A gronomy J our n al • Volume 102 , I s sue 2 • 2 010 707 O ver the last four decades, N fertilization has been an essential tool for increasing crop yield and quality, especially for cereals, and for ensuring maximum economic yield (Hirel et al., 2001). However, the energetic cost of synthesizing N fertilizers is very high (Smil, 2001), and N fertilization oft en represents the most expensive energy input in cereal-based cropping systems (Crews and Peoples, 2004). Moreover, because of its high mobility in the soil-plant-atmosphere system, N greatly contributes to agriculture-related pollution through leaching, volatilization, and denitrifi cation (Drinkwater et al., 1998;Limaux et al., 1999). Indeed, it has been estimated that oft en 50% or less of the N fertilizer applied to soil is recovered by cereals and that this percentage decreases as the N fertilizer rate increases (Foulkes et al., 1998;Raun and Johnson, 1999;Blankenau et al., 2002).Developing cropping systems and management practices that improve the ability of crops to absorb N could minimize the potential for N losses. Nitrogen use effi ciency is generally defi ned as the grain yield produced per unit of N available from the soil and fertilizer (Moll et al., 1982); it is the product of two physiological factors: (i) N uptake effi ciency (NUpE, defi ned as the amount of N uptake by the crop per unit of N available to the crop) and (ii) N utilization effi ciency (NUtE, defi ned as the grain yield per unit of N uptake by the crop). With regard to management practices, the choice of plant variety is particularly important; in fact, several studies have shown that many crop species have genetic variability for NUE (Fageria et al., 2008) and that the use of the best-adapted genotype can contribute to improved effi ciency in how cereal crops acquire and use soil N or fertilizer N. Foulkes et al. (1998) found that modern wheat varieties were less effi cient at recovering soil N than older varieties, which suggests that old varieties may be the best choice for low input and organic growing systems. In contrast, other researchers (Le Gouis et al., 2000;Brancourt-Hulmel et al., 2003;Guarda et al., 2004) have found that NUpE and NUE have increased with the introduction of improved varieties, and that modern varieties give the best results even under limited N availability. Sylvester-Bradley and Kindred (2009) state that wheat breeding has greatly increased grain yield and that this improvement has been associated with an increase in optimum N rate; the increase in N fertilizer use has counter-acted the improvement in grain yield, resulting in a static NUE at optimum N levels.Th e varieties suited for low input or organic systems should combine high N use effi ciency with superior competitive ability against weeds. To this end, it is also necessary to take into account the fact that N application can signifi cantly aff ect the competitive interactions between the crop and the weeds and that N application oft en increases the competitiveness of weeds more than tha...

Much research around the world has compared the performance of cereals grown under conventional and conservation tillage systems; however, relatively few long‐term experiments have been conducted in Mediterranean areas, and little attention has been given to interactions among tillage techniques and other system components across space and time. In this study, we investigated the effects of the long‐term (18‐yr) use of three tillage techniques (conventional tillage, CT; reduced tillage, RT; and no‐till, NT) on wheat (Triticum durum Desf.) grain yield and quality within three crop sequences: continuous wheat, faba bean (Vicia faba L.)–wheat, and berseem clover (Trifolium alexandrinum L.)–wheat. In addition, we investigated the effects of climatic variability on the treatments and evaluated whether cumulative effects occurred from continuous treatment. On average, NT resulted in a grain yield advantage over CT when water stress was high and, conversely, a disadvantage when water stress was low. The effect of the tillage system on grain yield varied by crop sequence. Grain yield differences between NT and CT when wheat was grown after faba bean or berseem clover were explained primarily by climatic variability without a cumulative effect over time. In contrast, in continuous wheat, NT resulted in a progressive decrease in grain yield compared with CT. On average, wheat grain protein content varied significantly by tillage system (CT > RT > NT). This suggests that fertilizer N requirements increase with NT compared with CT because of changes in N cycling that lead to a reduction in plant‐available soil N.

Arbuscular mycorrhizal fungi (AMF) can play a key role in natural and agricultural ecosystems affecting plant nutrition, soil biological activity and modifying the availability of nutrients by plants. This research aimed at expanding the knowledge of the role played by AMF in the uptake of macro- and micronutrients and N transfer (using a 15N stem-labelling method) in a faba bean/wheat intercropping system. It also investigates the role of AMF in biological N fixation (using the natural isotopic abundance method) in faba bean grown in pure stand and in mixture. Finally, it examines the role of AMF in driving competition and facilitation between faba bean and wheat. Durum wheat and faba bean were grown in pots (five pots per treatment) as sole crops or in mixture in the presence or absence of AMF. Root colonisation by AMF was greater in faba bean than in wheat and increased when species were mixed compared to pure stand (particularly for faba bean). Mycorrhizal symbiosis positively influenced root biomass, specific root length, and root density and increased the uptake of P, Fe, and Zn in wheat (both in pure stand and in mixture) but not in faba bean. Furthermore, AMF symbiosis increased the percentage of N derived from the atmosphere in the total N biomass of faba bean grown in mixture (+20%) but not in pure stand. Nitrogen transfer from faba bean to wheat was low (2.5–3.0 mg pot-1); inoculation with AMF increased N transfer by 20%. Overall, in terms of above- and belowground growth and uptake of nutrients, mycorrhization favoured the stronger competitor in the mixture (wheat) without negatively affecting the companion species (faba bean). Results of this study confirm the role of AMF in driving biological interactions among neighbouring plants.

Knowledge of the effects of agricultural practices on weed seedbank dynamics is essential for predicting future problems in weed management. This article reports data relative to weed seedbank structure after 18 years of continuous application of conventional tillage (CT, based on mouldboard ploughing) or no tillage (NT) within three crop sequences (continuous wheat, WW; wheat-faba bean, WF; and wheat-berseem clover, WB). Tillage system did not affect the size of the total weed seedbank, but altered both its composition and the distribution of seeds within the soil profile. In particular, the adoption of CT favoured some species (mainly Polygonum aviculare), whereas the continuous use of NT favoured other species (Papaver rhoeas, Phalaris spp. and Lactuca serriola). The effects of tillage system on weed seedbank size and composition were less pronounced in the WB cropping system than in either the WW or WF. Compared with WF and WB, WW resulted in an increase in total weed seedbank density (about 16 000 seedlings m-2 in WW, compared with 10 000 and 6000 seedlings m-2 in WF and WB, respectively) and a reduction in weed diversity, with a strong increase in some species (e.g. Polygonum aviculare). Our results for the effect of NT application on weed seedbank size and composition suggest that farmers should only apply such a conservative technique within an appropriate crop sequence

Arbuscular mycorrhizal fungi (AMF) have a major impact on plant nutrition, defence against pathogens, a plant’s reaction to stressful environments, soil fertility, and a plant’s relationship with other microorganisms. Such effects imply a broad reprogramming of the plant’s metabolic activity. However, little information is available regarding the role of AMF and their relation to other soil plant growth—promoting microorganisms in the plant metabolome, especially under realistic field conditions. In the present experiment, we evaluated the effects of inoculation with AMF, either alone or in combination with plant growth–promoting rhizobacteria (PGPR), on the metabolome and changes in metabolic pathways in the roots of durum wheat (Triticum durum Desf.) grown under N-limited agronomic conditions in a P-rich environment. These two treatments were compared to infection by the natural AMF population (NAT). Soil inoculation with AMF almost doubled wheat root colonization by AMF and decreased the root concentrations of most compounds in all metabolic pathways, especially amino acids (AA) and saturated fatty acids, whereas inoculation with AMF+PGPR increased the concentrations of such compounds compared to inoculation with AMF alone. Enrichment metabolomics analyses showed that AA metabolic pathways were mostly changed by the treatments, with reduced amination activity in roots most likely due to a shift from the biosynthesis of common AA to γ-amino butyric acid. The root metabolome differed between AMF and NAT but not AMF+PGPR and AMF or NAT. Because the PGPR used were potent mineralisers, and AMF can retain most nitrogen (N) taken as organic compounds for their own growth, it is likely that this result was due to an increased concentration of mineral N in soil inoculated with AMF+PGPR compared to AMF alone.