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

Abstract
Background and Objectives
Phosphorus (P) is a macronutrients that its deficiency severely limits plant growth and production. Because of the complexity of phosphorus chemistry in soil, less than 30% of applied phosphorus 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 phosphorus fixation and increase soil phosphorus availability because organic matter and its decomposition products (e.g., organic acids) occupy the surfaces of phosphate adsorbents in soils and prevent the formation of phosphorus precipitation. During pyrolysis of organic material a complex mixture of phosphorus (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. Crystalline P minerals that have been identified in biochars include whitlockite ((Ca, Mg)3(PO4)2) in biochars produced from manure at 500°C , dehydrated struvite (NH4MgPO4) in biochars produced from cattle manure and sewage sludge and hydroxyapatite (Ca5(PO4)3(OH)) in biochars 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 this increased the soluble P fraction although total P was diluted by doing so. 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. In this research, the effects of combined application of biochar pyrolysed at 300˚C and phosphorous (P) fertilizer on dry matter and uptake of calcium (Ca), magnesium (Mg), iron (Fe), zinc (Zn), copper (Cu), and manganese (Mn) nutrients by rapeseed (Brassica napus L.), Hyola 308 cultivar were studied in an alkaline loamy soil in greenhouse .
Methodology
After being washed with deionized water, wheat straw samples were milled, sieved < 1 mm, and then were oven-dried at 60 ◦C for 24 h. Biochars were produced by slow pyrolysis of wheat straw at 300 º C for 1 h under argon (Ar) gas flow at the heating rate of 10 ºC min− 1. The produced biochars were transported in plastic containers for later analysis. A factorial experiment was done in a completely randomized design with three replicates. The factors were organic matter (from two sources of biochar and wheat straw and three rates of 0, 20, and 40 g kg-1) and phosphorous (P) at three levels (0, 20, and 40 mg kg-1 as triple superphosphate). After the plant harvesting, shoot dry matter and the concentrations and content of Ca, Mg, Fe, Zn, Cu, and Mn in plant shoot were determined.
Findings
Applied biochar at both levels of two and four percent increased the rapeseed shoot dry matter by 46 and 56.5 percent, respectively, compared to the control. The interaction effects of biochar and P at both levels of used biochar decreased the Mg and Cu uptake by the shoots compared to the control, but increased the content of Ca, Fe, Zn, and Mn nutrients by the rapeseed shoot. The combined application of straw and P reduced the Ca, Mg, Fe and Cu uptake at both levels of used biochar (2 and 4 g 100 g-1) compared to the control. However, the content of Zn and Mn increased at the biochar level of two percent compared to the control, but significantly decreased at the biochar level of four percent compared to the control.
Conclusion
According to the results obtained from this research, the use of biochar produced from wheat straw at a temperature of 300˚C increased the uptake of Fe, Zn, and Mn nutrients, 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. In this research, application of biochar and P at 2 g 100g-1 and 20 mg kg-1 levels, respectively, is recommended to reduce the consumptions of P fertilizer without yield reduction in greenhouse conditions. Also, application of wheat straw at 2 and 4 g 100g-1 levels, is not recommended and future research should be done at lower levels. Finally, it was concluded that biochar may to play an important role in soil fertility and plant production, so further research should continue.