Biochar-coupled organic fertilizer reduced soil water-dispersible colloidal phosphorus contents in agricultural fields

Colloid phosphorus transport in agricultural soils: a critical review of content characteristics, migration mechanisms, and environmental drivers

Soil phosphorus (P) loss through runoff is a significant contributor to waterbody eutrophication, particularly in regions of intensive agricultural production. One critical pathway for P loss is the migration of soil colloid-facilitated P (Pcoll), which warrants attention. However, the mechanisms underlying the generation, release, and migration of soil Pcoll remain poorly understood. This paper comprehensively reviews recent advancements in the technical methods for the extraction, separation, and physicochemical characterization of soil Pcoll. Utilizing data from published literature, we compile Pcoll contents across various soil types, with a focus on comparing Pcoll levels in agricultural soils characterized by different soil textures, fertilization treatments, and land use practices. Additionally, we analyze the relationships between soil main physicochemical parameters including pH, total organic carbon (TOC), total phosphorus (total P), and available phosphorus (available P), colloidal organic carbon (OC), calcium (Ca), magnesium (Mg), iron (Fe), and aluminum (Al) and Pcoll content. This review also summarizes the pathways of soil Pcoll loss, including surface runoff and subsurface runoff, along with the major influencing factors such as colloid properties, soil aggregates, pore structure, soil solution chemistry, crop planting structure, and fertilization management. Finally, we discuss future research directions concerning soil Pcoll, which include the establishment of standardized methods for Pcoll collection, a deeper understanding of Pcoll composition and distribution at the molecular level, investigations into the coupling effects of soil OC turnover and Pcoll cycling, and elucidating Pcoll loss mechanisms across multiple scales. Overall, this review aims to address existing gaps and shortcomings in soil Pcoll research and provide theoretical guidance for future studies.

Effects of irrigation and organic fertilizer on pumpkin yield, quality, and water-fertilizer use efficiency in arid northwest China

Due to the increasing water scarcity and the need for sustainable agricultural practices in arid regions, optimizing water and fertilizer management is crucial for enhancing crop productivity and resource efficiency. Field experiments in 2022 and 2023 in northwestern China's arid region explored the impacts of irrigation volume, organic fertilizer use, and their coupling on pumpkin yield, quality, and water-fertilizer efficiency. The study included ten treatments with a completely randomized two-factor design, comprising three irrigation quotas, three organic fertilizer application rates and a control group (CK). The results showed that the organic fertilizer application significantly enhanced soil moisture content, which peaked at a depth of 50 cm. Irrigation quota and organic fertilizer application had a highly significant impact on pumpkin vine length and stem diameter (P < 0.01), with a significant interaction between the two factors (P < 0.05). The rate of dry matter accumulation in pumpkin peaked at 60 ~ 80 days after sowing, with a trend of F2 > F3 > F1 in dry matter accumulation at identical irrigation quota. The effects of irrigation volume, organic fertilizer application and water-fertilizer coupling on pumpkin yield, irrigation water use efficiency (IWUE), partial fertilizer productivity (PFP) and pumpkin quality were statistically highly significant (P < 0.01). Specifically, increasing the irrigation volume from W1 to W3 increased the yield by 17.36%. However, pumpkin yield initially increased and then decreased in response to increasing organic fertilizer application. IWUE increased and then decreased with the increase of organic fertilizer application, while PFP increased with the increase of irrigation volume. Regression analysis revealed that the optimal range for irrigation quota to ensure pumpkin quality was 430 ~ 506 m3·ha-1, and that for organic fertilizer application was 5,373 ~ 6,570 kg·ha-1. When only quality indicators were considered, the W2F2 treatment performed well. However, from the comprehensive evaluation of pumpkin yield, quality, and water and fertilizer use efficiency using the TOPSIS method, the W3F2 treatment was identified as the most suitable among the water- fertilizer coupling management modes considered in this study for pumpkin cultivation in the arid northwestern China.

Biochar for ameliorating soil fertility and microbial diversity: From production to action of the black gold

This article evaluated different production strategies, characteristics, and applications of biochar for ameliorating soil fertility and microbial diversity. The biochar production techniques are evolving, indicating that newer methods (including hydrothermal and retort carbonization) operate with minimum temperatures, yet resulting in high yields with significant improvements in different properties, including heating value, oxygen functionality, and carbon content, compared to the traditional methods. It has been found that the temperature, feedstock type, and moisture content play critical roles in the fabrication process. The alkaline nature of biochar is attributed to surface functional groups and addresses soil acidity issues. The porous structure and oxygen-containing functional groups contribute to soil microbial adhesion, affecting soil health and nutrient availability, improving plant root morphology, photosynthetic pigments, enzyme activities, and growth even under salinity stress conditions. The review underscores the potential of biochar to address diverse agricultural challenges, emphasizing the need for further research and application-specific considerations.

Restricted colloidal-bound phosphorus release controlled by alternating flooding and drying cycles in an alkaline calcareous soil

Colloid-facilitated phosphorus (P) migration plays an important role in P loss from farmland to adjacent water bodies. However, the dynamics of colloidal P (Pcoll) release as influenced by irrigation in alkaline calcareous soil remains a knowledge gap. The present study, monitored the dynamic change of Pcoll under different water management strategies: 1) control, 2) flooding, and 3) alternating flooding and drying cycles. Soil water-dispersible colloids (0.6 nm-1 μm) were extracted by combining filtration and ultrafiltration methods. The contents of P, cation and organic carbon in the water-dispersible colloids were determined and the stability and mineral composition of colloidal fractions were characterized. The results showed that Pcoll ranged from 16.5 to 25.5 mg kg-1 and represented 42.8%-64.9% of the water-extracted P in the control. Flooding significantly decreased the Pcoll content by 16.0%-62.1% (mean 32.7%) and it may be attributed to the dissolution of colloidal iron (Fe) bound P. The alternating flooding and drying treatment significantly reduced the Pcoll content by 11.6%-88.0% (mean 67.6%). The Pcoll content of the flooding event was always greater than the Pcoll content of the drying event during flooding and drying cycles. Redundancy analysis and random forest modeling showed that the colloidal calcium (Ca) and ionic strength in soil solutions had negative correlations with the Pcoll content, and pH, ionic strength and truly dissolved P were the critical factors affecting Pcoll. Drying of the flooded soil led to the decrease of pH and the increase of ionic strength, colloidal Ca content and positive charges of colloid surfaces, which promoted colloid aggregation and enhanced soil P sorption capacity. This restricted the loss potential of Pcoll. In summary, controlled flooding and drainage when managed correctly have a role to play in mitigating Pcoll loss from P-enriched calcareous soils.

Promotion of phosphate release from humic acid-iron hydroxide coprecipitates in the presence of citric acid

Phosphate (P) resources are expected to be depleted within a century. Therefore, promoting balanced phosphorus fertilizer use and understanding phosphorus dynamics in soils containing iron (III), organic acids, and iron (III)-organic molecule particulates is crucial. This study investigated the sorption of citric acid onto humic acid-iron hydr(o)xide coprecipitate (HAFHCP) and the reciprocal effects of citric acid and P sorption on HAFHCP with different C/Fe ratios. The results showed that the maximum sorption capacity (MSC) of citric acid on HAFHCP decreased with increasing C/Fe ratios in the HAFHCP. The P sorption on HAFHCP pre-sorbed with citric acids (denoted as C-P) decreased by 50% compared with that of the MSC on FH. However, citric acids could only reduce P sorption by 20% when P was pre-sorbed on HAFHCP (denoted as P-C). The results suggested that upon the formation of HAFHCP, citric acids might increase P availability, especially in the C-P system. Although citric acids initially inhibited P sorption on HAFHCP in the P-C system, P sorption increased with prolonged reaction time. The exposures of new sorption sites upon dissolution of Fe from HAFHCP by citric acids or/and the formations of Fe bridge between P and organic domains of HAFHCP might contribute to these results. Additionally, a number of large HAFHCP aggregates became smaller while sorbing P due to the increasing electric repulsion on the surfaces of FH, enabling the subsequent dissolutions of more Fe by citric acids from HAFHCP in the P-C system. By integrating these innovative and sustainable strategies, the recycling and reuse of P can be optimized, thereby minimizing the reliance on synthetic fertilizers and mitigating environmental impacts. This approach fosters the efficient utilization of phosphorus resources, improves soil fertility, and enhances the overall resilience of agricultural systems and ecosystems.

Reduced colloidal phosphorus release from paddy soils: A synergistic effect of micro-/nano-sized biochars and intermittent anoxic condition

Colloidal phosphorus (CP) has high mobility and great loss risk; their biogeochemical processes are influenced by agricultural management such as redox oscillation and biochar-amendment application. This study monitored CP concentration in pore-water, soil P species and P adsorption capacity, to investigate CP release from paddy soils as affected by the interactive effects of oxygen status (continuous anoxic/oxic for 12 days, CA/CO; intermittent anoxic for 2, 4, 6, 8, 10 days during the 12-day cycle, IA2-10) and management (soil only, CK; bulk/micro/nano-sized biochar with various properties: SBBulk, SBMicro, and SBNano). Compared to the control (0.25-0.84 mg L-1, CK-CA), the single intermittent anoxic treatment (CK-IA) reduced CP concentrations by 45 %, due to the rise of Eh and pH and the decline of the degree of P saturation along with the increased soil Fe/Al-P and organic-P. Longer anoxic duration under the CK-IA reduced CP release, probably donated from massive production of redox-stable amorphous Fe/Al-bound P. The single biochar treatment (SB-CA: SBBulk-CA > SBMicro-CA > SBNano-CA) decreased CP release by 37 % as compared to the CK-CA, ascribed to the increased soil pH, Eh, and P adsorption capacity. The combined treatment (SB-IA: SBBulk-IA2 > SBNano-IA10) synergistically reduced CP release by 68 % in comparison with the CK-CA, due to the increase of adsorption through interactions of soil Fe/Al/Ca- and organic-P. Therefore, nano-sized biochar and long intermittent anoxic duration are recommended for reducing CP release from paddy soils.