1. Introduction
Ecological stoichiometry provides a tool to investigate the interdependency and balance of nutrient elements (Michał et al., 2020), which reflects the nutrient limitation of organisms, especially carbon (C), nitrogen (N), and phosphorus (P). Soil extracellular enzyme activity is one of the important factors affecting ecological stoichiometry. Recent studies also emphasized that enzyme stoichiometry should be incorporated into biochemical models (Nannipieri et al., 2018). Soil microorganisms drive global nutrient cycling by producing a variety of extracellular enzymes (Singh and Kumar, 2021). Meanwhile, sinsabaugh et al. (2008) found that the logarithm transformed enzymes activities for β-1,4-glucosidase (βG), β-1,4-N-acetyl-glucosaminidase (NAG) + L-leucine aminopeptidase (LAP), and acid phosphatase (ACP) tend to be 1: 1: 1 at the global scale. Furthermore, soil enzyme stoichiometry is consistent on global and regional scales, so many studies predict the variation of soil microbial resource allocation by comparing the regional enzyme stoichiometry with global average levels (He et al., 2020), and guide the healthy development of agriculture in response to global changes.
The change of land use pattern is one of the important factors affecting the steady state of ecological stoichiometry (Gao et al., 2014). Firstly, different land use patterns lead to different fertilization rates, which will directly change the stoichiometric ratio of soil C, N, and P. Some results showed that long-term N addition could significantly improve the total nitrogen content and N: P ratios (Li et al., 2021), and high N input could change the soil community structure and microbial biomass (Muhammad et al., 2021). Secondly, the litter and root exudates of different vegetation types are different, which leads to changes in ecological stoichiometry. Many studies have found that litter changes the soil enzyme stoichiometry due to the alteration of C input (Liu et al., 2021a). Moreover, the root exudates significantly affected the stoichiometry of rhizosphere soil enzymes, resulting in the change of stoichiometric steady state (Xiao et al., 2021). Thirdly, different aboveground vegetation has different nutrient requirements, which affect the soil nutrient cycle (Zhang et al., 2020). For example, Cui et al. (2019) found that in natural grassland, shrubland, woodland, and cropland, the microbial community in natural grassland was the least limited by C and P.
The Yellow River Delta (YRD) is a modern sedimentary plain formed by the deposition of a large amount of sediment carried by the Yellow River (Li et al., 2014). It is one of the largest coastal saline-alkali lands in the warm temperate zone of China and has the characteristics of shallow groundwater, poor soil texture, and high evaporation-precipitation ratio (Cui et al., 2021). In recent decades, a large area of wilderness has been cultivated to promote the development of agriculture. However, the land use changed frequently due to the secondary salinization and human disturbance, resulting in serious soil quality reduction and land degradation. Studies had shown that under the combined long-term effects of sedimentation, reclamation, and fertilization, the average soil C: N: P ratio in the YRD is 64.5: 2: 1, which led to the common limitation of N and P in the local soil (Meng et al., 2021). Therefore, the study of ecological stoichiometry in the YRD is of great significance to guide the rational land use in the saline-alkali reclamation region. Although previous studies have revealed the soil stoichiometry at the regional scale of YRD, there are few studies on combined effect of soil C: N: P ratio and enzyme stoichiometry under different land use patterns. In this study, three representative land use patterns were selected to analyze the nutrient elements, enzyme activity, and stoichiometric characteristics, in order to provide a theoretical basis for the rational land use and sustainable development of agricultural ecosystem in the YRD.