Understory Plant Abundance Is More Important than Species Richness in Explaining Soil Nutrient Variation Following Afforestation on the Eastern Loess Plateau, China

Abstract

Afforestation plays an important role in controlling soil erosion and nutrient loss on the Loess Plateau in China. However, previous studies on soil nutrient changes have mainly focused on the effects of tree species, whereas little is known about how changes in shrubs and herbs caused by afforestation drive soil nutrient changes. In this study, we examined the variation characteristics of understory vegetation and soil nutrients for different vegetation types. The results showed that compared to abandoned farmland, plantations significantly increased soil organic carbon and total nitrogen but had no significant effect on total phosphorus. Robinia pseudoacacia L. forests were more effective than Pinus tabuliformis Carr. forests in increasing soil nutrient content. In addition, herbaceous vegetation in the R. pseudoacacia forest better explained the soil nutrient variation, and herb abundance was the best explanatory variable; however, shrub vegetation contributed more to soil nutrient variation in the P. tabuliformis forest, and shrub abundance contributed the most. Accordingly, we determined that understory plant abundance, rather than species richness, may be the most important factor driving soil nutrient changes. Specifically, herb abundance in the R. pseudoacacia forest may drive soil nutrient changes mainly by regulating herb biomass and litter biomass. By contrast, shrub abundance in the P. tabuliformis forest indirectly affected soil organic carbon mainly by altering shrub biomass. Furthermore, although the phylogenetic relationships had less effect on soil nutrients than species composition, they also made important contributions. Therefore, the phylogenetic relationships should also be considered in addition to species composition when assessing the impact of vegetation on soil properties in the future.