Evaluating spatial arrangement for durum wheat (Triticum durum Desf.) and subclover (Trifolium subterraneum L.) intercropping systems

Cover crops and mulches can be used for increasing sustainability in winter cereal cropping systems. We performed a 2-year field experiment in Central Italy with the aim of finding a suitable spatial arrangement for durum wheat (Triticum durum Desf.) and subclover (Trifolium subterraneum L.) as a living mulch system in order to provide a high cereal grain yield and a sufficient subclover reseeding following the wheat harvest. Experimental treatments consisted of: (i) five cropping patterns [wheat and subclover mixed in the same row, with rows 15 cm apart (same row); 2 rows of wheat and 1 row of subclover at a distance of 10 cm between rows (narrow rows); 2 rows of wheat and 1 row of subclover with a distance of 10 cm between the wheat rows and 17.5 cm between the wheat and subclover rows (wide rows); durum wheat sole crop and subclover sole crop, both in rows 15 cm apart]; (ii) two nitrogen fertilization levels (0 and 100 kg ha−1 of N); (iii) and two weed management levels (weed-free and weedy). The wheat grain yield was not reduced by the intercropped subclover in narrow rows, while it was around 14% lower in same row and wide rows compared to the one in the pure crop treatment. When intercropped with subclover, wheat was the competitively superior species and its competitive advantage was greater when it was closer to the legume and/or in presence of nitrogen fertilization. A strong negative relationship between wheat aggressivity and subclover seed production was observed. Following the wheat harvest, the legume reseeding was sufficient to regenerate a cover crop in the autumn of the second year regardless the spatial arrangement, even if the density of the subclover seedlings was almost twice in wide rows compared to the one in the same row. Although the intercropped systems were characterized by an increase in plant density compared to the sole crops (100% of wheat + 50% of subclover), the competitive ability of the wheat-subclover system against the weeds was higher than the wheat sole crop only in narrow rows where a significant reduction of both weed density and weed biomass was observed. When the subclover is used as living mulch in durum wheat, a moderate separation between the two species could be a suitable spatial arrangement for obtaining an adequate wheat grain yield, ensuring satisfactory subclover reseeding, controlling the weeds more effectively.