves which could be readily monitored. The second method used differences in 15N natural abundance and N concentration between legume and non-legume species to quantify the contribution of each species to root biomass of a pea–barley mixture. Each contribution was calculated using 15N abundance and N concentration of root mixtures and of subsamples of roots of individual species within mixtures. Both methods can indeed be used to distinguish roots of species in mixtures and thus to study belowground competition between associated species. The use of these methods demonstrated species differences in root system dynamics between species but also significant effects of interactions between species in mixtures. The rooting depth penetration rate was mainly species specific whereas root biomass was dependant on plant growth, allocation of dry matter between shoot and root components and growth factors such as N fertilization. Root biomass of each species may vary therefore with the level of competition between species.

Pea–barley intercropping and short-term subsequent crop effects across European organic cropping conditions

Grain legumes are known to increase the soil mineral nitrogen (N) content, reduce the infection pressure of soil borne pathogens, and hence enhance subsequent cereals yields. Replicated field experiments were performed throughout W. Europe (Denmark, United Kingdom, France, Germany and Italy) to asses the effect of intercropping pea and barley on the N supply to subsequent wheat in organic cropping systems. Pea and barley were grown either as sole crops at the recommended plant density (P100 and B100, respectively) or in replacement (P50B50) or additive (P100B50) intercropping designs. In the replacement design the total relative plant density is kept constant, while the additive design uses the optimal sole crop density for pea supplementing with ‘extra’ barley plants. The pea and barley crops were followed by winter wheat with and without N application. Additional experiments in Denmark and the United Kingdom included subsequent spring wheat with grass-clover as catch crops. The experiment was repeated over the three cropping seasons of 2003, 2004 and 2005. Irrespective of sites and intercrop design pea–barley intercropping improved the plant resource utilization (water, light, nutrients) to grain N yield with 25–30% using the Land Equivalent ratio. In terms of absolute quantities, sole cropped pea accumulated more N in the grains as compared to the additive design followed by the replacement design and then sole cropped barley. The post harvest soil mineral N content was unaffected by the preceding crops. Under the following winter wheat, the lowest mineral N content was generally found in early spring. Variation in soil mineral N content under the winter wheat between sites and seasons indicated a greater influence of regional climatic conditions and long-term cropping history than annual preceding crop and residue quality. Just as with the soil mineral N, the subsequent crop response to preceding crop was negligible. Soil N balances showed general negative values in the 2-year period, indicating depletion of N independent of preceding crop and cropping strategy. It is recommended to develop more rotational approaches to determine subsequent crop effects in organic cropping systems, since preceding crop effects, especially when including legumes, can occur over several years of cropping.

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.

Eco-functional Intensification by Cereal-Grain Legume Intercropping in Organic Farming Systems for Increased Yields, Reduced Weeds and Improved Grain Protein Concentration

Intercropping, i.e., simultaneously growing two (or more) species in the same field for a significant period of time but without necessarily concomitant sowing or harvest, is a practice aimed at eco-functional intensification.

Differences for traits associated with early N acquisition in a grain legume and early complementarity in grain legume–triticale mixtures

We grew a grain legume in rhizotrons, alone and mixed with a cereal. There were differences between some of the genotypes of the grain legume but they all demonstrated slow early growth and low ability to absorb mineral soil nitrogen compared to the cereal. The two species were complementary for nitrogen acquisition in the first 2 months of growth. Mixtures showed a higher total soil mineral nitrogen uptake than pure grain legume. Complementarity allowed higher cereal biomass production than in pure cereal, without compromising grain legume growth.