Intercropping improves soil structure by increasing large macro-aggregates in a newly reclaimed soils
There are large amount of reclaimable lands in arid areas of northwest China where irrigation is available. Most of these reclaimed soils are usually characterized by poor soil structure and low fertility. Thus, it is important to improve structure of the reclaimed soils. Two long-term field experiments initiated in 2009 were conducted at the soils which were reclaimed for 25 years at Jinyuan in Gansu (Exp. 1) and for 3 years at Hongsibu in Ningxia (Exp. 2), China. The Exp. 1 was two-factorial design, where main factor was P application rates (0, 40, 80 kg ha-1) and the second factor was cropping systems, including faba bean/maize, soybean/maize, chickpea/maize, turnip/maize, and their corresponding monocultures. The Exp. 2 was split-split-plot design, where main plot treatments included inoculation with rhizobium or not, and the sub-plot treatments included five N application rates (0, 75, 150, 225 and 300 kg N ha−1), and the sub-sub-plot treatments were continuous faba bean, maize, and faba bean/maize intercropping. Each treatment has three replications in both field experiments. Soil water stable aggregates were measured in the 4th (2012) and 5th (2013) year of both field experiments.
Maize/legume or turnip intercropping exhibited a significant 32-55% increase of large macro-aggregates (> 2 mm) concentration in comparison with legumes or turnip alone. Long term annual P additions and continuous turnip cropping could be less favorable for water stable aggregates in the Exp.1. Similarly, in the Exp. 2, amount of large macro-aggregates (> 2 mm) were substantially increased by faba bean/maize intercropping compared with monocropped faba bean (704 g kg-1 vs 583 g kg-1), which presented similar trends to the Exp. 1. While N fertilizer hardly affected water stable aggregates. Moreover, Rhizobium bacteria resulted in the reduction of large macro-aggregates and mainly influenced the free silt and clay particles. In summary, the formation of more stable aggregates was significantly improved after 3 years of intercropping compared with monocropping, especially in newly reclaimed soils.