How biochar suspension influences soil microbial activity and phosphorus availability

Biochar (BC) application to soil is a method of long-term carbon (C) sequestration in croplands. Along with contribution to climate change mitigation, agronomic benefits of biochar are widely accepted. Amelioration and nutritive benefits of biochar are mainly attributed to high applications rates, which may not be viable for farmers. We suggest a new approach: band application, which implements biochar in modern intensive crop rotations and optimizes both C and nutrients cycles (i.e. phosphorus (P)).

As maize is globally one of the most widely planted crops, corncobs (CC) may be utilized for BC production. Corncob biochar (CC BC) may be implemented into the farming practice as suspension with liquid phosphorus solution jointly applied in band in the close proximity to seeds.

We conducted an incubation experiment to evaluate the short-term effects (within 32 days) of corncob biochar and inorganic P application on P availability and microbial activity in a loamy Luvisol.

Corncobs were pyrolyzed to biochar (350^oC; 0.2-0.3^oC s^-1), grinded (<200 µm), suspended and mixed with phosphate solution (as monopotassium phosphate). We compared liquid phosphorus (P) and joint P and BC application (BC+P) to control (water). The application rates were: 30 kg ha^-1 P (200 mg kg^-1 P) and 300 kg ha^-1 BC (0.2% w/w). Soil was sampled on day 7, 14, and 32 to measure pH, available P, enzyme kinetics, microbial biomass C, N, and P.

Despite the alkaline pH of BC (pH 8.7), application of BC+P and P decreased soil pH (5.6-5.7) compared to soil in control (5.8). Acidifying effect of suspensions was attributed to used phosphate source and may differ for others. BC+P did not alter soil P availability compared to solely P applied. Thus, dynamics of soil available P within 32 days was mainly attributed to soil processes but not the BC effect.

Higher basal respiration in amended soil compared to control within first 7 days indicates that physiological status of microorganisms was affected by P and BC+P application. But this was independent on changes in microbial biomass C and dissolved organic C. Microbial biomass-specific activity of beta-glucosidase and leucine aminopeptidase (but not acid phosphatase) in BC+P soil were lower compared to only P application, that may be caused by adsorbing properties of BC. The application BC+P increased the catalytic efficiency (Ka) of all enzymes compared to only P, which indicates the positive effect of biochar on enzymatic efficiency of soil.

Sole P application resulted in acceleration of P immobilization on day 32, as microbial molar C:N:P ratio for only P (18:2:1) differed from control and BC+P (35:3:1 and 36:4:1, accordingly). Principal Component Analysis revealed that BC+P treatment differed from the control and solely P applied at all time points.

To conclude, joint application of corncob biochar suspension and phosphorus solution may not only increase soil P availability, but prevent high P immobilization by microorganisms and facilitate higher enzyme efficiency to potentially increase nutrient availability for seedlings in the application bands.