Does Intercropping improve soil aggregation and organic carbon protection? A case-study in the Semi-Arid Mediterranean

Intercropping has been claimed to improve the soil structure and soil quality, however its effects on soil fertility parameters in semi-arid Mediterranean agroecosystems remain unclear. The objective of this study was to assess whether intercropping and its combination with N fertilisation are adequate practices to improve the soil aggregate stability and organic matter quality. An irrigated on-farm experiment was established in northeastern Spain to evaluate the effect of seven cropping systems (faba bean-durum wheat, pea-durum wheat, and rapeseed-pea intercropping and the respective sole crops) and two N-fertiliser rates (0 vs. 75 kg N ha −1 ). Several soil variables were analysed in bulk soil: water-stable macro- and microaggregates, soil organic carbon (SOC), soil total nitrogen (STN), reduced permanganate (Per red ), particulate organic matter (POM) and carbon (POC), mineral-associated organic matter (MAOM) and carbon (MAOC) and microbial biomass carbon (MBC) and nitrogen (MBN). Also, SOC, Per red and nitrogen (N) storage within aggregates were determined. Intercropping did not increase the share of water-stable aggregates nor SOC levels, but SOC concentration was higher in macro- (2.34 g C 100 g −1 ) than in microaggregates (1.89 g 100 g −1 ). Intercropping did also not affect STN, but the mineral N-fertilised treatment increased both SOC (2.24 vs. 2.08 g 100 g −1 ) and STN (0.193 vs 0.177 g 100 g −1 ) significantly in bulk soil. This study showed that after a few years upon establishment, intercropping had not enhanced the soil aggregate stability significantly and as a sole practice intercropping does not improve soil C and N concentrations nor their active fractions. • Intercropping effects on soil fertility were measured in an on-farm experiment. • No increase of soil organic carbon under intercropping but overall highest concentrations in macroaggregates. • Higher nitrogen-levels and microbial biomass went along with formation of MAOM and reduced permanganate. • Nitrogen fertiliser improved soil organic carbon due to microbial alteration. • Soil total nitrogen binds to silt and clay surfaces outside of aggregates.