China is the biggest apple producing country in the world (Liu Y et al., 2013), with a cultivated area of 2.41 × 10⁶ ha (46% of the world surface devoted to apple) producing 39.68 × 10⁶ t of apples (49% of the world’s total production) in 2013 (http://faostat3.fao.org/home/E). In China, the Loess Plateau is a large area of apple production that represents 28.09 and 25.73% of the country’s apple cultivation area and total yield, respectively (Yang & Chen, 2013). The apple products of the Loess Plateau area have been exported to more than 30 countries in Europe, North America and Southeast Asia (Li et al., 2008; Yang & Chen, 2013; Qian et al., 2015). However, the Loess Plateau is also the largest dryland rain-fed agricultural area in China, and water resource deficiencies seriously restrict apple production (Huang & Gallichand, 2006). In 2010, the apple yield per hectare in the Loess Plateau (with annual mean rainfall and potential evaporation of 200–700 and 763–963 mm, respectively) was 14.22 t/ha, only 47% of that of the Bohai Coastal area (30.15 t/ha) (with annual mean rainfall and potential evaporation of 550–950 and 450–600 mm, respectively) (Pu et al., 2010; Yang & Chen, 2013; Zhang et al., 2014). Thus, decreasing water loss from evaporation and increasing yield per unit area is a great challenge for improving apple production of the Loess Plateau.

In the Loess Plateau, almost 70% of rainfall usually occurs from the hot summer to early autumn (July–September), and much of this is lost through evaporation due to seasonal high temperatures (Ren et al., 2008). Thus, limited annual precipitation and high evaporation in the hot season are the main constraints for apple production. Additionally, apple trees have more extensive root systems and canopy than annual crops, so apple orchards have higher evapotranspiration and lower available soil water in comparison with local cereal crops (e.g. winter wheat) (Huang & Gallichand, 2006). Evaporation during hot summers may cause soil moisture deficits of various degrees in apple orchards when precipitation is limited. Low soil fertility is another constraint. Soil organic matter (SOM) in most apple orchards of the Loess Plateau is usually in the range of 1.0–1.5% due to the application of inorganic fertilizer without organic inputs, which is much lower than that of American apple orchards (> 2.0%) (Chen et al., 2014; Zhao & Tong, 2014). The stability of macro-aggregates and moisture retention capacity are also low due to the application of chemical fertilizer alone (Sarkar et al., 2003). Therefore, development of suitable water management measures and rational fertilization are needed to sustain apple production in the Loess Plateau.

As a perennial legume, crown vetch (Coronilla varia L.) is a common intercropping plant in apple orchards with many benefits, including controlling weeds, decreasing soil erosion, fixing atmospheric N₂, increasing soil enzyme activities, and improving the soil micro-ecological environment (Jarvis, 1983; Cardina et al., 1986; Qian et al., 2015). However, intercropping of crown vetch might compete with apple trees for water (Li et al., 2014). Mulch application could help to conserve soil water and decrease evaporation (Sauer et al., 1996; Burt et al., 2005; Monzon et al., 2006; Yuan et al., 2009; Ward et al., 2009). Thus, intercrop-mulch (IM), a process involving intercropping with crown vetch at first and then mowing it (the residues are left on the soil surface as mulch) in the rainy hot season, is recommended. However, the effect of IM on water use efficiency (WUE) in apple production is unclear, especially with regard to differences in water consumption before and after mulch application. In addition, although mulch can significantly decrease evaporation, the WUE can still be very low due to a lack of adequate fertilizer use (Li et al., 2000). Many studies have shown the effects of straw mulch and plastic film or combinations of mulch with varied nitrogen (N) rates on the WUE of cereal crops (Li et al., 2001; Khurshid et al., 2006; Chen et al., 2015; Li et al., 2015). However, little is known about the effect of applying IM of crown vetch with different kinds of fertilization on the WUE of apple trees.

Current apple production in the Loess Plateau can be substantially increased by rational water conservation and fertilization measures. Thus, a field experiment was conducted aiming to: (1) evaluate the combined effects of IM and different kinds of fertilization on soil water storage, evapotranspiration, yield components, yield, and WUE of apple trees under dryland conditions; and (2) find the best combination of these strategies for apple production in the rain-fed area of the Loess Plateau.

About 70% of yearly precipitation usually occurs from the middle of the hot summer to early autumn (July–September) in rain-fed dryland apple orchards in the study area; thus, this period has sufficient rainwater resources. However, the high temperature in this period induces a high amount of water evaporation. Additionally, this period coincides with the fruit expansion stage when adequate soil water is imperative. Therefore, decreasing ineffective water evaporation during this period could improve apple tree growth, yield, and WUE.

In the present study, under IM with crown vetch, single fruit weight and fruit number per tree were significantly increased in comparison with the NIM treatment (Table 5). The apple yield and WUE under IM increased by 27.4% and 14.6% during the three years studied, respectively; particularly in the dry year, they significantly increased by 69.5% and 73.3%, respectively, although the increase in WUE was not obvious due to a lack of water stress in 2011 and 2014, during which there was enough precipitation. The yield and WUE of IM were increased due to the application of mulch three times during June–September, which decreased evaporation from topsoil and improved soil water storage. The water stored through mulch application surpassed or at least compensated for the water consumed in crown vetch growth. However, Du et al. (2015) found that apricot yield decreased due to water competition between erect milk vetch and apricot in an intercrop-mulching system, even though the mulch was applied after mowing, probably because mulch was applied too late (in autumn) to be effective for seasonal fruit growth as apricots had already ripened (in July). In the present study, successive monthly mowing and mulching during July–September would substantially restrain the competition capacity of crown vetch and decrease water evaporation in the hot rainy season. Especially in the dry year, mowing and mulching significantly decreased ET_mow by 17.3% in comparison with that of CK (Table 3). Thus, more water was stored for apple tree growth and WUE_mow in the rainy hot summer was significantly increased (Table 6). Additionally, although growth of crown vetch would consume some soil water before mowing (from late March to early July) and so compete for soil water with apple trees and increase ET_tot, the apple yield still increased. A possible explanation for this observation was the adaptive spatial complementarity of the two root systems, which may have prevented water competition (Fetene et al., 2003; Morlat & Jacquet, 2003). Monteiro & Lopes (2007) found that the water depletion that was observed in the sward treatments at bloom time may have induced death of vine roots in the upper layers and development of a deeper root system to explore deeper layers. Li et al. (2011) also reported that apple roots extended more into deep soil layers, while fine roots extended more deeply into soil layers than thick roots did, and white clover fine roots extended more into surface soil layers in the apple-white clover intercropping system. After mowing and mulching from July-September, IM treatment can result in more water storage for fruit growth in comparison with NIM. Taken together, these results indicate that IM increased apple yield, showing that there is great potential for decreasing evaporation and increasing apple yield and WUE through IM with crown vetch if crown vetch is well managed and competition with apple trees prevented.

Although IM of crown vetch can decrease evaporation and store more water for transpiration of apple trees, adequate fertilization is still needed in combination with IM to increase WUE. In our experiment, soil fertility was improved after applying IM with NPKM for seven years. The SOM of NPKM (with IM) at depths of 0–20 and 20–40 cm was increased by 15.8 and 20.3% in comparison with that of CK under IM, respectively (Table 7). Thus, single fruit weight and fruit number per tree were obviously increased. Therefore, apple yield for NPKM (with IM) was increased by an average of 73.25% in the three studied years; the WUE_mow of NPKM (with IM) was also significantly increased by 75.86% in comparison with that of CK (with IM). These results suggest that NPKM fertilization supplies sufficient available nutrients for tree growth and improves soil fertility in the long term, in comparison with NPK fertilization (Lönhárd-Bory & Németh, 1990; Zhao et al., 2014). As a result, we suspect that apple trees subjected to NPKM treatment likely had larger canopies (Li TT et al., 2013), while soil water, which was conserved by mulch, was efficiently used for transpiration, so apple yield and WUE_mow increased. Previous research had reported that intercropping might decrease yields because of nutrient competition (Du et al., 2015). In our experiment, NPKM treatment improved soil fertility more than other fertilization treatments. Therefore, NPKM treatment was more effective than other treatments as a means of mitigating nutrient competition between crown vetch and apple trees. In addition, due to the manure addition, which could improve the mean weight diameter of aggregates, total porosity, and water holding capacity (Karami et al., 2012; Liu CA et al., 2013), NPKM fertilization could induce water storage more effectively than chemical fertilizer alone, mitigating water competition between crown vetch and trees. As a result, NPKM fertilization could greatly compensate for nutrient deficiency with IM and applying the two together could result in greater apple yields compared to applying IM with other fertilizers, benefiting farmers (Table 8). In the Loess Plateau, farmers always apply plastic film and straw mulch to decrease the evaporation. However, plastic film does not increase the abundance of soil nutrients and has negative environmental impacts, and both plastic film and straw mulch impose transportation expenses and other costs each year. As a perennial legume, crown vetch can live for several years if well managed and sprout in late March each year after being sown in the first year, and it can form root nodules, control weeds, prevent soil erosion, and improve soil fertility (Wheeler, 1974; Symstad, 2004). As a legume, crown vetch can mitigate nitrogen competition due to its ability to fix atmospheric N₂ via root nodules, compared to other non-legume species, and other similar studies found that nitrogen competition was mitigated by legume grass cover systems (King & Berry, 2005; Messiga et al., 2015). In our experiment, the total net return (for three years) of IM was 31.5% higher than that of NIM, and that of IM combined with NPKM was 72.35% higher than that of IM alone. All of these reasons indicate that IM of crown vetch combined with NPKM is a good practice for apple production, although it may compete for soil water with apple trees before the first mowing and mulching.

In summary, applying IM conserved more soil water in the hot rainy season for transpiration during the fruit expansion stage of apple trees, particularly in the dry year, although the effect was not obvious when enough precipitation prevented water stress. Thus, single fruit weight and fruit number per tree increased, as well as apple yield and WUE. Additionally, in comparison with other fertilization treatments, the water and nutrients consumed by crown vetch were more effectively compensated by NPKM application. Hence, IM with NPKM resulted in the highest yield and WUE of the tested treatments. For these reasons, and considering agricultural, soil and economic factors, IM combined with NPKM is expected to be a beneficial practice for farmers engaged in apple production in the Loess Plateau.