4.2. Effects of different land use patterns on soil stoichiometry
Soil C: N: P ratio is an important index that reflecting the vegetation nutrient limitation and biochemical cycle. The C: N ratio is used to characterize the mineralization degree of organic matter, and low C: N ratio (< 25) indicates that mineralization of soil organic matter is greater than fixation (Muhammad et al., 2021). And C: P ratio is used to measure P availability. The lower C: P ratio is, the higher P availability is (Zhang et al., 2019e). Meanwhile, soil C: P ratio is negatively correlated with soil C emissions (Liu et al., 2021b). In this study, the C: N ratios of AG were lower than that of WM and PC in all soil layers, and the C: N ratios of WM in 40-60cm soil layer and PC in 20-60cm soil layer were higher than 25. The soil C: P ratios were WM > AG > PC in 20-60cm soil layers. The results indicated that AG was beneficial to microbial decomposition of soil organic matter and improvement of soil P availability, but it also increased the risk of C emissions, while WM was beneficial to soil C sequestration in 40-60cm soil layer. In addition, PC was beneficial to soil C sequestration in 20-60cm soil layer, which was caused by no disturbance of PC to soil (Topa et al., 2021). Soil N: P ratio can be used as an effective index to measure plant nutrient limitation. With the increase of soil N: P ratios, the limitation of P on plant growth also increases (Xiao et al., 2021). In this study, soil N: P ratios were AG > WM > PC and were less than 14. The results showed that when the soil N: P ratio was less than 14, there was nitrogen limitation in plants (Xi and Jiang, 2019). Therefore, the aboveground vegetation growth of the three land use patterns in the YRD was limited by N, and AG was more limited by P than WM and PC.
In this study, the average soil stoichiometric ratios of AG, WM, and PC were 31.32: 1.74: 1, 34.05: 1.46: 1, and 26.58: 1.14: 1, respectively. Compared with the soil stoichiometry in the world of grassland (C: N: P=169: 12: 1) (Cleveland and Liptzin, 2007) and farmland (C: N: P=64: 5: 1) (Zheng et al., 2021), it showed remarkably narrow C: N: P ratios in the YRD, which was due to lower C and N levels. Meanwhile, there was little difference between AG and WM, which indicated that the fixation effect of AG on soil C was far lower than expected. This is due to a large amount of human disturbance in AG, such as mowing, which will lead to the loss of soil C (Wang et al., 2020). Compared with the average soil stoichiometry of China (C: N: P=60: 5: 1) (Tian et al., 2010), AG, WM, and PC showed lower C and N contents, while the N content of AG was closer to the average level of China. Therefore, N limitation is a common problem in the YRD. Among the three land use patterns, AG is more conducive to alleviate the N limitation.