Effects of biochar and phosphorus on dry matter and uptake of calcium, magnesium, iron, zinc, copper, and manganese by rapeseed in a calcareous soil

Phosphorus (P) is a macronutrient that its deficiency severely limits plant growth and production. Because of the complexity of P chemistry in soil, less than 30% of applied P fertilizer is absorbed by plants and the rest in the soil converts to unavailable forms. Organic matter can be applied to the soil to reduce soil P fixation and increase soil P bioavailability because organic matter and its decomposition products (e.g., organic acids) occupy the surfaces of phosphate adsorbents in soils and prevent the precipitation of phosphate compounds. During pyrolysis of organic material a complex mixture of P species is formed, which may include amorphous, semi-crystalline, and crystalline constituents, along with organic constituents. However, organic P forms will tend to disappear while inorganic P forms will subsequently be formed and crystallinity will increase with increasing pyrolysis temperature. The crystalline P minerals that have been identified in biochars include whitlockite [(Ca, Mg)3(PO4)2] pyrolysed from manure at 500 °C, dehydrated struvite (NH4MgPO4) produced from cattle manure and sewage sludge, and hydroxyapatite [Ca10(PO4)6(OH)2] made from slaughterhouse waste and from mixtures of that waste with either corn residue or wood. The crystallinity of hydroxyapatite was lower when corn residues or wood were added to the slaughterhouse waste and as result increased the soluble P fraction. To the best of the authors’ knowledge, the evidence to support the impact of biochar on P sorption and desorption in Iranian soils is scarce while taking account of the fact that P deficiency is dominant in approximately 72% of the arable soils. So, in this research, the effects of combined application of biochar pyrolysed at 300 ˚C and P fertilizer on dry matter and uptake of calcium (Ca), magnesium (Mg), iron (Fe), zinc (Zn), copper (Cu), and manganese (Mn) by rapeseed (Brassica napus L.), Hyola 308 cultivar, were studied in an alkaline loamy soil under greenhouse conditions.

After being washed with deionized water, wheat straw samples were milled, sieved < 1 mm, and then were oven-dried at 60 ºC for 24 h. The biochars were produced by slow pyrolysis of wheat straw at 300 ºC for 1 h under argon (Ar) gas flow at the heating rate of 10 ºC per minute. The produced biochars were transported in plastic containers for later analysis. Afterward, a factorial experiment was done on the basis of a completely randomized design with three replications. The factors were organic matter at 5 levels (no organic matter application, wheat straw 2%, wheat straw 4%, biochar 2%, and biochar 4%) and phosphorous (P) fertilizer at 3 levels (0, 20, and 40 mg/kg) as triple superphosphate. After the plant harvesting, shoot dry matter and the concentration and content of Ca, Mg, Fe, Zn, Cu, and Mn in plant shoot were determined by Atomic Absorption Spectrophotometer.

Wheat straw application significantly reduced rapeseed shoot dry matter compared to the control. At each level of wheat straw (2 and 4%), P application significantly increased shoot dry matter as compared to the control treatment. Under wheat straw application conditions, the using both levels of P (20 and 40 mg/kg) significantly increased shoot dry matter. This finding indicates the possible immobilization of P due to wheat straw application and the positive effect of P fertilizer under these conditions. Using biochar at both levels of 2 and 4 percent increased the rapeseed shoot dry matter by 46 and 56.5 percent, respectively, compared to the control. The combined application of straw and P reduced the Ca, Mg, Fe, and Cu uptake at both levels of biochar (2 and 4 %) compared to the control. However, the content of Zn and Mn increased at the biochar level of 2% compared to the control, but significantly decreased at the biochar level of 4% compared to the control.

According to the results obtained from this research, the use of biochar produced from wheat straw at the temperature of 300˚C increased the uptake of Fe, Zn, and Mn, which led to the improvement of the growth characteristics of rapeseed. In this research, the effect of biochar on the Cu concentration and content in shoot unlike other nutrients was negative. According to the results, the wheat straw drived biochar at 300 °C improved rapeseed plant growth characteristics and nutrients uptake except Cu. Also, application of biochar and P at 2% and 20 mg/kg levels, respectively, is recommended to reduce the consumptions of P fertilizer without yield reduction of rapeseed in under similar conditions. In addition, application of wheat straw at 2 and 4 % levels is not recommended and future research should be done at its lower levels. Finally, it was concluded that biochar may play an important role in soil fertility and plant production, so further research should continue.