Yves Crozat

Mixing plant species in cropping systems: concepts, tools and models. A review

The evolution of natural ecosystems is controled by a high level of biodiversity, In sharp contrast, intensive agricultural systems involve monocultures associated with high input of chemical fertilisers and pesticides. Intensive agricultural systems have clearly negative impacts on soil and water quality and on biodiversity conservation. Alternatively, cropping systems based on carefully designed species mixtures reveal many potential advantages under various conditions, both in temperate and tropical agriculture. This article reviews those potential advantages by addressing the reasons for mixing plant species; the concepts and tools required for understanding and designing cropping systems with mixed species; and the ways of simulating multispecies cropping systems with models. Multispecies systems are diverse and may include annual and perennial crops on a gradient of complexity from 2 to n species. A literature survey shows potential advantages such as (1) higher overall productivity, (2) better control of pests and diseases, (3) enhanced ecological services and (4) greater economic profitability. Agronomic and ecological conceptual frameworks are examined for a clearer understanding of cropping systems, including the concepts of competition and facilitation, above- and belowground interactions and the types of biological interactions between species that enable better pest management in the system. After a review of existing models, future directions in modelling plant mixtures are proposed. We conclude on the need to enhance agricultural research on these multispecies systems, combining both agronomic and ecological concepts and tools.

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.

Methodological Progress in On-Farm Regional Agronomic Diagnosis: A Review

The development of sustainable cropping systems is a key priority for agronomists and crop scientists. A first step involves understanding the relationship between cropping system performance and farmers’ practices. To complete this step, a methodological framework entitled Regional Agronomic Diagnosis (RAD) has been developed. During the last ten years, the scope of the regional agronomic diagnosis has been enlarged to include several factors describing crop quality and the environmental impact of cropping systems. Regional agronomic diagnosis has led to several major advances such as (1) the assessment of the effect of preceding crop and soil structure on malting barley quality in France and (2) the assessment of the effects of ploughing, nematicide use and fertilisers on soil properties in intensive banana plantations in the West Indies. Improvements have also been gained in methodology, particularly by the selection of indicators for assessing the effects of crop management, soil and weather conditions, and data analysis. Finally, regional agronomic diagnosis has been integrated into more general approaches of agricultural development. We review here this methodological progress.

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.

Yield performance and seed production pattern of field-grown pea and soybean in relation to N nutrition

Abstract The aim of this study was to compare the yield and seed production of well irrigated indeterminate pea and soybean in relation to N nutrition and plant N status at the beginning of seed-filling (BSF stage). Plant N status was characterized by a N nutrition index (NNI). A large range in nitrogen accumulation and dinitrogen fixation was obtained by means of N fertilizer application (pea and soybean), inoculation (soybean) and increasing soil compaction (pea) carried out at various sites and in different years. Pea displayed a higher dinitrogen fixation ability than soybean and accumulated a larger proportion of nitrogen prior to the BSF stage. The nitrogen requirement for the production of 1 ton of soybean seed was twice that for pea and its maximum grain yield was half that of pea. In response to increasing NNI at the BSF stage, the two species showed differences in seed establishment and seedfilling. These differences in yield response to the treatments are discussed in relation to the seed composition and N nutrition of both species.

Use of the fluorescent antibody technique to characterize equilibrium survival concentrations of Bradyrhizobium japonicum strains in soil

SummaryThe ability of Bradyrhizobium strains to survive saprophytically in soil was studied by means of fluorescent antibodies (FA). It was found that the recovery rate may be considered a constant value although the limit of detection by the FA technique is approximately 103 bacteria g−1 soil. By studying the survival kinetics of B. japonicum strains introduced into soils, we observed that whichever soil-strain combination was tested in a given soil during the incubation all the different populations of a strain reached the same survival balance level, generally about 103–104Bradyrhizobium g−1 soil. When we reintroduced strains into a soil containing rhizobia of the same specificity, the new inhabitants reached the same equilibrium level as that of the initial population. The balance threshold level does not appear to be a very sensitive way to classify, strains on their saprophytic ability. We suggest that survival kinetics should be characterized by the rate at which the population reaches equilibrium.

Early remobilization : a possible source of error in the ureide assay method for N2 fixation measurement by early maturing soybean

Abstract The ureide assay method has been found to be suitable for soybean (Glycine max) to assess nitrogen ixation in a wide range of genotypes and Bradyrhizobium strains. The relationships between the relative abundance of ureide-N in xylem-sap and the proportion of plant N derived from nitrogen fixation were studied for early maturing soybeans grown under greenhouse and field conditions. Plants grown under greenhouse conditions were either supplemented with N-free nutrients or supplied with different levels of 15N-nitrate. In the field, soybeans were grown with or without inoculation, in the presence or absence of nitrogen fertilizer. Treatments were applied in order to obtain a wide range of nitrogen nutrition levels. The effect of the source and level of nitrogen nutrition on growth and nitrogen accumulation in plants, proportion of fixed nitrogen in the xylem-sap and in the shoots, and the proportions of N-compounds in the xylem-sap was determined. The greenhouse study demonstrated that the relative...

Modelling of survival kinetics of some Bradyrhizobium japonicum strains in soils

SummaryThe existence of a balance survival rate of different B. japonicum strains in soils has been well established. Balance levels (M) were shown to be independent of inoculum size but differences between strains in a soil could not be discussed partly because of the instability of the recoverys efficiency in the area of the plateau. In this paper, a dynamic model, the Gompertz equation, is fitted to the data, to estimate the rate (A) at which equilibrium is reached. For the three strains in the three soils previously studied, this rate is also independent of inoculum size and the soil-strain interaction is highly significant. Some data from the literature are also well fitted by the model. We propose to characterize the saprophytic potential of a strain with the pair of parameters A-M. An optimal experimental procedure is proposed to obtain the most precise estimates for these parameters. Their biological significance is discussed, but the fact that they are not dependent upon inoculation level is of basic importance for agronomic practice.