Impacts of long-term jujube tree/winter wheat–summer maize intercropping on soil fertility and economic efficiency—A case study in the lower North China Plain

Agroforesry is a common traditional practice in China, especially in the saline-alkaline regions, like the lower North China Plain (LNCP) characterized by lower yields of food crops. Adding trees to the agricultural land creates additional fruitsets or woody biomass besides food crops, enabling farmers to diversify the provision of farm commodities. However, the productivity of many agroforestry systems has been lower than expected in recent years, highlighting the need for a mechanistic understanding of below- or above-ground interactions. The study combined investigation and experimental data together to evaluate the effects of long-term intercropping agroforestry system [jujube tree (Zizyphus jujuba Mill. var. inermis (Bunge) Rehd.)/winter wheat–summer maize] on soil fertility balance, crop production and system economic efficiency over the past 22 years in LNCP, with a view to developing an effective fertilization management for the moderately alkaline soils. Except remain higher pH, the soils are basically free of sodic and soil salinity is not the major restriction factor for intercrops, even through there are some fluctuation with season and distance from jujube tree. The intercropping system significantly reduced soil nutrient contents, like soil organic C (SOC), total N (TN), available P (avail. P) and K (avail. K) in most parts of the ecotone of the system, but increased those nutrients in the belt of underneath the edge of tree canopy, The growth of intercrops at the belt of 3.5m from tree was severely negative stressed by jujube tree in term of lower soil moisture, nitrate, avail. P and K although receiving more photosynthetically active radiation (PAR), whereas the winter wheat growing at the 2.5m row had more water and nutrients supplied and thus produced more grain yield. Uneven fertilization to the ecotone (about 1–2.5m of the intercrop field boundary) could partly offset the consumption and competition for nutrients between the tree and the intercrops, and improved the grain yields by 12.1% and 14.5% in the ecotone regions (distance from jujube trees) of 1.5m for winter wheat and 2.5m for summer maize by increasing respective yield components. Although the mean grain yield of intercropped winter wheat and summer maize was reduced by 35.6 and 35.2%, respectively, compared to sole cropping system, the intercropping system proved to be a profitable land use type based on net income and economic returns, in addition to the wood and ecologi