Biochar and urea co-application regulates nitrogen availability in soil.

Overuse of nitrogenous fertilizers especially urea has been considered a significant source of reactive N causing acute environmental impacts through leaching, volatilization, and N2O gas emission from fertilized crop fields. However, some recent studies have proposed that such environmental losses of N can be ignored by adapting an alternative way of combining nitrogenous fertilizer with pyrolyzed biomass (biochar). Therefore, the effect of co-application of rice-residue biochar (RB) or poultry manure biochar (PB) along with urea on N dynamics was investigated by conducting a 60-day incubation experiment. The results showed that urea led to greater N mineralization (0.2 µg N g-1 soil day-1) due to the easy availability of ammonical-N (NH4+-N) produced from hydrolysis that acted as a substrate for nitrification. Sole application of biochars (RB or PB) or their co-application with urea resulted in 38-45% and 19-28% lower N mineralization than the sole urea amended soil, respectively. The lower N mineralization in sole biochar or biochar plus urea amended soil was most likely caused due to (1) increased C:N ratio of the biochar amended soil, (2) adsorption of NH4+-N by biochar, (3) microbial immobilization of the nitrogen in the amended soil, and (4) lower urease activity in the treatments amended with biochar. Thus, it may be concluded that the co-application of biochar with urea can reduce N losses through moderation of N mineralization and make it available to plants for longer periods.

Sorption of dissolved organic matter in salt-affected soils: effect of salinity, sodicity and texture.

Loss of dissolved organic matter (DOM) from soils can have negative effects on soil fertility and water quality. It is known that sodicity increases DOM solubility, but the interactive effect of sodicity and salinity on DOM sorption and how this is affected by soil texture is not clear. We investigated the effect of salinity and sodicity on DOM sorption in soils with different clay contents. Four salt solutions with different EC and SAR were prepared using combinations of 1M NaCl and 1M CaCl(2) stock solutions. The soils differing in texture (4, 13, 24 and 40% clay, termed S-4, S-13, S-24 and S-40) were repeatedly leached with these solutions until the desired combination of EC and SAR (EC(1:5) 1 and 5dSm(-1) in combination with SAR <3 or >20) was reached. The sorption of DOC (derived from mature wheat straw) was more strongly affected by SAR than by EC. High SAR (>20) at EC1 significantly decreased sorption in all soils. However, at EC5, high SAR did not significantly reduce DOC sorption most likely because of the high electrolyte concentration of the soil solution. DOC sorption was greatest in S-24 (which had the highest CEC) at all concentrations of DOC added whereas DOC sorption did not differ greatly between S-40 and S-4 or S-13 (which had higher concentrations of Fe/Al than S-40). DOC sorption in salt-affected soil is more strongly controlled by CEC and Fe/Al concentration than by clay concentration per se except in sodic soils where DOC sorption is low due to the high sodium saturation of the exchange complex.