Acid-modified cotton straw biochar has instructive for the improvement of saline-alkali soil

Soil salinization severely hinders the development of agriculture. Pristine biochar (BC) has a considerable impact on improving soil quality and maximizing crop yield. However, applying biochar has the risk of aggravating soil salinization. Acid-modified biochar may have a good effect on the improvement of saline-alkali soil.

Nitric acid and phosphoric acid were used to modify the pristine biochar (BC). The pristine biochar (BC), nitric acid-modified biochar (NC) and phosphoric acid-modified biochar (PC) were used at 1%, 2%, and 3% (w/w), respectively. There were ten treatments in total: CK, BC₁, BC₂, BC₃, NC₁, NC₂, NC₃, PC₁, PC₂, and PC₃.

Our results indicated that the application of biochar further aggravated soil salinization. Soil pH and EC significantly increased by 0.08–0.42 units and 215.51–362.07 μS.cm⁻¹, respectively. Additionally, the BC application reduced the activities of C-, N- and P-acquiring enzymes in saline-alkali soil. Compared with BC, NC and PC decreased the alkaline metal elements by 84.34% and 79.29%, respectively, and enhanced the specific surface area. Acid-modified biochar significantly reduced the content of Na⁺, Cl⁻, SO₄²⁻, CO₃²⁻ and HCO₃⁻. Meanwhile, the application of acid-modified biochar reduced soil pH and EC by 0.16–0.59 units, 228.38–788.93 μS.cm⁻¹, respectively. Furthermore, the application of acid-modified biochar enhanced N fixation ability and the activity of C-acquiring enzymes and P-acquiring enzymes.

The application of pristine cotton straw biochar will further aggravate soil salinization, thereby inhibiting the soil enzyme activities. In contrast, acid-modified cotton straw biochar can reduce soil salinity, improve soil available nutrients and also enhance the enzymatic activities, which is advantageous to the restoration of saline-alkali soil.