Does Biochar Particle Size, Application Rate and Irrigation Regime Interact to Affect Soil Water Holding Capacity, Maize Growth and Nutrient Uptake?

The purpose of this study was to investigate the sole and combined effects of biochar particle size (BPS), biochar application rate (BAR) and irrigation (IRR) on soil water holding capacity, maize total dry matter (TDM) and maize uptake of nitrogen (N) and phosphorus (P) in a sandy clay loam soil. Two biochar particle sizes of < 2 mm and 2–4 mm in combination with four biochar application rates of 0 t ha⁻¹ (B0), 20 t ha⁻¹ (B20), 40 t ha⁻¹ (B40) and 80 t ha⁻¹ (B80) were tested under either well-irrigated (I_w) or intermittent drought (D_i) conditions. The BPS acting alone or in synergy with IRR and BAR did not significantly affect pot field capacity water content (FC_pot), maize TDM, N and P uptakes. However, IRR and BAR significantly interacted to affect P uptake but not N uptake and FC_pot. The B80 treatment recorded significantly higher TDM, N uptake and P uptake values of 119 g pot⁻¹, 1833 mg pot⁻¹ and 501 mg pot⁻¹, compared with the counterpart B0 values of 99 g pot⁻¹, 1487 mg pot⁻¹ and 429 mg pot⁻¹, respectively. The B20 and B40 treatments recorded intermediate values of the TDM, N and P uptakes, and they were not significantly different from the counterpart variables for the B0 treatment. The I_w treatment increased maize TDM, N and P uptakes by 28, 24 and 30%, respectively, relative to the D_i treatment. Altogether, well-irrigated maize grown in sandy clay loam soil amended with either < 2 mm or 2–4 mm sized biochar, applied at 80 t ha⁻¹, is likely to improve TDM as well as N and P uptakes.