GROWTH, NITROGEN UPTAKE AND MAIZE NITROGEN RECOVERY OF COVER CROPS IN CONSERVATION AGRICULTURE

Planting winter cover crops has several benefits compared to keeping the soil bare. The choice of the cover crop species and sowing date is crucial to have the best cover crops establishment and weed suppression. The seeds germination of cover crops is affected by the sowing date with a preference of early sowing. However, the appropriate date of cover crops sowing is not known. Also, cover crops nitrogen dynamics is variable among species. In a conservation agriculture context, we conducted two field experiments in Northern Italy and one laboratory experiment under controlled conditions of temperature and soil moisture. The objectives were to (i) assess the growth and nitrogen uptake of five pure winter cover crops (black oat, Avena strigosa Schreb.; cereal rye, Secale cereale. L.; white mustard, Sinapis alba L.; Egyptian clover Trifolium alexandrinum L.; and hairy vetch, Viccia villosa Roth) as influenced by plant species from three botanical families and two sowing dates (SD1 and SD2), (ii) assess the effect of cover crops presence/absence (bare soil) on weed suppression and maize productivity, (iii) estimate and assess the cover crops contribution to the following main crop (maize) in terms of nitrogen recovery and immediate availability, (iv) establish the course of nitrogen mineralization from pure cover crops in laboratory incubation conditions and (v) assess the effect of three managements of winter-hardy cover crops termination methods and control of weed in maize (chemical vs. mechanical) on maize productivity. The field experiments were carried out in Orzinuovi, Brescia, Italy. Relevant differences in cover crops growth were observed among species, with white mustard SD1 having the highest biomass in November (5.3 and 3.2 t ha-1, respectively for the first and the second year) and Egyptian clover the lowest (less than 1 t ha-1). Also, we demonstrated that hairy vetch SD1 had the highest nitrogen uptake in November (114 kg N ha-1). The presence of cover crops reduced weed infestation compared to a bare soil. Sowing cover crops at end of August, instead of mid-September, had a positive effect on production, establishment, nitrogen uptake, and weed suppression. Maize yield following cover crops was not affected by the cover crop sowing dates and species during the two years of experiment. The maize nitrogen recovery was variable within years; the highest recovery was for maize following hairy vetch SD2 (+67%). The importance of sowing cover crops was demonstrated by the higher nitrogen recovery of maize following cover crops compared to maize following no cover crop treatment. In a laboratory incubation experiment of 84 days, cover crop shoots were collected from cover crops grown in the field, mixed with soil and kept under controlled temperature of 20 °C and soil moisture of 100% field capacity. We demonstrated differences in nitrogen mineralization among the five pure cover crops and weed shoots with hairy vetch, collected in March (C/N ratio of 10.1), having the highest and immediate net nitrogen mineralization from the beginning of incubation until 84 days after start of incubation. Black oat collected in March (C/N ratio of 19.8), had also an immediate net nitrogen mineralization during the whole incubation period but at a lower rate compared to hairy vetch. Egyptian clover collected in November (C/N ratio of 11.4), started nitrogen mineralization 7 days after start of incubation. White mustard collected in November (C/N ratio of 17.7), had a low rate of nitrogen mineralization. Shoots of weed, cereal rye, white mustard and black oat collected in March immobilized nitrogen during the whole incubation period at different rates; cereal rye had the highest rate of immobilization and was not able to start nitrogen mineralization 84 days after start of incubation. In the second field experiment our results indicated that a “post-glyphosate” scenario (mechanical termination of cover crops and chemical control of weed in maize) is the best management to produce the highest yield of maize compared to a “business-as-usual” management (chemical termination of cover crops and weed control in maize) and “organic” management (mechanical termination of cover crops and weed control in maize).