2.1. Site description
The study field was located in the reclamation region of the YRD (37°54′19″-37°57′46″N, 118°40′52″-118°43′02″E), Dongying City, Shandong Province, China. The region belongs to the continental monsoon climate zone in warm temperate, with an annual average temperature of 12.1°C and annual average precipitation of 690 mm, mostly concentrated in summer, accounting for 63.9% of the total annual precipitation. The annual sunshine time is 2571–2865 h and the annual average frost-free period is 210 days. As a piece of young land in the YRD, the soil is mainly silt particle type and soil texture is light loam. The soil types are saline soil and fluvo-aquic soil. The average pH is 8.31 and the average salt content is 5.84 g kg-1 (0-20cm of soil layer) (Wang et al., 2021b). The soil had a poor fertilizer conservation due to short development time. But the sense data and vector data for more than 30 years about land use change have shown that the area of new reclaimed land for forage land and arable increased fast (Zhang et al., 2021a). The comprehensive agriculture was planned to develop in the YRD by Chinese central government due to the anthropogenic expansion (Zhang et al., 2011). Therefore, land use conversion was frequent and secondary salinization was occurred seriously due to human activities, which threated the food production and environment safety.
The native representative species in the study site consist ofPhragmites communis (Cav.) Trin. ex Steud. and Suaeda salsa (L.) Pall.. The large part with native vegetation has been reclaimed for gaining new cultivated land in 2001 (Jiao et al., 2014). The new cultivation vegetation mainly consists of wheat (Triticum aestivum L.) and maize (Zea may L.), cotton (Gossypiumspp), and alfalfa (Medicago sativa L.). The wheat-maize rotation is a main cultivated model with conventional tillage in the saline-alkali reclamation region. The winter wheat was sown in early October and harvested in early June next year, and the fertilizer amounts of N, P2O5, and K2O were 240 kg hm-2, 120 kg hm-2, and 90 kg hm-2, respectively. The summer maize was sown in mid-June and harvested in late September, and the fertilizer amounts of N, P2O5, and K2O were 225 kg hm-2, 75 kg hm-2, and 90 kg hm-2, respectively. The cotton with 85% coverage was applied with basal fertilizer and after fertilizer (CO(NH2)2: (NH4)2HPO4 = 3: 1; 600 kg hm-2) every year. The alfalfa as leguminous forage is the important green forage source harvested four times in late May, early July, late August, and early October each year, respectively, and did not receive any fertilization. The conversion between cotton and alfalfa happened frequently due to the secondary salinization and waterlogging.