Low nutrient reserves and poor soil fertility management in coconut cultivation have limited coconut productivity. Biochar is one of the best solutions to improve soil nutrient reserves and fertility over time. This study explored the best rate of biochar application in coconut-growing soils to reduce leaching losses and macronutrient enrichment. Two major coconut-growing soil series; Madampe (Sandy) and Andigama (Sandy loam) were used in the experiment. Gliricidia wood biochar (BC) pyrolyzed at 700 °C was used to amend the 0−15 cm topsoil in PVC columns at the rates of 0% BC, 1% BC, 2% BC and 3% BC (w/w) with recommended doses of inorganic fertilizer. Columns were arranged in Completely Random Design (CRD) with three replicates and leached two weeks intervals over five months. Our findings revealed two distinct scenarios for sandy and sandy loam soils. Considering the retention of macronutrients in sandy loam soil treated with 2% and 3% biochar, the order of retention was P > K > Mg > Ca. However, soil N was not altered substantially (P > 0.05) with the amount of biochar amendment. Similarly, in sandy soil, 3% BC treatment demonstrated the sequence P > Mg > N = Ca, although soil K was not differed significantly (P > 0.05) with biochar amendment rate. Albeit a once-in-a-lifetime application of 3% biochar could be recommended on coconut-growing sandy soils, application of this rate would be unprofitable. Thus, the 2% rate will be more appropriate for both sandy and sandy loam soils. These application rates must be validated through long-term field experiments under major coconut-growing soils.

Climate change threatens long‐term soil health because of increased severity and frequency of drought periods. Applying biochar to soils before a drought can increase non‐biochar soil carbon (C) and water storage over the long term and sustain crop yield. However, the on‐farm benefit of buried solid biochar and applied liquid biochar at low rates remains uncertain. This study examined the effects of two novel biochar‐based soil amendments on soil C, water storage and crop yield. The biochar‐based amendments included a biochar reactive barrier (RB) made by layering wood‐based biochar, straw mulch and cow manure into a series of open surface trenches, and a liquid biochar mineral complex (BMC) applied twice, at low rate (200 kg ha⁻¹) to one side of RB (fertilised area), while the other side of RB received no treatments (non‐fertilised area). Moisture concentration within the RB ranged from 6.76% up to 56.68% after large rainfall, more than double the surrounding soils and gradually started migrating from the RB outwards. Soil within 50 cm distance of the RB showed a 24.5% increase in non‐biochar soil C compared with soil at 600 cm distance of the RB, 2.54% versus 2.04%, respectively, in the non‐fertilised area, which was supported with lowering soil microbial activity. Pasture yield increase was associated with liquid BMC fertiliser rather than proximity to the RB. Pasture yield was 44% higher in the fertilised area compared with the non‐fertilised area 27.89 t ha⁻¹ versus 19.31 t ha⁻¹. Approximately 158 kg CO₂e was removed from the atmosphere for each cubic meter of RB and an annual removal of 150 kg CO₂e ha⁻¹ was estimated by liquid BMC application. Income earned by increased yield was still profitable even though applied liquid BMC could cost between USD 400–520 ha⁻¹ including shipping costs. Overall, our study suggested biochar‐based RB and BMC fertilisers can effectively increase soil moisture retention while building non‐biochar soil C storage in the surrounding soil. The adoption of biochar‐based techniques has the potential to improve drought resilience while increasing soil C in wide range of non‐irrigated cropping systems.

Sustaining crop yield is a growing challenge due to soil fertility loss and climate change. As a result, fertiliser application is increasing to sustain yield and counteract low fertiliser use efficiency. Developing nutrient-efficient fertilisers, for example by combining biochar with fertilisers, is thus crucially needed. In this context, the development of biochar-mineral complex (BMC) fertilisers into liquefied products suitable for broadacre application is highly innovative. This study aimed to explore the agronomic benefits and economic viability of four novel liquefied BMCs designed for application within a pasture cropping system. The different forms of developed BMCs included two non-enriched BMCs (BMC1 and BMC2) that were micronised using different techniques, a phosphorus (P)-enriched BMC (BMC3), and a nitrogen (N)-enriched BMC (BMC4), all applied both with- and without co-fertilisation. Soil biochemistry, plant NP uptake, NP balances, plant yield and cost–benefit were examined. Pasture yield was significantly higher using BMC1 and BMC3 with co-fertilisation than without, most likely due to improved P availability, as evidenced by positive P balances. Yield was significantly enhanced under BMC4 at 42.20 t ha ⁻¹ , compared with BMC1 at 21.90 t ha ⁻¹ , BMC2 at 19.41 t ha ⁻¹ , BMC3 at 25.81 t ha ⁻¹ , the fertilised control at 18.75 t ha ⁻¹ and non-fertilised control at 11.53 t ha ⁻¹ . Only BMC4 showed positive NP balances, suggesting that pasture did not absorb N and P from the existing soil reserve. Soil microbial composition and abundance were not affected by the applied BMC treatments in short term. The benefit–cost ratio ranged from 1.94 to 2.54, indicating economic viability of the BMC products and suggesting a great potential of biochar-based fertilisers for wide adoption to other crops. Graphic abstract

Biochar mineral complex (BMC) can improve nutrient use efficiency (NUE) and reduce nutrient leaching, but the optimal application form and rate of application in soilless cropping systems remain unclear. This study evaluated co-applying BMC, either in solid form and placed to form a horizontal laminar layer approximately halfway up the potting media column, or as a liquid BMC suspension combined with reduced fertiliser rates in a protected ginger cropping system using pine bark potting media. Six treatments were examined: full-rate (15 t ha⁻¹) and half-rate (7.5 t ha⁻¹) organic fertiliser blend, BMC at high-rate (1 t ha⁻¹) and low-rate (0.5 t ha⁻¹) co-applied with half-rate fertiliser, BMC at high-rate without fertiliser, and BMC suspension at high-rate co-applied with half-rate fertiliser. Aboveground biomass, rhizome yield, nitrogen (N) and phosphorus (P) leaching, NUE, and net income were assessed. Fertiliser costs were reduced by 28% to 30% by applying half-rate fertiliser with or without BMC, respectively, and without decreasing rhizome yield, aboveground biomass, and NUE, compared to applying full-rate fertiliser. Co-applying solid BMC as a layer and at a low rate with half-rate fertiliser reduced N and P leaching without affecting yield or profitability. The BMC suspension showed no significant difference from full-rate fertiliser in yield and leaching outcomes while offering advantages for mechanised application in labour-intensive cropping systems. Overall, co-applying BMC either as a solid layer or as a liquid suspension with reduced fertiliser rates offers an economically and environmentally sustainable alternative to conventional fertiliser use in soilless cropping systems.

Nitrogen (N) and phosphorus (P) are two of the most essential nutrients for plants, but overapplication of N and P to cropping systems can lead to eutrophication and groundwater contamination through leaching. Biochar has been used to improve N and P use efficiency, reducing leaching, and the effects of biochar on N and P leaching and availability have been widely studied. Biochar is often applied with fertilisers; however, combined effects of biochar with fertilisers on N and P leaching and availability have not been adequately addressed. This meta‐analysis synthesised 1798 data points from 48 peer‐reviewed articles to evaluate the effects of applying biochar with fertilisers on N and P leaching and availability. Our results showed that applying biochar with fertilisers decreased NO ₃ ⁻ , NH ₄ ⁺ and P (as PO ₄ ³⁻ ) leaching by 13.2% ± 0.1%, 1.5% ± 0.1% and 16.5% ± 0.7%, respectively (Bootstrap 95% CI). Method of biochar application emerged as the primary factor affecting N and P leaching. For NO ₃ ⁻ leaching, time after biochar application was the next most important predictor, whereas NH ₄ ⁺ leaching was predicted by the rate of biochar application and soil organic C concentration. P leaching was moderated by the feedstock type and pyrolysis temperature used to make biochars. Applying biochar with fertilisers did not affect NH ₄ ⁺ and NO ₃ ⁻ availability but increased P availability by 21.9% ± 0.5%. This study provides strong empirical evidence that co‐applying biochar with fertilisers is an effective management practice that can reduce nutrient leaching and minimise environmental impacts of cropping systems.