# Afforestation Enhances Soil Ecosystem Multifunctionality by Improving Soil Quality and Enzyme Activities in Coastal Saline–Alkali Land

## TL;DR

Planting trees in salty coastal areas improves soil health and ecosystem functions by boosting soil quality and enzyme activity.

## Contribution

This study reveals how afforestation improves soil quality and ecosystem multifunctionality in saline–alkali coastal regions.

## Key findings

- Afforestation increased soil quality index by up to 184% in 36-year-old stands.
- Soil enzyme activities for carbon, nitrogen, and phosphorus acquisition improved with afforestation.
- Soil salinity and nutrients were the strongest predictors of ecosystem multifunctionality.

## Abstract

Afforestation is widely recognized as an effective strategy for enhancing soil properties and ecosystem functions in coastal saline–alkali regions. However, the dynamic relationship between soil quality and ecosystem multifunctionality, as well as the underlying mechanisms driving this relationship, remains poorly understood. To fill this knowledge gap, we investigated four stand ages (6, 12, 22, and 36 years) of Robinia pseudoacacia in the Yellow River Delta, a representative coastal ecosystem in China. Our findings indicate that afforestation significantly enhanced soil quality compared to non-afforested sites. Afforestation also stimulated the activities of carbon-, nitrogen-, phosphorus-acquiring enzymes, and alleviated microbial N limitation with stand age. Additionally, EMF showed significant improvement across the four stand ages. Notably, we found a strong and positive correlation between SQI and EMF at all stand ages, with soil salinity and nutrients emerging as the most significant predictors of EMF. These results provide useful guidance and scientific support for improving forest management and ecological restoration efforts in this region.

Afforestation is critical for enhancing soil quality and optimizing land use in coastal saline–alkali regions. However, the dynamics of soil quality and ecosystem multifunctionality (EMF) during afforestation, as well as their coupling relationships, remain poorly understood. In this study, the effects of four stand ages (6, 12, 22, and 36 years) of a leguminous tree species (Robinia pseudoacacia) on the soil quality index (SQI), enzymatic stoichiometry, and soil ecosystem multifunctionality (EMF) were investigated in coastal saline–alkaline lands of the Yellow River Delta, China. Results showed that compared to non-afforestation, afforestation increased the SQI by 81%, 74%, 146%, and 184% in the 6-, 12-, 22-, and 36-year-old stands, respectively. Concurrently, afforestation increased the activities of the carbon acquisition (Cacq), nitrogen acquisition (Nacq), and phosphorus acquisition (Pacq) enzymes and alleviated microbial N limitation. Afforestation increased the soil EMF by 182%, 243%, 263%, and 295% in the 6-, 12-, 22-, and 36-year-old stands, respectively. The SQI was significantly positively correlated with soil EMF, regardless of afforestation age. Soil salinity, SOC, TN, and TP were the best predictors of soil EMF. The afforestation of Robinia pseudoacacia improved the soil quality and stimulated enzyme activity, thereby increasing the soil EMF. Our findings provide a theoretical foundation for the sustainable management of stands in coastal saline–alkali lands.