Different efficiencies of grain legumes in crop rotations to improve soil aggregation and organic carbon in the short-term in a sandy Cambisol

Abstract

Legume plants can promote C storage by enhancing the formation and stabilization of soil aggregates that protect soil organic C (SOC) from mineralization. However, this effect can vary with soil biochemical properties, environmental conditions and legume species. This work aimed to test if including grain legumes in rotation with cereals in a sandy acid soil in a Mediterranean climate can enhance aggregation and SOC storage. Triticale (× Triticosecale ‘Alter’ cv.) fodder was monocultivated with and without synthetic-N fertilizer and in rotation either with irrigated cowpeas (Vigna unguiculata’ Fradel’ cv.), rainfed faba beans (Vicia faba ‘Favel’ cv.) or peas (Pisum sativum ‘Grisel’ cv.) in a sandy loam Cambisol for three years. Legume residues were incorporated into the soil after grain harvest. Crop yields and respective organic C inputs were determined. Topsoil (0–20 cm) samples were analyzed for clay-size minerals mineralogy and mass and C distribution in aggregate-size fractions obtained by wet-sieving. SOC stocks were calculated before and after the trial and in non-cultivated controls. Despite differences in C inputs to the soil, SOC storage was not enhanced with legume cultivation. This may be due to the short duration of the experiment or to the low clay content (10%) and very low reactivity of the clay-size minerals (kaolinite-dominated) of the soil, which seem to have weakened SOC protection. However, non-cultivated controls had up to three times higher SOC stocks, indicating that organic C can be stored in this soil under adequate conditions. Nonetheless, a legume-effect on soil aggregation was observed. Introducing irrigated cowpea in the rotation maintained soil structure, as evidenced by a similar macroaggregate (>250 μm, Magg) content to the baseline, which was deteriorated in the fertilized cereal monoculture in the respective site (less Magg than the baseline). Furthermore, including rainfed faba bean in the rotation improved the soil structure (more Magg than the baseline), which was similar to the baseline when pea was used instead or when cereal was monocropped with synthetic N fertilizer. We conclude that including legumes in rotation with cereal can benefit soil structure in the short-term in the studied edaphoclimatic conditions, but these benefits depend on legume species and agricultural management. However, beneficial effects in SOC stock seem not to be attainable in these conditions in such a short period.