Biochar soil amendment has proven to be beneficial for soil fertility regulation, while the long-term mechanisms under different soil conditions remain largely unexplored. This study was based on two long-term field experiments in calcareous soils to explore the effect of biochar on fertility and phosphorus availability, as well as potential mechanisms. Experiments were conducted on Fluvo-aquic soil (6 years) and Cinnamon soil (5 years), respectively. Biochar was applied once at 20 t/ha level and mixed with the topsoil. We evaluated soil fertility and phosphorus fractions with and without biochar amendment and analyzed the related extracellular enzyme activity and functional gene abundance. The results indicate a persistent increase in crop yield by 10.8–24.3% in calcareous soils after 5–6 years of treatment. Biochar enhanced soil fertility, including increased SOC, available phosphorus, and potassium content. Moreover, it boosted the activity of extracellular enzymes related to soil carbon, nitrogen, and phosphorus cycling. This effect may be associated with the stability of morphological features and mineral element regulation on biochar surfaces. Biochar soil amendment led to a long-term increase in phosphorus availability, of which available phosphorus increased by 13.1–49.1%. Potential mechanisms included an increase in labile phosphorus forms and biological mechanisms of raising alkaline phosphatase activities and increasing the copy numbers of the phosphatase genes (phoD, phoX, and nifH). In conclusion, biochar soil amendment resulted in sustained improvement in soil fertility in calcareous soils and consistently increased phosphorus availability via biochemical mechanisms.

Greenhouse gas and NH₃ emissions are exacerbated by the inappropriate timing and excessive application of nitrogen (N) fertilizers in wheat cultivation in China. In this study, the impacts on N₂O, CO₂, and NH₃ emissions of a delayed and reduced N application regime on the Huang-Huai-Hai Plain were investigated. The treatments comprised the control (N₀), conventional N at 270 kg N ha⁻¹ (N₂₇₀) and optimized N application of 180 kg N ha⁻¹ (N₁₈₀), N₁₈₀ + biochar at 7.5 t ha⁻¹ (N₁₈₀B_7.5), N₁₈₀ + biochar at 15 t ha⁻¹ (N₁₈₀B₁₅), N₁₈₀ + DMPP (a nitrification inhibitor; N₁₈₀D), N₁₈₀D + biochar at 7.5 t ha⁻¹ (N₁₈₀DB_7.5), and N₁₈₀D + biochar at 15 t ha⁻¹ (N₁₈₀DB₁₅). Reduced N application (N₁₈₀) lowered N₂O and NH₃ emissions. Biochar application resulted in a 4–25% and 12–16% increase in N₂O and NH₃ emissions, respectively. Application of DMPP significantly decreased N₂O emissions by 32% while concurrently inducing a 9% increase in NH₃ emissions. Co-application of DMPP and biochar significantly reduced the activity of nitrification enzymes (HAD, NOO), resulting in a reduction of 37–38% in N₂O emissions and 13–14% in NH₃ emissions. No significant differences in CO₂ emissions were observed among the various N treatments except the N₀ treatment. Application of DMPP alone did not significantly affect grain yield. However, biochar, in combination with DMPP, effectively increases grain yield. The findings suggest that the N₁₈₀DB₁₅ treatment has the potential to reduce emissions of N₂O and NH₃ while concurrently enhancing soil fertility (pH, SOC) and wheat yield.

Nitrogen is a vital element for soil fertility and crop productivity. The transformation of nitrogen is directly affected by tillage practices for the disturbing soil. The characteristics of different nitrogen forms under different tillage modes are still unclear. A 3-year cycle tillage experiment was carried out to assess the combination of rotary tillage (RT), deep tillage (DT), and shallow rotary tillage (SRT) on nitrogen transformation and distribution, wheat yield and nitrogen balance in fluvo-aquic soil from Huang-Huai-Hai Plain in China. The results showed the rotation tillage cycle with deep tillage in the first year increased the total nitrogen (TN), and the main nitrogen form content in 0–30 cm compared with continued rotary tillage (RT-RT-RT). Moreover, the nitrate (NO₃⁻-N) and ammonium nitrogen (NH₄⁺-N) content were improved in 20–40 cm by deep tillage practice with the highest value as 39.88 mg kg⁻¹ under DT-SRT-RT. The time, tillage, and depth significantly affected the different nitrogen forms, but there was no effect on dissolved organic carbon (DON) and soil microbial biomass nitrogen (SMBN) by the interaction of time and tillage. Moreover, compared with RT-RT-RT, the rotation tillage promoted the spike number and kernels per spike of wheat, further increasing the wheat yield and nitrogen partial productivity, and with a better effect under DT-SRT-RT. The NO₃⁻-N and NH₄⁺-N trended closer and positively correlated with wheat yield in 0–40 cm in 2019. The rotation tillage with deep tillage improved the different forms of nitrogen in 0–30 cm, wheat yield, and nitrogen partial productivity, and decreased the apparent nitrogen loss. It was suggested as the efficiency tillage practice to improve nitrogen use efficiency and crop yield in this area.

Optimum tillage practices can create a suitable soil environment, and they improve the soil nutrient status to ensure crop development and yield. In this study, we evaluated the influences of six tillage practices on soil nutrients and maize yields from 2017 to 2019 in fluvo-aquic soil in the North China Plain. The field experiment was carried out by a split design with rotary tillage (RT) and deep tillage (DT) in wheat season in the main plot and no-tillage (NT), subsoiling between the row (SBR), and subsoiling in the row (SIR) in maize season in the subplot. The results showed that the soil nutrient content was higher under the treatments with rotary tillage in the wheat season in the 0–20 cm soil layer, while in the 20–40 cm soil layer, the soil nutrient content was higher under the treatments with deep tillage in the wheat season. The integrated principal component scores indicated that the soil nutrients had improved in the second year. The ecosystem multifunctionality (EMF) index was higher with the treatments with rotary tillage in wheat season in the 0–20 cm soil layer, while it was the highest under DT-SIR at 20–40 cm. Correlation analysis showed that the soil EMF index correlated significantly (p < 0.05) with the soil nutrient content mainly in the 0–40 cm soil layer. The higher maize yield was under the treatments with deep tillage compared to that under the treatments with rotary tillage in the wheat season. The yield-increasing effect was higher under the treatments with subsoiling than those utilizing no-till in the maize season, with the highest average yield of 13,910 kg hm⁻² in the DT-SIR during the three years. Maize yield was strongly correlated with nutrients in the subsoil layer. The higher yield stability was found under RT-NT. To sum up, during the three-year experiment, rotary tillage in the wheat season combined with subsoiling in the maize season improved the soil nutrient content and the EMF index in the 0–20 cm layer, while the combination of deep tillage in the wheat season and subsoiling in the maize season improved those indices in the 20–40 cm soil layer, and increased the maize yield, the best one was under DT-SIR.

Selenium is an important indicator for the evaluation of tea quality. However, the relationship between selenium uptake by tea plants (Camellia sinensis) and soil properties, as well as selenium transport and distribution in a tea plantation soil–tea plant–tea infusion production system, remain unclear. In this study, 12 tea plantations situated in a typical selenium-rich area of China were selected, and the characteristics and crucial factors influencing the uptake and transport of selenium were analyzed using a plantation soil–tea plant–tea infusion production system. The soil total selenium content ranged from 1.12 to 6.67 mg kg⁻¹, with an average of 2.57 mg kg⁻¹. The average available selenium content was 53.56 µg kg⁻¹, and the activation rate of soil selenium was 2.27%. Soil-available selenium was significantly positively correlated with total selenium, available potassium, and soil organic matter contents, and was significantly negatively correlated with soil pH. The selenium content in old leaves ranged from 0.29 to 2.73 mg kg⁻¹, which met the standard for selenium-rich tea, whereas only 33% of young leaves met this standard. The selenium enrichment factor was highest in the fibrous root and lowest in the young leaves. The average selenium transport factors from fibrous roots to main roots, from main roots to main stems, from main stems to lateral stems, from lateral stems to young leaves, and from lateral stems to old leaves were 0.53, 0.92, 0.67, 0.97, and 2.30, respectively. The selenium concentration of tea infusion ranged from 1.88 to 12.49 μg L⁻¹, and the average selenium dissolution rate was 22.62% after one brewing. This study identified critical factors that influence soil-selenium availability. The selenium content in tea plant organs is indicated to be strongly associated with the selenium content in the main roots.

Tea trees (Camellia sinensis) growing on selenium (Se)-rich soils often exhibit the phenomenon of cadmium (Cd) accumulation. However, the transport of Cd in the soil–tea tree–tea infusion continuum in such areas, as well as the impact of Se on Cd in this system, remains enigmatic. In this study, we investigated the migration of Cd from soil to tea tree and from tea tree to tea infusion, as well as the influence of Se on the Cd. The Cd content of the soil was 0.37 mg kg⁻¹, which was approximately 3.81 times higher than the background value. The average activation rate of soil Cd was 20.93%, and was significantly negatively correlated with soil pH and significantly positively correlated with available potassium. The Cd enrichment coefficients in tea tree organs showed a gradually decreasing trend from fibrous roots to taproots, lateral stems, main stems, old leaves, and young leaves. The Cd transport coefficients from fibrous roots to taproots, taproots to main stems, and main stems to lateral stems progressively increased, whereas from lateral stems to old and young leaves significantly decreased. The maximum potential carcinogenic health risk from Cd in the tea infusion was 1.60 × 10⁻⁷ to 5.03 × 10⁻⁷, thus drinking Se-enriched tea had a low health risk from Cd intake. Our findings revealed a notable threshold effect on the accumulation of Cd in fibrous roots. The primary factor contributing to the low Cd concentration in tea leaves lies in the reduced Cd transport efficiency from fibrous roots to taproots and from lateral stems to young leaves.