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.