Synthesis of struvite-enriched slow-release fertilizer using magnesium-modified biochar: Desorption and leaching mechanisms

To improve the retention and slow-release abilities of nitrogen (N) and phosphorus (P), an 82 %-purity struvite fertilizer (MAP-BC) was synthesized using magnesium-modified biochar and a solution with a 2:1 concentration ratio of NH4+ to PO43- at a pH of 8. Batch microscopic characterizations and soil column leaching experiments were conducted to study the retention and slow-release mechanisms and desorption kinetics of MAP-BC. The slow-release mechanism revealed that the dissolution rate of high-purity struvite was the dominant factor of NP slow release. The re-adsorption of NH4+ and PO43- by biochar and unconsumed MgO prolonged slow release. Mg2+ ionized by MgO could react with PO43- released from struvite to form Mg3(PO4)2. The internal biochar exhibited electrostatic attraction and pore restriction towards NH4+, while magnesium modification and nutrient loading formed a physical antioxidant barrier that ensured long-term release. The water diffusion experiment showed a higher cumulative release rate for PO43- compared to NH4+, whereas in soil column leaching, the trend was reversed, suggesting that soil's competitive adsorption facilitated the desorption of NH4+ from MAP-BC. During soil leaching, cumulative release rates of NH4+ and PO43- from chemical fertilizers were 3.55-3.62 times faster than those from MAP-BC. The dynamic test data for NH4+ and PO43- in MAP-BC fitted the Ritger-Peppas model best, predicting release periods of 163 days and 166 days, respectively. The leaching performances showed that MAP-BC reduced leaching solution volume by 5.58 % and significantly increased soil large aggregates content larger than 0.25 mm by 24.25 %. The soil nutrients retention and pH regulation by MAP-BC reduced leaching concentrations of NP. Furthermore, MAP-BC significantly enhanced plant growth, and it is more suitable as a NP source for long-term crops. Therefore, MAP-BC is expected to function as a long-term and slow-release fertilizer with the potential to minimize NP nutrient loss and replace part of quick-acting fertilizer.

Meat and bone meal biochar can effectively reduce chemical fertilizer requirements for crop production and impart competitive advantages to soil

Safe and effective circulation of nutrient-rich meat and bone meal (MBM) could become a carbon-based alternative to limited chemical fertilizers (CFs). Therefore, MBM biochars (MBMCs) were produced at 500, 800, and 1000 °C to evaluate their effects on plant growth, nutrient uptake, and soil characteristics. The results revealed that MBMC produced at 500 °C (MBMC500) contained the maximum amount of C, N, and phytoavailable P. All additional MBMC doses with recommended CF increased sorghum shoot yield (6.7-16%) and significantly improved P uptake. Additional experiments were conducted with decreasing doses of CF (100-0%) with or without MBMC500 (7 t/ha) to quantify its actual fertilizing value. MBMC500 showed the capability to reduce CF requirement by 20% without compromising the optimum yield (by 100% CF) while increasing pH, CEC, total-N, available-P, Mg, and microbial population of post-harvest soil. Although a δ¹⁵N analysis confirmed MBMC500 as a source of plant N, a reduction in N uptake by MBMC500 + 80% CF treatment compared to 100% CF might have limited further sorghum growth. Thus, future studies should concentrate on producing MBMC with better N utilization capability and achieving maximum CF reduction without negative environmental impacts.

Preparation of a new biochar-based microbial fertilizer: Nutrient release patterns and synergistic mechanisms to improve soil fertility

The contradiction between population growth and soil degradation has been increasingly prominent, such that novel fertilizers (e.g., biochar and microbial fertilizers) should be urgently developed. Biochar is a promising fertilizer carrier for microbial fertilizers due to its porous structure. However, the preparation and mechanisms of the effects of biochar-based microbial fertilizers have been rarely investigated. In this study, biochar, Bacillus, and exogenous N-P-K fertilizers served as the raw materials to prepare biochar-based microbial fertilizers (BCMFs) by optimizing the preparation methods and the process parameters. Moreover, the release patterns of N-P-K were analyzed. A pot experiment was performed on pakchoi to examine the effect of the BCMFs and explore its synergistic effect on soil fertility. The results of this study indicated that adsorption by biochar maintained bacterial activity, whereas the granulation process reduced bacterial activity. The adsorption-granulation process increased the content of total nitrogen and organic matter in the soil while enhancing the slow-release effect of the BCMFs. The Elovich model was capable of describing the nitrogen release of the BCMFs, including the diffusion and chemical processes. As indicated by the result of the column leaching experiment, the BCMFs stopped nutrient leaching more significantly than the conventional fertilizers (CF), especially in stopping N and P leaching. The use of the BCMFs improved the available soil nutrients and soil quality while enhancing the abundance of bacteria correlated with carbon and nitrogen metabolism in the soil. Moreover, a 20 % reduction in the use of the BCMFs did not significantly affect the soil available N and P and the growth status of pakchoi. The result of redundancy analysis indicated that the cation exchange capacity (CEC), NH₄⁺-N, NO₃^--N, β-glucosidase (BG), urease (URE), and alkaline phosphatase (AlkP) were the six critical environmental factors for the microbial community structure and could explain 94.8 % of the variance. The BCMFs up-regulated the levels of the above six factors, especially CEC and BG, thus improving the soil quality and enhancing the soil fertility.

Effects of biochar-based fertilizer on nitrogen use efficiency and nitrogen losses via leaching and ammonia volatilization from an open vegetable field

It is essential for the sustainable development of agriculture to enhance nitrogen use efficiency (NUE) of crop plants by increasing yield and reducing nitrogen (N) losses. Biochar-based fertilizer (BF) has received increasing attention because of its full play to the advantages of chemical compounds with sufficient N and less N loss risk with good adsorption characteristics, but this potential was seldom reported for open-field vegetable crops, NUE of which were significantly lower than cereal crops. A field trial was conducted to investigate the efficacy of BF on NUE in vegetable cropping system by comparison with chemical fertilizer (CF) and partial substitution of organic fertilizers to chemical fertilizers (COF). The yield, plant N uptake, residual soil mineral N, and N losses via leaching and ammonia volatilization from an open vegetable (water spinach, Ipomoea aquatica L.) field were analyzed. The results indicated that BF treatment had significantly higher yield, plant N uptake, and NUE (agronomic efficiency and recovery efficiency as the NUE indicators), compared with those of CF and COF treatments. N losses via leaching were respectively accounted for 53.30%, 37.74%, and 33.39%; and N losses via ammonia volatilization were respectively accounting to 1.13%, 0.78%, and 1.54% of N fertilizer applied (at a rate of 200 kg N/ha) in CF, COF, and BF treatments. Despite the increasing ammonia volatilization due to the alkalinity of biochar, BF treatment significantly enhance NUE by increasing N uptake by water spinach and minimizing N losses via leaching. This study suggested that BF could serve as a promising slow-release N fertilizer for sustainable N management in field vegetable production and provided critical information for the development and dissemination of BF management guidelines.

Impacts of biochar-based fertilization on soil arbuscular mycorrhizal fungal community structure in a karst mountainous area

The application of biochar-based fertilizer can improve soil properties in part by stimulating microbial activity and growth. Karst ecosystems, which make up large areas of Southwest China, are prone to degradation. Understanding the response of arbuscular mycorrhizal fungal (AMF) community structure to biochar-based fertilizer application is of great significance to karst soil restoration. A field experiment was conducted in a typical karst soil (calcareous sandy loam) in Southwest China. A high-throughput sequencing approach was used to investigate the effect of biochar-based fertilization on AMF community structure in the karst soil. With the control (CK), compost with NPK fertilizer (MF), biochar (B), a lower amount of biochar with compost and NPK fertilizer (B1MF), biochar with compost and NPK fertilizer (BMF), and a higher amount of biochar with compost and NPK fertilizer (B4MF), the field trials were set up for 24 months. Soil amendments increased soil nutrient content and AMF diversity. The composition and structure of the AMF community varied among the treatments. AMF community composition was significantly impacted by soil chemical properties such as TC (total carbon), TN (total nitrogen), TP (total phosphorus), and AP (available phosphorus). Furthermore, network analysis showed that biochar-based fertilization increased the scale and complexity of the microbial co-occurrence network. Biochar-based fertilization enabled more keystone species (such as order Diversisporales and Glomerales) in the soil AMF network to participate in soil carbon resource management and soil nutrient cycling, indicating that biochar-based fertilizer is beneficial for the restoration of degraded karst soils.

Biochar-based fertilizer amendments improve the soil microbial community structure in a karst mountainous area

Biochar-based fertilizer amendment can improve soil properties partly due to stimulated microbial activities and growths. The karst ecosystem is prone to degradation and accounts for a large proportion of southwest China. Understanding of the response of the microbial community structure to biochar-based fertilizer application is of great significance in karst soil restoration. A field experiment located in southwest China was conducted in typical karst soil, and a high-throughput sequencing approach was used to investigate the effect of biochar-based fertilizer application on microbial community structure in karst soil. Field trials were set up for 24 months using the following treatments: control (CK), compost plus NPK fertilizer (MF), biochar (B), less biochar (half the quantity of biochar in B) plus compost and NPK fertilizer (B1MF), biochar plus compost and NPK fertilizer (BMF), and more biochar (double the quantity of biochar in B) plus compost and NPK fertilizer (B4MF). The results elucidated that BMF and B4MF treatments had higher contents of soil carbon and soil nutrients N, P, and K than the other treatments. Soil microbial abundance and diversity were significantly increased by biochar-based fertilizer amendments (BMF and B4MF), compared to CK (P < 0.05). BMF and B4MF treatments significantly increased the relative abundance of dominant microorganisms, compared to CK (P < 0.05). The difference in the composition of indicator microbes between each treated group indicated that soil amendments altered the microbial community structure. There was a strong correlation between soil properties (soil C-, N-, and P-fractions) and microbial community structure. Furthermore, network analysis revealed that the addition of biochar-based fertilizer increased the scale and complexity of the microbial co-occurrence network. To summarize, the application of biochar-based fertilizer enabled more keystone species in the soil microbial network to participate in soil carbon resource management and soil nutrient cycling, indicating that biochar-based fertilizer is beneficial for the restoration of karst-degraded soils.

A potential Mg-enriched biochar fertilizer: Excellent slow-release performance and release mechanism of nutrients

A potential Mg-enriched biochar fertilizer (MBF) was successfully synthesized via pyrolysis of MgCl₂-enriched corn straw and high-efficiency reclaiming of N- and P-containing nutrients from biogas effluent. Mathematical modeling and column leaching method demonstrated that the MBF exhibited excellent slow-release performances of total P and N with sustainable release rates. Leaching experiment indicated that the final accumulative release ratios of N and P from MBF were 7 times and 6 times lower than those of chemical fertilizer (CF), respectively. The mechanism study reveals that the P-release performance of MBF was not only controlled by the low solubility of MgP precipitates formed on the biochar surface, but also enhanced by the 'P-trap' effect of MgO through re-precipitation process of PO₄^3-. Meanwhile, the N-release behavior of MBF was dominated by the multi-effects of biochar carrier, including the confinement effect and electrostatic attraction for NH₄⁺, as well as the hydrogen bonds and pore-filling effect for N-containing organic matter. In addition, MBF significantly promoted the corn growth and enhanced the nutrient uptake efficiency of corn. These results suggested that MBF may therefore have promising potential in sustainable agriculture application with multiple environmental benefits.