Soil physical properties and soil organic carbon (SOC) are considered as important factors of soil quality. Arable land, grassland, and forest land coexist in the saline-alkali reclamation area of the Yellow River Delta (YRD), China. Such different land uses strongly influence the services of ecosystem to induce soil degradation and carbon loss. The objective of this study is to evaluate the variation of soil texture, aggregates stability, and soil carbon affected by land uses. For each land use unit, we collected soil samples from five replicated plots from “S” shape soil profiles to the depth of 50 cm (0–5, 5–10, 10–20, 20–30, and 30–50 cm). The results showed that the grassland had the lowest overall sand content of 39.98–59.34% in the top 50 cm soil profile. The content of soil aggregates > 0.25 mm (R0.25), mean weight diameter and geometric mean diameter were significantly higher in grassland than those of the arable and forest land. R0.25, aggregate stability in arable land in the top 30 cm were higher than that of forest land, but lower in the soil profile below 20 cm, likely due to different root distribution and agricultural practices. The carbon management index (CMI) was considered as the most effective indicator of soil quality. The overall SOC content and CMI in arable land were almost the lowest among three land use types. In combination with SOC, CMI and soil physical properties, we argued that alfalfa grassland had the advantage to promote soil quality compared with arable land and forest land. This result shed light on the variations of soil properties influenced by land uses and the importance to conduct proper land use for the long-term sustainability of the saline-alkali reclamation region.

Enhancing nitrogen (N) use efficiency (NUE) and reducing urea N losses are major challenges in ensuring sustainable agriculture. The aim of this study was to investigate the effect of humic acid urea on N losses, soil microbial nutrient balance and NUE through leaching experiments, soil incubation experiments and field experiments of maize-wheat rotation. We set up four N gradients (240 kg N hm−2, 216 kg N hm−2, 192 kg N hm−2, 168 kg N hm−2) and two N fertilizer types (urea and humic acid urea) to make up five treatments, with no N application as the control. The results showed that humic acid urea reduced the fertilizer N losses by 25.51%, 23.07% and 23.08% in the three pathways of N leaching, NH3 volatilization and N2O emission, respectively, compared with urea. Humic acid urea significantly increased soil ammonium N, nitrate N and available phosphorus contents, and brought the enzyme stoichiometry ratio closer to 1:1:1, which promoted microbial nutrient balance. Application of humic acid urea significantly increased yield, NUE and annual net economic profit of maize and wheat. Among all treatments, the application of humic acid urea at 216 kg N hm−2 maximized NUE, reduced environmental pollution and increased yield.

The ecological impact of nitrogen (N) deposition has gained significance since the advent of the industrial revolution. Although numerous studies have examined the impact of N deposition on soil organic carbon (SOC), certain arid and barren rocky mountainous regions, which experience more pronounced N limitations, have been overlooked. This study was conducted in the Yimeng Mountains, examining eight treatments created by four N addition levels (0 kg N ha−1 yr−1, 50 kg N ha−1 yr−1, 100 kg N ha−1 yr−1 and 200 kg N ha−1 yr−1) and two tree species (Quercus acutissima Carruth. and Pinus thunbergii Parl.). The research revealed variations in the effect of N addition on leaf litter decomposition and SOC density (SOCD) between different tree species. Notably, N addition stimulated the decomposition of leaf litter from Quercus acutissima Carruth. However, the decomposition of Pinus thunbergii Parl. leaf litter was enhanced at N addition levels below 100 kg N ha−1 yr−1, while it was hindered at levels exceeding 100 kg N ha−1 yr−1. In the Quercus acutissima Carruth. forest, the N addition levels of 50 kg N ha−1 yr−1, 100 kg N ha−1 yr−1 and 200 kg N ha−1 yr−1 resulted in decreases in SOCD by 10.57%, 22.22% and 13.66%, respectively, compared to 0 kg N ha−1 yr−1. In the Pinus thunbergii Parl. forest, the N addition levels of 50 kg N ha−1 yr−1, 100 kg N hm−2 ha−1 and 200 kg N ha−1 yr−1 led to increases in SOCD by 49.53%, 43.36% and 60.87%, respectively, compared to 0 kg N ha−1 yr−1. Overall, N addition decreases the SOCD of Quercus acutissima Carruth., but it increases the SOCD of Pinus thunbergii Parl., attributed to the alteration in soil enzyme stoichiometry and nutrient cycling by N addition. This study fills a theoretical gap concerning leaf litter decomposition and SOC sequestration in arid and barren rocky mountainous regions under global climate change.

The reclamation area of the Chinese Yellow River Delta (YRD) has experienced frequent land use changes in recent decades. The consequence of such land use changes on the stocks and allocation of ecosystem-scale carbon is not known. Here, we assessed carbon stocks and allocation of four representative land uses in the YRD area: (1) purple alfalfa (LAL), (2) reed and Aeluropus littoralis (RAE), (3) cotton (ECO), and (4) Chinese tamarisk (CTA). The results showed that the overall carbon stocks, and carbon stocks of aboveground, litter, roots, and soil were notably different among different land uses. The native CTA land had the largest overall carbon stock (belowground and aboveground) and had the strongest potential to allocate the carbon to the soil carbon pool (95.72%), followed by the natural grassland (RAE). Alfalfa grassland (LAL) also had a large carbon stock due to its large aboveground biomass, litter, and roots, but the relative allocation proportion of soil carbon stock was lower than that of the other land uses examined. Cotton (ECO) had the lowest soil carbon and total carbon stocks among four land uses. In combining our data on the carbon pool with changes of land use in the YRD area, we argued that land reclamation in the YRD area was likely to turn this area from a carbon sink to carbon source with the release of soil organic carbon. Therefore, cautions should be taken to reduce carbon release, if the reclaimed land be used to plant crops.