Long-term afforestation strongly changes the soil’s physicochemical and biological properties. However, the underlying mechanism of different tree species driving change in soil nutrients is still unclear in the long-term dryland plantations of the Loess Plateau, China. In this study, samples of surface soil (0–20 cm) and woody litter were collected from five plantations (≥50 years) of Caragana korshinskii, Armeniaca sibirica, Populus hopeiensis, Platycladus orientalis, and Pinus tabulaeformis and a natural grassland, and tested for the carbon, nitrogen, phosphorus, and potassium contents, as well as the soil sucrase (SC), urease (UE), and alkaline phosphorus (ALP) activities. We found that soil nutrients, enzyme activities, and the litter’s chemical properties obviously varied among five tree species. C. korshinskii significantly increased the soil’s TC, organic carbon (OC), total nitrogen (TN), available nitrogen (AN), and available potassium (AK) by 28.42%, 56.08%, 57.41%, 107.25%, and 10.29%, respectively, and also increased the soil’s available phosphorus (AP) by 18.56%; while P. orientalis significantly decreased soil TN (38.89%), TP (30.58%), AP (76.39%), TK (8.25%), and AK (8.33%), and also decreased soil OC (18.01%) and AN (1.09%), compared with those in grassland. The C. korshinskii plantation had higher quality litter and soil enzyme activities than the P. orientalis plantation. Moreover, 62.2% of the total variation in soil nutrients was explained by the litter’s chemical properties and soil enzyme activities, and the litter phosphorus (LP) and soil ALP had a more significant and positive impact on soil nutrients. Therefore, tree species, LP, and soil ALP were key factors driving soil nutrient succession in dryland plantations. The significantly positive nitrogen–phosphorus coupling relationship in the “litter–enzyme–soil” system revealed that the improving nitrogen level promoted the phosphorus cycle of the plantation ecosystem. Our results suggest that leguminous tree species are more suitable for dryland afforestation through the regulation of litter quality and soil enzyme activities.

As an important means of curbing soil degradation, afforestation has a profound impact on regional soil properties and quality. However, it is still unclear regarding how to conduct a systematic assessment of soil properties and soil quality and the impact of vegetation characteristics and plant functional traits in leguminous plantations with different afforestation years in drylands. Therefore, we investigated the vegetation characteristics and determined the functional traits of leaves and roots and the soil physicochemical properties of Caragana korshinskii plantations with 13, 35, and 55 years. The results showed that tree height; crown diameter (CD); root dry matter content; root water content; soil clay, silt, and sand contents; bulk density (BD); soil water content; soil organic carbon (SOC); total nitrogen (TN); available nitrogen; total phosphorus (TP); available phosphorus (AP); and soil quality index (SQI) changed significantly with an increase in afforestation years. Although the specific leaf area did not show a significant variation, it had a significant negative effect on soil properties and SQI, except for soil sand and BD. Soil sand and BD decreased with the afforestation succession, but the succession pattern of soil clay, silt, SOC, TN, TP, and AP was 13 years < 35 years < 55 years, and SQI increased from 0.20 (13 years) to 0.77 (55 years). This indicated that long-term legume afforestation led to the transformation of soil texture from silty loam to silt and significantly improved the soil properties and quality in the study area.

Carbon sinks in terrestrial ecosystems can be significantly increased by afforestation, which will slow global warming. However, it is still unclear how different plantations influence the carbon sink and how they respond to environmental factors, especially in drylands. In this study, eddy correlation method (EC) was used to measure carbon and water fluxes and environmental factors of two artificial forests (Larix principis-rupprechtii and Pinus tabulaeformis) in the dryland of Northwest China, and the responses of evapotranspiration (ET), net ecosystem exchange (NEE), gross primary productivity (GPP), and ecosystem respiration (RECO) to environmental factors were also assessed. Results showed that the L. principis-rupprechtii forest ecosystem had higher water use efficiency (WUE), light use efficiency (LUE), GPP, and RECO than the P. tabulaeformis forest ecosystem. However, the proportion of net ecosystem production (NEP) to GPP in the P. tabulaeformis forest ecosystem (62.89%) was higher than that in the L. principis-rupprechtii forest ecosystem (47.49%), indicating that the P. tabulaeformis forest ecosystem had the higher carbon sequestration efficiency. In addition, the CO₂ and H₂O fluxes in the L. principis-rupprechtii forest ecosystem were more sensitive to environmental factors, compared with the P. tabulaeformis forest ecosystem. Further, the RECO of the L. principis-rupprechtii forest ecosystem was more sensitive to temperature changes, which implies that the L. principis-rupprechtii forest ecosystem will release more CO₂ than the P. tabulaeformis forest ecosystem with a warming climate. Therefore, the P. tabulaeformis forest ecosystem may have better carbon sequestration potential. These results are important for understanding the effects of climate change on the CO₂ and H₂O cycles in coniferous plantation ecosystems in drylands.

The environmental heterogeneity caused by altitude can lead to trade-offs in nutrient utilization and allocation strategies among plant organs; however, there is still a lack of research on the nutrient variation in the “flower–leaf–branch–fine root–soil” systems of native shrubs along altitude gradients in China’s unique karst regions. Therefore, we analyzed the carbon (C), nitrogen (N), and phosphorus (P) contents and their ratios in flowers, leaves, branches, fine roots, and surface soil of Hypericum kouytchense shrubs across 2200–2700 m altitudinal range in southwestern China’s karst areas, where this species is widely distributed and grows well. The results show that H. kouytchense organs had higher N content than both global and Chinese plant averages. The order of C:N:P value across plant organs was branches > fine roots > flowers > leaves. Altitude significantly affected the nutrient dynamics in plant organs and soil. With increasing altitude, P content in plant organs exhibited a significant concave pattern, leading to unimodal trends in the C:P of plant organs, as well as the N:P of leaves and fine roots. Meanwhile, plant organs except branches displayed significant homeostasis coefficients in C:P and fine root P, indicating a shift in H. kouytchense’s P utilization strategy from acquisitive-type to conservative-type. Strong positive relationships between plant organs and soil P and available P revealed that P was the key driver of nutrient cycling in H. kouytchense shrubs, enhancing plant organ–soil coupling relationships. In conclusion, H. kouytchense demonstrates flexible adaptability, suggesting that future vegetation restoration and conservation management projects in karst ecosystems should consider the nutrient adaptation strategies of different species, paying particular attention to P utilization.