4.1. Effects of different land use patterns on soil physical and chemical properties
Different land use patterns, due to differences in fertilization, vegetation types, and tillage methods, will lead to significant changes in soil physicochemical properties. In our study, AG and WM significantly increased WSA (> 0.25 mm) content compared with PC, and WSA (> 0.25 mm) was considered to play an important role in soil stability (Zhang et al., 2021b). This is because AG and WM have more litter, root biomass, and root exudates, which can provide important binders for soil aggregates and promote the formation of soil macroaggregates (Hazra et al., 2019). The application of chemical fertilizer has little effect on the size distribution of soil aggregates (Liao et al., 2021), while long-term cultivation and frequent disturbance will directly reduce the stability of WSA (Li et al., 2009), which also explains the reason why the WSA (> 0.25 mm) content of AG is higher than that of WM.
The research showed that soil salinity in the YRD was affected by groundwater depth, soil evaporation, soil water holding capacity, and vegetation canopy density (Zhang et al., 2019a). The soil salinization degree in this study increased with the deepening of soil layer, which was due to the increase of rainfall in summer, increasing salt leaching (Xie et al., 2011). Meanwhile, the soil salinity of AG and WM was higher than that of PC, which was due to the salt enrichment of rhizosphere soil caused by the ”salt island” effect of alfalfa and the ”fertilizer island” effect of wheat and maize in the face of the poor environment (Li et al., 2019c). The soil salinity of AG was less than that of WM, which was due to the higher ground coverage and canopy density of alfalfa, which reduced soil evaporation and inhibited the accumulation of salt on the soil surface (Xia et al., 2019).
The effects of different land use patterns on soil nutrient cycle are also different. The results showed reasonable N management and straw returning could promote soil C sequestration in wheat-maize rotation (Zhao et al., 2021). In this study, the WM was more conducive to increasing soil TC content compared with AG, while cutting alfalfa did not increase soil TC content (Bell et al., 2012). The AG was more conducive to the increase of SOC content, which was due to its higher content of macroaggregates, and the C in macroaggregates was easier to be decomposed by microorganisms (Bhattacharyya et al., 2021). In this study, AG significantly increased the contents of TN, NH4+-N, and NO3--N in soil compared with WM and PC, which was consistent with the previous research conclusions (Li et al., 2019b). Leguminous forage contributed 40-70kg N hm-2 per quarter (Sanginga, 2003), and continuous alfalfa cropping could promote N mineralization and improve N availability (Li et al., 2019b). Contrary to the trend of N in this study, the contents of TP and AP in AG were lower than that in WM. This is because leguminous forage such as alfalfa need more P than traditional crops. After all, P plays an important role in the energy transformation of rhizobia (Wang et al., 2021a). The research showed that soil P content decreased with the increase of alfalfa continuous cropping years (Jiang et al., 2006).