Performance of field vegetable cropping systems under organic farming: effects of tillage and cover crop management

Conservation tillage in organic farming has been increasingly promoted to preserve soil fertility, decrease greenhouse gas emissions and improve the sustainability of agricultural systems. However, concerns regarding weed suppression, nutrients supply and soil compaction are limiting the expansion of these practices. For this, a two-year field experiment (2015-2017) implemented under Mediterranean conditions (Pisa, Italy) was conducted to evaluate the combined effects of tillage and cover crop management on weed suppression, plant growth, productivity, fruit quality and resource use efficiency in organic tomato production. We compared for this purpose systems consisting of i.) tomato transplanted on conventionally-tilled soil following or not a cover crop (Trifolium squarrosum L.) with/without a biodegradable plastic mulch; ii.) no-till where the cover crop was rolled as dead mulch and tomato directly transplanted. Initially, the living clover provided 20% reduction of weed biomass. In 2016, weeds were able to regrow shortly after transplanting and no weed control by the dead mulch was provided during the season. In contrast, the dead mulch of clover reduced weed abundance across the whole tomato growing season in 2017. However, no-till systems did not ensure the same weed control level of conventionally-tilled systems. During the growing season, residues of clover incorporated outperformed the dead mulch and the fallow conventionally-tilled system in weed control. Supplemental mechanical weeding on dead mulched plots reduced weed biomass and succeeded to improve plant performance. In both years, no effects were noticed on perennial weeds abundance. No-till systems had higher weed richness and different weed community structure compared to conventionally-tilled systems in 2017. N and P uptake were the highest in plastic mulch systems mainly when preceded by the clover and the lowest on the dead mulch. Conventionally-tilled systems without the legume green manure had the highest Nitrogen use efficiency. The best performance in terms of yield was obtained where the preceding clover was incorporated into the soil although it varied whether managed under plastic mulch or without it according to the site-year. The potential yield reduction in dead mulch systems accounted for at least 82% in 2016 and 65% in 2017. Greater soil nitrates availability was provided by the clover as green manure compared to dead mulch. No evidence of growth limiting soil compaction was seen in no-till systems although a slight soil resistance was seen in the first few cm up to fifteen cm of the topsoil. Soil visual quality (soil structure, aggregate dimension, rooting, residues decomposition) analysis did not reveal substantial differences between the management systems. The basic quality of red tomato (firmness, pH and TSS) was little influenced by tillage and cover cropping. Fruit vitamin C content however increased in tomato grown on clover dead mulch. Under cold storage, tomato fruits from the different agricultural practices behaved similarly. All results pointed out to the importance of N availability and weed control in reduced tillage systems. Based on that, the complete removal of tillage in a crop growing cycle may not be a successful solution in short term under some conditions, unless an effective weed management is provided both by highly performing cover crops i.e., high biomass and allelopathic action, and subsequent weeding and a good management of N supply during the season. Cover crops are an integral component of organic farming systems due to their benefits for soil fertility and their contribution to weed management. Allelopathic cover crops could be a tool in the multi-tactic approach used for weed management and could be potentially interesting for organic reduced tillage systems where cover crops are key for this purpose. In another study, we investigated two cover crop species, rye (Segale cereale L.) and squarrose clover (Trifolium squarrosum L.) used pure or in mixture for their allelopathic activity on some of common arable weeds. For this purpose, we prepared aqueous extracts with a series of dilutions from each cover crop type and we tested them through in-vitro tests i.e., seed germination and growth bioassay, on some selected weed species: Conyza canadensis (L.) Cronq., Amaranthus retroflexus L. and Digitaria sanguinalis (L.) Scop. The three aqueous extracts at different concentrations affected the germination and the early growth of almost all tested weed species although the magnitude of the effects was species-specific. An inhibition of Conyza canadensis radicle growth was obtained with all three extracts, reaching 75 to 80% with rye and mixture extract. Clover extract had the highest phytotoxic effect on Amaranthus retroflexus germination and growth whereas Digitaria sanguinalis growth was mostly affected by the mixture. Besides, the extracts had the potential of delaying germination onset of the dicot weeds. The phytotoxic effects of rye and mixture cover crops obtained in laboratory tests were also detected in field with a reduction in weed density almost forty days after their incorporation as green manure. Cover crops managed as green manure were more effective in reducing weed density than dead mulches. Our results showed all three cover crop types as potential cover crops for an ecological weed management, although additional studies should be done to test the effectiveness of squarrose clover in field conditions.