Soil potassium relations in an arid soil treated with biochar and chemical fertilizers after cultivation with safflower (Carthamus tinctorius L.) under water stress

Abstract Safflower cultivation has been developed in arid regions of Iran in the recent decade. Potassium (K) is an important plant nutrient and its content has been decreased in arid soils due to the intensive agriculture and no K fertilizer application. We tried to monitor K pools distribution and K release in a calcareous soil cultivated with safflower (Carthamus tinctorius L.) under normal irrigation and water stress when treated with wheat biochar (WB), cotton biochar (CB), nitrogen + phosphorus (N + P), N + P + WB and N + P + CB. All fertilizer treatments except N + P increased the contents of soluble, exchangeable, non-exchangeable and HNO3-extractable K by 81-124, 64-98, 14-28 and 31-50%, respectively. The N + P treatment decreased soluble and non-exchangeable K by 10 and 6%, respectively and increased exchangeable K by 12%. Ten times extraction of control soil with 0.01 M CaCl2, 0.01 M oxalic acid and 1% wood vinegar released 270, 250 and 256 mg K kg−1, respectively and cumulative K release increased with all fertilizer treatments except N + P. The CaCl2 solution extracted more K from soil samples, as compared to the oxalic acid and wood vinegar, and this difference was considerable in the first stages of K extraction. Analysis of Al and Ca release proves that exchange reaction is the main mechanism of K extraction with all solutions. Water stress increased soluble K content by 17%. Cumulative K released from soil samples under normal irrigation and water stress were 404 and 417 mg kg−1, respectively. It is concluded that irrigated agriculture in arid regions may decrease soil available K and use of different biochars improves soil K availability and its K release potential.