Effects of Dodonaea viscosa Afforestation on Soil Nutrients and Aggregate Stability in Karst Graben Basin

Dodonaea viscosa is widely cultivated in the karst graben basin and is crucial for recovering land after rocky desertification. However, the effect of long–time D. viscosa afforestation on changes in the quality of soil remains unclear. Soil nutrients and aggregate composition can be used to evaluate the beneficial effects of afforestation of D. viscosa in improving soil functional stability. In this study, soil nutrients and aggregate stability were investigated using cropland, 10–year, 20–year, and 40–year D. viscosa afforestation and secondary succession shrub. Compared to the cropland, D. viscosa afforestation significantly increased the soil water content (WC), soil organic carbon (SOC), and total nitrogen (TN) contents, with an enhanced effect observed with prolonged afforestation. Soil nutrient contents under D. viscosa afforestation rapidly reached the level of the shrub. Dodonaea viscosa afforestation promoted the formation of >2 mm aggregates and decreased the ratio of 0.053–0.25 mm aggregates, which varied with afforestation years. Compared to the cropland, the content of >0.25 mm water–stable aggregates (R>0.25), mean weight diameter (MWD), and geometric mean weight diameter (GMD) of soil increased exponentially. However, soil erodibility factor (K) and unstable aggregates index (EIt) decreased exponentially with prolonged D. viscosa afforestation, and the latter two indicators did not reach the level of the shrub. These results indicated that soil nutrients, aggregate stability, and erosion resistance increased with prolonged D. viscosa afforestation. However, the aggregate stability and erosion resistance exhibited by D. viscosa could not reach the level of secondary shrub for a long time

Land Use Change from Natural Tropical Forests to Managed Ecosystems Reduces Gross Nitrogen Production Rates and Increases the Soil Microbial Nitrogen Limitation

Understanding the underlying mechanisms of soil microbial nitrogen (N) utilization under land use change is critical to evaluating soil N availability or limitation and its environmental consequences. A combination of soil gross N production and ecoenzymatic stoichiometry provides a promising avenue for nutrient limitation assessment in soil microbial metabolism. Gross N production via 15N tracing and ecoenzymatic stoichiometry through the vector and threshold element ratio (Vector-TER) model were quantified to evaluate the soil microbial N limitation in response to land use changes. We used tropical soil samples from a natural forest ecosystem and three managed ecosystems (paddy, rubber, and eucalyptus sites). Soil extracellular enzyme activities were significantly lower in managed ecosystems than in a natural forest. The Vector-TER model results indicated microbial carbon (C) and N limitations in the natural forest soil, and land use change from the natural forest to managed ecosystems increased the soil microbial N limitation. The soil microbial N limitation was positively related to gross N mineralization (GNM) and nitrification (GN) rates. The decrease in microbial biomass C and N as well as hydrolyzable ammonium N in managed ecosystems led to the decrease in N-acquiring enzymes, inhibiting GNM and GN rates and ultimately increasing the microbial N limitation. Soil GNM was also positively correlated with leucine aminopeptidase and β-N-acetylglucosaminidase. The results highlight that converting tropical natural forests to managed ecosystems can increase the soil microbial N limitation through reducing the soil microbial biomass and gross N production