Evaluating Plant Functional Diversity in an Organic Intercropping System



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Overyielding, a phenomenon whereby plant production in mixture exceeds that of production in monoculture, has been attributed to complementary use of resources by different plant functional types. Few studies have examined the role of plant functional diversity and the concept of overyielding in food production systems. Different combinations of peanut (Arachis hypogaea L.), watermelon (Citrullus lanatus Thunb.), okra (Abelmoschus esculentus Moench.), cowpea (Vigna unguiculata L.), and pepper (Capsicum annuum L.) planted alone or in various intercropping combinations were investigated over two growing seasons in an organic system in the peak of summer heat in Texas. Results from land equivalent ratio (LER) indicate that the within-row intercropping combination of peanut, watermelon, and okra (Wpwo) and the four species combination with the addition of cowpea (Wpwoc) consistently overyielded both growing seasons, despite a reversal in dominance patterns exhibited by watermelon and okra between years. There was no effect of intercropping system on changes in total nitrogen (N) or soil organic carbon (SOC). However, soil microbial biomass carbon (SMB-C) was lower in Wpwo as compared to okra grown in monoculture. Although there was no difference in root length density (RLD) between cropping system, there was a significant positive linear relationship between RLD and SMB-C. Low leaf area index (LAI) values in cowpea and peanut monocultures resulted in higher daily soil temperatures and an increase in weed biomass. There was a strong inverse relationship between LAI and soil temperature, particularly with daily maximum soil temperature. No differences in physiological parameters were detected. In 2012, when watermelon was a subordinate crop, specific leaf area (SLA) and leaf N content were highest in the multispecies systems, particularly Wpwoc. Carbon to nitrogen ratio (C:N) was also lowest in Wpwo and Wpwoc as compared to watermelon grown in monoculture indicating watermelon underwent morphological changes at the leaf level due to competition for light and allocated less C to leaves when competition was reduced. Overall findings suggest that three and four species intercropping combinations, whereby each crop is selected to perform a specific function within the system, may provide small-scale sustainably-minded producers a model system that can be utilized in the peak of summer in southern climates and allow them to reduce inputs while increasing overall yields.