Tree-based Intercropping Research Articles

Cumulative effects of tree-based intercropping on arbuscular mycorrhizal fungi

In tree-based intercropping system (agroforestry), the role of perennial trees in maintaining active populations and mycelial networks of arbuscular mycorrhizal fungi (AMF) is well documented. Agroforestry positively influences the AMF community, but complete studies regarding mycorrhization in such systems are scarce. The present study was conducted to assess the effect of tree introduction in agriculture fields on mycorrhization. In particular, we investigated the effect of trees on AMF colonization of intercrops and vice versa, the effect of canopy management of trees on their root colonization, and the cross-infectivity of AMF isolated from tree rhizosphere in intercrops and vice versa. The results of the field study suggest that in agroforestry systems, trees acted as AMF inoculum reservoir for intercrops, especially during the rainy season. Intercropping (Phaseolus mungo and Triticum aestivum in the rainy and winter seasons, respectively) increased mycorrhization, i.e., root colonization and spore population in the rhizosphere of Albizia procera and Eucalyptus tereticornis. Canopy management, i.e., shoot pruning, reduces root colonization in A. procera, Anogeissus pendula, Dalbergia sissoo, Hardwickia binata, and Tectona grandis, especially in April 2005 (late spring), but during subsequent periods, differences among the treatments were at par. Results from greenhouse suggest that AMF are nonspecific in their selection of host since species isolated from tree rhizosphere could colonize the roots of crops and vice versa.

Nitrogen and phosphorus economy of a legume tree-cereal intercropping system under controlled conditions

Considerable amounts of nitrogen (N) and phosphorus (P) fertilizers have been mis-used in agroecosystems, with profound alteration to the biogeochemical cycles of these two major nutrients. To reduce excess fertilizer use, plant-mediated nutrient supply through N2-fixation, transfer of fixed N and mobilization of soil P may be important processes for the nutrient economy of low-input tree-based intercropping systems. In this study, we quantified plant performance, P acquisition and belowground N transfer from the N2-fixing tree to the cereal crop under varying root contact intensity and P supplies. We cultivated Acacia senegal var senegal in pot-culture containing 90% sand and 10% vermiculite under 3 levels of exponentially supplied P. Acacia plants were then intercropped with durum wheat (Triticum turgidum durum) in the same pots with variable levels of adsorbed P or transplanted and intercropped with durum wheat in rhizoboxes excluding direct root contact on P-poor red Mediterranean soils. In pot-culture, wheat biomass and P content increased in relation to the P gradient. Strong isotopic evidence of belowground N transfer, based on the isotopic signature (δ15N) of tree foliage and wheat shoots, was systematically found under high P in pot-culture, with an average N transfer value of 14.0% of wheat total N after 21days of contact between the two species. In the rhizoboxes, we observed limitations on growth and P uptake of intercropped wheat due to competitive effects on soil resources and minimal evidence of belowground N transfer of N from acacia to wheat. In this intercrop, specifically in pot-culture, facilitation for N transfer from the legume tree to the crop showed to be effective especially when crop N uptake was increased (or stimulated) as occurred under high P conditions and when competition was low. Understanding these processes is important to the nutrient economy and appropriate management of legume-based agroforestry systems.

Tree‐Based Intercropping in Southern Ontario, Canada

Tree‐based intercropping (TBI) integrates tree production within annual grain cropping. The system is widely used in tropical regions, but is not common in temperate regions. This study evaluates the annualized return from TBI systems in southern Ontario, Canada against annual grain crop production. The TBI systems include hybrid poplar, Norway spruce, and red oak. The annualized return for all TBI systems is less than for annual cropping using the base prices; however, when tree prices are high and grain prices low the hybrid poplar TBI system has a higher return than annual cropping. Grants for planting trees, technologies to reduce the cost of establishing and maintaining trees, and improving the returns from tree production will be required for producers in temperate regions to adopt TBI systems.Un système de cultures intercalaires (SCI) intègre la production d’arbres dans la culture annuelle de céréales. Ce système est largement utilisé dans les régions tropicales, mais peu courant dans les régions tempérées. La présente étude évalue le rendement annualisé des SCI dans le sud de l’Ontario, au Canada, par rapport à celui de la production annuelle de céréales. Les SCI intègrent le peuplier hybride, l’épinette de Norvège et le chêne rouge. Le rendement annualisé des SCI est inférieur à celui des cultures annuelles si l’on utilise les prix de référence. Toutefois, lorsque les prix des arbres sont élevés et que les prix des céréales sont faibles, le SCI qui intègre le peuplier hybride obtient un rendement supérieur à celui des cultures annuelles. Pour que les agriculteurs des régions tempérées adoptent les SCI, il faudra offrir des subventions à la plantation d’arbres, offrir des technologies qui permettront de réduire les coûts de plantation et d’entretien des arbres et améliorer les rendements/revenus de la production d’arbres.

Effects of Shade on Nitrogen and Phosphorus Acquisition in Cereal-Legume Intercropping Systems

With increasing economic and environmental costs associated with fertilizer use, the need for low-input agroecological systems is on the rise. It is well documented that intercropping legumes can increase the supply of nutrients, through N2-fixation and P mobilization. Concurrently, the integration of trees in the agricultural landscape shows increasing evidence of nutrient inputs through efficient biogeochemical cycles. However, little is known about the effects shade has on legume-crop performance. This research aims to determine whether intercropping of the legumes soybean (Glycine max L. Merr.) and alfalfa (Medicago sativa) with wheat (Triticum turgidum durum) is beneficial for performance, particularly under shady conditions associated with tree-based intercropping. Test species were cultivated in hydroponics with a broad nutrient solution and steady state addition of N for 3 weeks. Individual plants were transferred to rhizoboxes with a 2 mm zone of soil for 16 days under (i) full sun or (ii) shade to mimic light levels at the tree-crop interface. Under monocropping, shading was found to significantly decrease wheat biomass. Intercropping wheat with alfalfa under full sun had no negative effect on growth but did increase wheat P uptake as compared to monocropped wheat. In contrast, intercropping wheat with soybean under full sun decreased wheat biomass, suggesting competition. However, under shade, this competitive effect was mitigated, as wheat exhibited similar biomass and higher N and P shoot concentration when associated with soybean as compared to monocropped wheat under lower light levels. This effect may be attributed to reduced biomass of soybean combined with higher soybean N2-fixation under shade. Legume-based intercrops may increase nutrient supply and growth but these beneficial effects will be dependent on matching species selection to light levels under tree-based intercropping.

Temporal and compositional differences of arbuscular mycorrhizal fungal communities in conventional monocropping and tree-based intercropping systems

Previous research has found that conventional agricultural systems adversely affect arbuscular mycorrhizal (AM) fungi. However, there is little information on how more ecologically sustainable agricultural practices such as tree-based intercropping (TBI) influence AM fungal communities. In this study, we investigated whether TBI promotes a more abundant and diverse AM fungal community compared to conventional monocropping (CM). Abundance was estimated by measuring spore abundance and hyphal length in soil, and AM fungal colonization of corn (Zea mays) roots. Overall, AM fungal abundance was similar in both systems as corn roots from the CM and TBI systems were heavily colonized (>50%) by AM fungi throughout the growing season. Additionally, soil samples from the CM and TBI systems contained similar spore densities and hyphal length. Molecular analysis of the AM fungal community was assessed using terminal restriction fragment length polymorphism (T-RFLP) analysis of large subunit rRNA genes amplified from roots in the two cropping systems. A total of fourteen AM fungal phylotypes that belonged to the Glomeraceae were found in the two cropping systems. The TBI system had a higher AM fungal richness and contained several taxa not found in the CM system. Molecular analysis of AM fungal communities also revealed significant temporal and compositional differences between the TBI and CM systems. Within the TBI system, tree species differentially influenced the AM fungal community composition in the alley cropping regions. Future research should focus on determining whether compositional differences among AM fungal communities in CM and TBI systems have a functional effect on crop growth and productivity.

Influence of trees on the spatial structure of arbuscular mycorrhizal communities in a temperate tree-based intercropping system

Tree-based intercropping (TBI) is an ecologically sustainable agricultural practice that may promote a more diverse arbuscular mycorrhizal (AM) fungal community compared to conventional systems, but the influence of the dynamics of these systems on AM fungi has not been established. Soil and root samples were collected in the intercropping alleys along transects perpendicular to tree rows occupied by white ash ( Fraxinus americana), poplar ( Populus deltoids × nigra), Norway spruce ( Picea abies), and rows without trees (control). Molecular analysis of the AM fungal community at the TBI site revealed 17 phylotypes belonging to the Glomeraceae. Overall, the AM fungal community in the TBI site was comparable to other conventional agricultural systems; with the majority of phylotypes belonging to Glomus group A. AM fungal phylotype richness and community composition significantly differed among the treatments in the TBI site. AM fungal communities were more diverse in cropping alleys adjacent to trees that associate with AM fungi than trees that do not associate with AM fungi. Norway spruce had a negative influence on the AM fungal community as tree rows and bordering intercropping alleys had a significantly lower phylotype richness and different community composition. These results suggest that to maintain a diverse AM fungal community throughout TBI systems, it may be best to incorporate tree species that associate with AM fungi.

Reduced soil nutrient leaching following the establishment of tree-based intercropping systems in eastern Canada

Tree-based intercropping (TBI) systems, combining agricultural alley crops with rows of hardwood trees, are largely absent in Canada. We tested the hypothesis that the roots of 5–8 years old hybrid poplars, growing in two TBI systems in southern Quebec, would play a “safety-net” role of capturing nutrients leaching below the rooting zone of alley crops. TBI research plots at each site were trenched to a depth of 1 m on each side of an alley. Control plots were left with tree roots intact. In each treatment at each site, leachate at 70 cm soil depth was repeatedly sampled over two growing seasons using porous cup tension lysimeters, and analyzed for nutrient concentrations. Daily water percolation rates were estimated with the forest hydrology model ForHyM. Average nutrient concentrations for all days between consecutive sampling dates were multiplied by water percolation rates, yielding daily nutrient leaching loss estimates for each sampling step. We estimated that tree roots in the TBI system established on clay loam soil decreased subsoil NO3− leaching by 227 kg N ha−1 and 30 kg N ha−1 over two consecutive years, and decreased dissolved organic N (DON) leaching by 156 kg N ha−1 year−1 in the second year of the study. NH4+ leaching losses at the same site were higher when roots were present, but were 1–2 orders of magnitude lower than NO3− or DON leaching. At the sandy textured site, the safety net role of poplar roots with respect to N leaching was not as effective, perhaps because N leaching rates exceeded root N uptake by a wider margin than at the clay loam site. At the sandy textured site, significant and substantial reductions of sodium leaching were observed where tree roots were present. At both sites, tree roots reduced DON concentrations and the ratio of DON to inorganic N, perhaps by promoting microbial acquisition of DON through rhizodeposition. This study demonstrated a potential safety-net role by poplar roots in 5–8 year-old TBI systems in cold temperate regions.

Shoot pruning of a hedgerow perennial legume alters the availability and temporal dynamics of root-derived nitrogen in a subtropical setting

There is increasing interest in the ecology of tree-based intercropping systems, particularly alley cropping, in temperate settings. Shoot pruning of the tree component is an integral practice to alley cropping. Shoot pruning likely results in an input of belowground C and N to soil, with potential, but unknown impacts on the time pattern of N availability. We evaluated the impacts of pruning of Amorpha fruticosa L. on (a) its root dynamics; (b) mineral N and microbial N, and (c) the fate of root-derived N and its importance in determining N availability. For this, we conducted repeated partial pruning and foliar labeling with 15N under field conditions in Georgia, USA. Pruning altered root dynamics by initially decreasing biomass and later preventing increases, likely due to root death. Pruning modified the time pattern of mineral N by substantially increasing concentrations during the 3 months following pruning. 15N labeling indicated that root-derived N was an important source of this extra N and that pruning caused the availability of root-derived N to microbes to be extended until the following growing season. Greater concentrations of mineral N, a greater pool of microbial N and prolonged microbial access and storage of root-derived N with pruning has the potential to positively impact system productivity.

Potential Greenhouse Gas Mitigation through Temperate Tree-Based Intercropping Systems

Increasing awareness of global climate change has pressured agricultural producers to reduce greenhouse gas (GHG) emissions while at the same time encouraging them to maintain food production needed for an increasing population. Tree-based intercropping (TBI) systems are believed to be useful in climate change mitigation, especially in temperate regions, due to their potential to reduce GHG emissions from agricultural practices. The purpose of this paper is therefore to review some of the research conducted on GHG mitigation in TBI in southern Ontario and Quebec, Canada. Research conducted at the University of Guelph Agroforestry Research Station (GARS) indicated that TBI systems had the potential to lower N2O emissions by 1.2 kg ha-1 y-1 compared to a conventional agricultural field cropping system. Trees can assimilate residual nitrate (NO3-) left from nitrogen (N) fertilizer applications, thereby leaving less NO3- available for denitrification and subsequently reducing N2O losses. Carbon sequestration is also enhanced in TBI systems as carbon (C) is stored in both above and below ground tree components. Soil Organic Carbon (SOC) is higher in systems incorporating trees because tree litter decomposes slowly, therefore reducing CO2 loss to the atmosphere. The C sequestration potential of TBI systems and the possibility to include fast-growing tree species for bioenergy production in TBI systems make it a valid solution to mitigate climate change in temperate regions. The opportunity of C trading credits to offset the costs of implementing a TBI system and provide additional income to farmers could facilitate the adoption of TBI amidst agricultural producers in temperate regions.

Editorial- The Role of Complex Agroecosystems in Climate Change Mitigation

Editorial- The Role of Complex Agroecosystems in Climate Change Mitigation

Arbuscular mycorrhizal fungi in tree-based intercropping systems: A review of their abundance and diversity

Arbuscular mycorrhizal (AM) fungi are important components of agroecosystems as they form symbiotic associations with the majority of agricultural crops. The mycorrhizal association is normally mutualistic and can provide a number of benefits to the host plant including increased nutrient uptake, improved water relations, and protection from pathogens. However, conventional agricultural practices have been shown to have a negative impact on the abundance and diversity of AM fungi. The use of more diverse and sustainable land use practices such as tree-based intercropping can have the potential to reduce the negative impact of agricultural practices on AM fungi. This paper reviews the literature to investigate the effect of temperate and tropical tree-based intercropping systems on the abundance and diversity of AM fungi. Evidence from these studies suggests that tree-based intercropping systems support a more abundant and diverse AM fungal community compared to conventionally managed systems. However, there are studies that observed zero or negative significant effects on the AM fungal community as a result of incorporating trees into agricultural systems. The variable effect of tree-based intercropping systems on AM fungi observed may be a function of the different cultivation techniques, climatic variation, or diverse tree–crop combinations used within the different tree-based intercropping systems. To further our understanding of AM fungal dynamics in tree-based intercropping systems, future research should focus on the influence of tree species with varying mycorrhizal associations and the functional role of common mycelial networks in these systems, while utilizing applicable molecular techniques.

Intercropping hybrid poplar with soybean increases soil microbial biomass, mineral N supply and tree growth

We hypothesized that tree-based intercropping in southwestern Quebec, Canada, would stimulate soil microbial activity and increase soil nutrient supply, thereby benefiting the growth of trees. Our experimental design comprised alternating rows of hybrid poplar (Populus nigra L. × P. maximowiczii A. Henry) and high-value hardwood species spaced 8 m apart, between which two alley treatments were applied 5–6 years after planting the trees. The first alley treatment consisted of a fertilized soybean (Glycine max (L.) Merr.) intercrop grown over two consecutive years, while the second consisted of repeatedly harrowing to minimize vegetation in the alley. Tree rows were mulched with a 1.5 m wide polythene mulch. Microbial respiration and biomass, and mineral N concentrations and mineralization rates were measured on five or six dates at 0, 2 and 5 m from hybrid poplar rows. On some of the sampling dates, we found significantly higher soil microbial biomass, mineral N concentrations and nitrification rates, and a significantly lower microbial metabolic quotient (qCO2), in the soybean intercropping than in the harrowing treatment. Over the 2 year period, hybrid poplar biomass increment and N response efficiency (NRE) were significantly higher (51 and 47%, respectively) in the intercropping than in the harrowing treatment. Microbial biomass and mineral N supply were significantly lower beneath the polyethylene mulch than in the alleys, and we posit that this may stimulate the growth of tree roots into the alley. We conclude that soybean intercropping improves nutrient turnover and supply for hybrid poplar trees, thereby increasing the land equivalent ratio (LER).

Tree-based intercropping does not compromise canola ( Brassica napus L.) seed oil yield and reduces soil nitrous oxide emissions

Recent concerns over rising oil prices and greenhouse gas emissions have sparked an interest for the production of first generation biofuels on marginal agricultural land in Eastern Canada. Field trials were established to compare canola seed oil yield and soil nitrous oxide (N 2O) emissions in tree-based intercropping (TBI) and conventional monocropping (CM) systems. The 4–5 year-old TBI system comprised alternating rows of hybrid poplar and high-value hardwood species, with 8 m wide alleys. Each cropping system was planted with six canola cultivars, grown at four fertilizer N rates. Seed oil concentrations decreased linearly with fertilizer N, while seed oil yields increased either linearly or following a quadratic trend. An optimal fertilization rate was estimated at 80 kg N ha −1. Seed oil concentrations were higher in the CM than in the TBI system, but the two systems did not differ significantly in terms of seed oil yield. N 2O emissions were three times higher in the CM than in the TBI system, probably as a result of higher soil moisture. The cultivar that produced the highest seed oil yield also produced significantly more N 2O, probably as a result of greater available C in the rhizosphere. Our results may be useful to future life cycle assessments for analyzing the net environmental impacts of producing and distributing fertilizer N to biofuel crops, and the choice of cropping system and canola cultivar that minimize N 2O emissions. In a first instance, we conclude that our model TBI system did not compromise canola seed oil yields, and substantially reduced soil N 2O emissions.

Spatial heterogeneity of soil organic carbon in tree-based intercropping systems in Quebec and Ontario, Canada

Land use affects the carbon sequestration potential of soils across landscapes. Tree-based intercropping (TBI) systems where annual crops are grown between established tree rows are expected to exhibit spatial heterogeneity in the soil organic carbon (SOC) content due to differences in carbon input and decomposition rates of litter from trees and herbaceous plants. This study aimed to quantify variability in the SOC of TBI systems, compare the SOC content of TBI and nearby conventional agroecosystems, and determine if SOC was related to soil fertility. The TBI research sites were established 4 years (St. Paulin and St. Edouard, Quebec, Canada), 8 years (St. Remi, Quebec, Canada) and 21 years (Guelph, Ontario, Canada) before soil samples were collected for this study. The SOC content was greater within 0.75 m of the tree row than in the intercropped space at the St. Edouard and St. Remi sites. At the Guelph site, the SOC was spatially heterogeneous in plots with Norway spruce (Picea abies L.) but not hybrid poplar (Populus deltoides × P. nigra clone DN-177), probably due to litterfall distribution. Formerly a tree plantation, the TBI system at St. Remi contained 77% more SOC than a nearby conventional agroecosystem, while there was 12% more SOC in the TBI system than the conventional agroecosystem at Guelph. There was no difference in the SOC content of 4-year old TBI sites and nearby conventional agroecosystems. However, an increase in SOC at all TBI sites was positively related to the plant-available N concentrations, indicating the benefit of temperate TBI systems for soil fertility.

Tree-based intercropping systems increase growth and nutrient status of hybrid poplar: A case study from two Northeastern American experiments

Tree-based intercropping is considered to be a potentially useful land use system for mitigating negative environmental impacts from intensive agriculture such as nutrient leaching and greenhouse gas emissions. Rapid early growth of trees is critical for rapidly accruing environmental benefits provided by the trees. We tested the hypothesis that intercropping increases the growth and nutrient status of young hybrid poplars ( Populus spp.), compared to a harrowing alley treatment (i.e., no intercrop), in two experimental sites (St-Rémi and St-Édouard) in southern Québec, Canada. Three hybrid poplar clones (TD3230, Populus trichocarpa × deltoides; DN3308, P. deltoides × nigra; and NM3729, P. nigra × maximowiczii) were planted at St-Rémi. Clones DN3333 and DN3570 were planted at St-Édouard. At St-Rémi, intercropping comprised a 4-year succession of three crops of soybean ( Glycine max (L.) Merr.) and barley ( Hordeum vulgare L.). At St-Édouard, intercropping comprised a 3-year succession of buckwheat ( Fagopyrum esculentum Moench) , winter rye ( Secale cereale L.), and winter wheat ( Triticum aestivum L.). At St-Rémi, four years after treatment began, leafless aboveground biomass of hybrid poplars in the intercropping treatment was 37% higher compared to that in the harrowing treatment. At St-Édouard, after the third growing season, leafless aboveground biomass of hybrid poplars in the intercropping treatment was significantly higher by 40%. Vector analysis of foliar nutrient concentrations and comparison with critical concentration values showed that N (both sites) and K (St-Édouard) were the most limiting nutrients. Hybrid poplar clones responded similarly to treatments, with no consistent differences in tree growth observed between clones. We conclude that tree-based intercropping systems may offer an effective means of improving the early growth of hybrid poplars planted to provide both environmental services and high-value timber.

Photosynthesis, growth and yield of soybean and maize in a tree-based agroforestry intercropping system on the Loess Plateau

Agroforestry is the most effective way to restore the disturbed lands on the Loess Plateau and to develop the poor local economy. In order to maximize the potential benefits of tree-based intercropping systems, photosynthesis, growth and yield of soybean and corn were studied by measuring photosynthetic active radiation (PAR), plant water deficit and soil moisture in a 4-year-old plantation of walnut (Juglans regia L.) and plum (Prunus salicina) grown at a spacing of 5 m × 3 m on the Loess Plateau. The effects of tree competition significantly reduced PAR, net assimilation (NA), growth and yield of individual soybean or corn plants growing nearer (1 m near tree row) to tree rows. NA was highly correlated with growth and yield of the both crops. These correlations were higher for corn than soybeans, with corn, rather than soybeans being more adversely impacted by tree shading. Plum, rather than walnut had the greatest competitive effect on PAR and NA. Daily plant water deficits were non-significantly and poorly correlated with NA and growth and yield of the both crops. However, soil moisture (20 cm depth) was significantly correlated with biomass and yield of both crops. Possible remediation strategies are discussed to reduce tree competitive interactions on agricultural crops.

Molecular analysis of arbuscular mycorrhizal community structure and spores distribution in tree-based intercropping and forest systems

Due to their potential to enhance soil biodiversity and fertility, tree-based intercropping (TBI) systems are recognized as promising agrotechnologies. However, few studies have examined the effects of TBI on the diversity of arbuscular mycorrhizal (AM) fungi. To investigate this aspect, and to compare TBI with a more typical forest plantation (FO) system, a field experiment using soybean ( Glycine max (L.) Merr. cv. SO3-W4) and poplar ( Populus nigra L. × P. maximowiczii cv. A. Henry ‘Max 5’) was conducted on an arable field site in southwestern Québec. The species of AM fungi colonizing the roots were assessed by PCR-RFLP and DNA sequence analyses of SSU rRNA genes. A total of 13 different phylotypes were identified. Phylogenetic analyses demonstrated that these belonged to the genera Glomus (11), Acaulospora (1) and Scutellospora (1). The AM fungal diversity, as expressed by the Shannon–Wiener indices, were 0.82 ± 0.08 for the soybean and 0.70 ± 0.11 for poplar under the TBI system, and 0.53 ± 0.08 for poplar under the FO system. The study also investigated the distribution of AM fungal spores in the soil, and this differed in the two systems. Under the TBI system, spore abundance increased with increasing distance from the poplar trunk, while it remained relatively constant under the FO system. Our results also show that neighbouring trees and soybean plants hosted different AM fungal communities, suggesting that TBI systems may enhance AM fungal richness compared to monocultures.

Do tree-based intercropping systems increase the diversity and stability of soil microbial communities?

There is a need for research projects to provide scientific arguments favouring a greater implementation of tree-based intercropping (TBI) systems in Canada’s rural landscape. We tested the hypothesis that TBI increases the beta-diversity and stability of soil microbial communities compared to conventional monocropping (CM) systems. Soil from TBI research plots in St-Rémi (Québec) and Guelph (Ontario) were intensively sampled along 56-point grid patterns and compared to soil sampled in a similar manner in adjacent CM systems. Phospholipid fatty acids (PLFAs) were extracted from each sample, purified and methylated, and subsequently analyzed by gas chromatography. The spatial heterogeneity (i.e., microbial beta-diversity) of whole PLFA profiles from each field were analyzed using multivariate statistical procedures (PCA and PERMDISP), and those of individual PLFAs were analyzed using Levene’s and Moses’ tests. Microbial beta-diversity, based on a measure of dispersion among PLFA profiles within each sampling grid, was significantly higher in the TBI than in the CM system at the St-Rémi site only. Multivariate multiple regression analyses failed to reveal any significant relationship between soil physico-chemical variables and shifts in soil PLFA profiles. We compared the concentrations of broad microbial groups in both cropping systems using Tukey’s test and found a higher incidence of arbuscular mycorrhizal fungi (AMF) in TBI systems at both sites, and a greater ratio of gram positive-to-gram negative bacteria in the TBI system at the St-Rémi site only. In order to determine microbial stability, we monitored changes in microbial biomass of bulked soil samples from the sampling grids after these had been exposed to varying concentrations of a heavy metal (Cu) contaminant. Data were then fitted to decreasing exponential functions and model parameters were used to derive an index of microbial stability. Higher microbial stability was found in the TBI than in the CM system at the St-Rémi site only. Because our hypotheses regarding soil microbial beta-diversity and stability were confirmed at only one of two sites, our conclusions warrant prudence and discretion. Future research should explore the possible role of tree roots in maintaining AMF and other beneficial soil organisms for alley crops.

Production of soybean associated with different hybrid poplar clones in a tree-based intercropping system in southwestern Québec, Canada

In tree-based intercropping systems, trees, especially hybrid poplars (HP), can compete with crops for light, water and nutrients, resulting in a decrease in crop production. Some tree management characteristics, such as plantation density and tree species selection, may help to partially control this competition. The aim of this study was to analyse the effect of HP competition on yield and yield components of a soybean intercrop. Soybean was added to a HP–hardwood plantation which was established in 2000 and composed of alternate rows spaced 6 m (field A) and 8 m (field B) apart. The experimental design permitted the comparison of the combined effects of HP clones (TD-3230, DN-3308, NM-3729,), orientation with respect to HP row (East, West) and distance from the HP row. A thinning was performed at the beginning of 2006 to increase the initial 2 m-spacing between HP within each row to a final 6 m-spacing. Percent total light transmittance (PTLT) was measured from soybean emergence to grain filling (VE-R6). Soil water content (WC) and soil N mineralisation (NMIN) were determined at two different periods corresponding to distinct soybean reproductive stages: (1) from flowering to pod formation (R1–R4) and (2) during grain filling (R5–R6). In 2005, PTLT, soil WC, soil NMIN (for R1–R4), soybean yield and yield components were significantly reduced near the HP row (2 m). The number of pods per m 2 contributed more to the variation in soybean yield than did the 100-seed size. Yield and number of pods per m 2 were highly correlated to PTLT, soil WC (R1–R4) and NMIN (R1–R4). An interaction between clone and orientation was found for field A. On the east side of the HP row, soybean yield with NM-3729 or DN-3308 was significantly higher than that with TD-3230, whereas it did not differ from that with the clone on the west side of the field. In field B, a significant interaction between orientation and distance was observed for PTLT and the number of pods per m 2. PTLT and number of pods per m 2 showed less variability on the west side compared to the east side. In 2006, a more regular PTLT distribution with respect to different orientations and distances from the HP row was observed and compared with that in 2005. Similarly, soil WC, soil NMIN, soybean yield and yield components were more uniform in 2006. These results suggest that HP clone selection, tree spacing within the rows, row spacing, orientation, and silvicultural treatments such as thinning may be useful to control the negative effects of HP competition on the intercrop.

ACCLIMATION OF PHOTOSYNTHESIS AND GROWTH OF BANANA (MUSA SP.) TO NATURAL SHADE IN THE HUMID TROPICS

SUMMARYGrowth and photosynthetic performance of banana (Musa sp.) grown in three levels of natural shade (33, 55 and 77% reduction in incoming radiation) were compared to an unshaded control treatment. Net CO2 assimilation rates generally decreased with increasing shade. Chlorophyll fluorescence revealed short-term dynamic photoinhibition under high light conditions but no evidence of sustained photoinhibitory damage to photosystem II. Dynamic photoinhibition decreased with increasing shade, with the greatest depression in the variable to maximal fluorescence ratio (Fv/Fm) occurring in unshaded plants during the middle of the day. Specific leaf area and leaf area ratio increased proportionately with increasing shade, whilst the chlorophyll a/b ratio decreased, reflecting a greater efficiency of light utilization under shady conditions. The optimum shade level for photosynthetic productivity would be one at which the level of photosynthetic photon flux density (PPFD) is high enough to saturate CO2 assimilation but low enough to induce shade acclimation and to reduce photoinhibition. Under the conditions studied here, the saturation level of PPFD was around 1000 μmol m−2 s−1, a light level typical of the tree-based intercropping systems in which banana is commonly grown in the tropics.