This study aimed to determine the effects of the nitrogen (N) application period and level on the fate of fertilizer N and the contribution of N absorption and translocation to apple organ N. Two N application periods (labeled by the ¹⁵N tracer technique in spring and summer, represented by SP and SU, respectively) and three N levels (N0, MN, and HN) were used to determine the physiological indexes and aboveground, root, and soil ¹⁵N content of 4-year-old dwarf (‘Red Fuji’/M9T337) and arborized (‘Red Fuji’/Malus hupehensis Rehd.) apple trees. The results showed that HN led to shoot overgrowth, which was not conducive to the growth of the apple root system (root length, root tips, root surface area, and root volume) or the improvement of root activity. The contribution of soil N to apple organ N accounted for more than 50%, and the contribution of N application in summer to fruit N was higher than that in spring. Under HN treatment, the proportion of soil N absorbed by trees decreased, while that of fertilizer N increased; however, the highest proportion was still less than 50%, so apple trees were highly dependent on soil N. Under MN treatment, fertilizer N residue was similar to soil N consumption, and soil N fertility maintained a basic balance. Under HN treatment, fertilizer N residue was significantly higher than soil N consumption, indicating that excessive N application increased fertilizer N residue in the soil. Overall, the ¹⁵N utilization rate of arborized trees (17.33–22.38%) was higher than that of dwarf trees (12.89–16.91%). A total of 12.89–22.38% of fertilizer ¹⁵N was absorbed by trees, 30.37–35.41% of fertilizer ¹⁵N remained in the soil, and 44.65–54.46% of fertilizer ¹⁵N was lost. The ¹⁵N utilization rate and ¹⁵N residual rate of summer N application were higher than those of spring N application, and the ¹⁵N loss rate was lower than that of spring N application. High microbial biomass N (MBN) may be one of the reasons for the high N utilization rate and the low loss rate of N application in summer.

Velvet ash (Fraxinus velutina Torr.) is a dioecious tree species, which is widely used as a part of urban greeneries in saline land of North China. Female and male trees have different nutrient allocation trade-offs in dioecious species. As the fruit production consumes a lot of nutrients, female F. velutina plants grow slowly and are vulnerable to insects and diseases. Ecological stoichiometry can be used to study the physiological mechanism of the growth difference between female and male plants. The purpose of this study was to compare the seasonal patterns of C, N and P stoichiometry and their trade-offs in different organs of female and male F. velutina plants planted in urban green spaces. The fruit C, N and P contents of female F. velutina plants were all lower than those of leaves in the early growing season, but higher than those of leaves in the middle and late growing season. During most months, the leaf C and P contents of females were higher than those of males, while the leaf N content was lower than that of males, which was consistent with the sex-specific resource requirements for reproduction (i.e., high carbon requirements for ovules and high nitrogen demands for pollen). Compared to the females, there were more significant correlations between the stoichiometric indices (element contents and their ratios) of branches and leaves in male plants, and this difference may be related to the fact that the male plants were not involved in nutritional allocation for fruits. The leaf N/P of F. velutina was lower than 14 in the whole growing season, which indicated N limitation. The female and male plants of F. velutina had different sex-specific resource requirements for sex organ formation.

Coastal saline–alkaline land has considerable potential for soil carbon sequestration, but how different land-use types affect microbial resource limitation and carbon-use efficiency (CUE) in coastal saline–alkaline soils remains unclear. Four representative land-use types, namely bare land (BL), wetland (WL), grassland (GL), and forest land (FL), were investigated in the coastal saline–alkaline soils of the Yellow River Delta. Soil physicochemical properties, microbial biomass, and extracellular enzyme activities were measured, and ecoenzymatic stoichiometry, microbial resource limitation, and CUE were subsequently calculated. Compared with BL, vegetated land-use types decreased electrical conductivity by 52.1–95.8%, while soil water content, soil nutrient indicators, and microbial biomass indicators increased by 47.1–77.5%, 2.6–136.8%, and 2.2–274.4%, respectively. WL was mainly phosphorus-limited, whereas BL, GL, and FL were primarily nitrogen-limited. Despite relatively high soil organic carbon and nutrient availability, GL showed the strongest N limitation and was the only land-use type showing C limitation. Model-estimated CUE ranged from 0.544 to 0.579 and followed the order FL > BL > WL > GL. Random forest analysis showed that soil physicochemical properties contributed most to CUE variation (42.9%). Structural equation modeling further indicated that soil physicochemical properties were indirectly associated with CUE, mainly through stoichiometric characteristics and microbial resource limitation, showing positive and negative associations, respectively. These findings provide microbial evidence for optimizing land-use patterns, vegetation restoration, and carbon-oriented ecological restoration in coastal saline–alkaline land.

To investigate the combined effects of a reduced nitrogen application rate and biochar amendment on fruit yield and quality, photosynthetic carbon allocation, and soil carbon retention in apple orchards, a two-year field experiment was conducted using Malus domestica Borkh. cv. ‘Red Fuji’/M9T337. Six treatments were established: three nitrogen levels (conventional nitrogen [TN], 20% nitrogen reduction [TN80%], 40% nitrogen reduction [TN60%]) with and without biochar amendment (+B). The TN80%+B treatment led to the highest anthocyanin, soluble solid, and soluble sugar values and sugar/acid ratio. The yield and economic benefits of TN80%+B were not significantly different from those of TN, while its agronomic efficiency was the highest among all treatments. Moderate nitrogen reduction combined with biochar increased ¹³C accumulation in fruits and roots, enhanced the ¹³C distribution rate in fruits (41% in TN80%+B), and promoted the transfer of photosynthetic carbon from aboveground parts to roots and soil. Soil organic carbon, microbial biomass carbon, particulate organic carbon, mineral-associated organic carbon, and their corresponding ¹³C-labeled carbon fractions all increased following application of nitrogen reduction combined with biochar, peaking under the TN80%+B treatment. Soil CO₂ emissions decreased with declining nitrogen inputs, with the lowest cumulative emissions observed under TN60%+B (1937.31 kg C ha⁻¹). Biochar also suppressed cellobiohydrolase activity and cbh abundance while enhancing α-glucosidase, β-glucosidase, and carbon-fixation gene abundances (cbbL, accA, and aclA). Overall, TN80%+B provides the most favorable balance among outcomes, delivering concurrent benefits of improved fruit quality, stable yield, enhanced soil carbon retention, and reduced CO₂ emissions.