Effect of biochar amendment on nitrate retention in a silty clay loam soil

Introduction

Nitrate is highly soluble in soil water and is only weakly held by many soil particles. It can therefore move below the active root zone with percolating water, reducing nitrogen-use efficiency and creating a potential pathway to aquatic systems. Improving nitrate retention could support soil fertility and more efficient crop nutrition.

Biochar is a carbon-rich solid produced when biomass is processed through pyrolysis or gasification. Research has examined its potential effects on carbon storage, soil physical properties, microbial activity, water retention and nutrient availability. Its influence on nitrate, however, varies with the feedstock, production temperature, soil and application rate.

Some experiments have reported lower nitrate movement after biochar application, while others found little adsorption. Proposed mechanisms include physical entrapment in pores, interaction with positively charged surface sites, changes in nitrification and greater water retention. This study therefore evaluated a specific combination of fir-wood biochar and silty clay loam before considering field application.

Materials and methods

Soil and biochar

Topsoil from the 0–20 cm layer of an agricultural field in southern Italy was characterised before testing. It contained 27.6% clay, 55.8% silt and 16.6% sand. Field capacity was 38.0% of dry weight and wilting point was 20.3%. The soil was alkaline, with pH 8.4 and electrical conductivity of 0.3 dS m⁻¹.

The commercial biochar was produced by high-temperature pyrogasification of fir-wood chips. Analysis recorded pH 9.4, fixed carbon of 88.0%, volatile solids of 8.7% and ash of 3.3%. Carbon represented 83.2% of its mass. Both soil and biochar were air-dried and passed through a 1 mm sieve before mixing.

Biochar was applied at 2%, 4% and 8% of dry soil weight. These treatments were identified as B2, B4 and B8, with unamended soil serving as control B0. The mixtures were held at 22±0.5°C until use.

Nitrate sorption experiments

Each treatment was exposed to nitrate solutions at 5, 10, 50, 100, 200, 300 and 400 mg L⁻¹. Soil samples were mixed with solution for 24 hours, filtered and analysed by ion-exchange chromatography. Experiments were completed in duplicate.

The amount of nitrate adsorbed at equilibrium was calculated as:

Here Q_e represents nitrate adsorbed per unit mass, V is liquid volume, W is sample mass, C_i is the initial nitrate concentration and C_e is the concentration at equilibrium.

The relationship between adsorbed and dissolved nitrate was described with the Freundlich model:

K_f describes initial adsorption capacity and 1/n represents sorption intensity. Log-transformed results were assessed with analysis of covariance to compare regression intercepts and slopes across treatments.

Results and discussion

Nitrate retention was higher in every soil–biochar mixture than in the unamended control throughout the tested concentration range. The B2 and B4 treatments were not statistically distinct from one another and showed the greatest adsorption capacity. B8 differed from the two lower-rate treatments and did not deliver a proportional increase in retention.

The fitted lines converged as equilibrium nitrate increased, indicating that the relative retention benefit was most evident at lower solution concentrations. This pattern supports the interpretation that amendment rate and nitrate concentration interact rather than producing a simple dose-dependent response.

High-temperature biochar can have a large surface area and may develop sites that interact with nitrate. Nitrate can also move into biochar pores or associate with functional groups and positively charged cations at the surface. The findings are consistent with research showing some nitrate adsorption by selected high-temperature biochars, but they do not imply that every biochar will behave similarly.

Model coefficients

Treatments sharing the same grouping in the reported analysis were not significantly different at P≤0.05. RMSE means root mean square error.

Conclusions

The laboratory assessment found that fir-wood biochar improved nitrate retention in the tested silty clay loam. Amendments of 2% and 4% produced the strongest adsorption response; increasing the rate to 8% did not further improve the result.

These findings are preliminary and specific to the tested material and soil. Longer-term field experiments are necessary to examine how biochar properties, soil conditions, water movement and crop management interact under practical conditions.

Selected references

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