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Weed suppression enhanced by increasing functional trait dispersion and resource capture in forage ley mixtures

Suter, M., D. Hofer and A. Lüscher. 2017.

Abstract

Weeds in intensively managed grassland can result in substantial losses of forage yield and quality. We studied whether weeds could be suppressed sustainably by use of functionally diverse grassland mixtures with high resource capture and biomass yield. To this aim, growth of planted phytometers and of spontaneously establishing, unsown species were evaluated. To represent weeds, seedlings of five model species were grown up as phytometer plants in stands of forage leys under two precipitation regimes. Ley stands comprised monocultures and mixtures of two and four species with distinctly differing plant functional traits (PFTs) related to nitrogen (N) acquisition and rooting depth, including: Lolium perenne L. (PFT: non-N2-fixing, shallow-rooted), Cichorium intybus L. (non-N2-fixing, deep-rooted), Trifolium repens L. (N2-fixing, shallow-rooted), and Trifolium pratense L. (N2-fixing, deep-rooted). Five PFTs were measured from these ley species: leaf dry matter content, specific leaf area, leaf N and carbon (C) content, and the leaf C:N ratio. Survival of phytometer plants and biomass of unsown species was measured under rainfed conditions and during an experimentally induced, nine-week summer drought with complete rain exclusion. Mixtures with high functional dispersion (calculated from PFTs) and biomass yield suppressed phytometer plants and unsown species significantly more than mixtures with low functional dispersion. Under rainfed conditions, fourteen weeks after planting, survival of phytometer plants was 32% in the four-species mixture with the highest functional dispersion, while survival was 68% averaged across monocultures, the stands with the lowest functional dispersion. Surviving individuals had significantly smaller tiller number and rosette diameter in mixtures than in monocultures. Biomass of unsown species was 3.6 kg ha-1 in the four-species mixture, but was 13.0 kg ha-1 in averaged monocultures. The suppressive effect was equally apparent under drought conditions, where, at the end of the drought period, phytometer survival was 16% and 47% in the four-species mixture and averaged monocultures, respectively. During a six-week post-drought period, the suppressive effect of ley mixtures on phytometer plants was sustained. The strong suppressive effect of ley mixtures was explained by their superior capture for light and plant-available soil N. We conclude that cropping functionally diverse grassland mixtures could be a sustainable strategy to enhance weed suppression under rainfed and drought conditions.