Phosphorus dynamics in soil supplemented with burnt crop residues

Crop residues are valuable agricultural resources as they contain a significant amount of nutrients that can be returned to soil with incorporation of residues. However, crop residues are often considered agricultural waste and are frequently burnt in the field. Various reasons behind crop residue burning include: (a) ease and timeliness of farming operations, (b) better control of weeds and diseases, (c) lack of alternate uses of residues or lack of machinery to handle large amounts of residues, and (d) to return nutrients to soil. The nutrients stored in the residues can be lost during burning or they undergo irreversible transformations. Depending on the intensity of burn, most of the carbon and nitrogen are lost, whereas majority of phosphorus (P) remained in the burnt residue and can potentially contribute to P dynamics in soil.

The first experiment determined the total and water-soluble elemental composition and distribution of P fractions in different crop residues as affected by burning temperature. Approximately 99% of total carbon and more than 95% of total nitrogen were lost with burning residues at 550⁰C. Around 80 to 98% of total phosphorus was recovered in residues burnt at various temperatures. The total concentration of other elements (potassium, sodium, calcium, magnesium, zinc and manganese) increased with burning, but water solubility decreased, except for sodium and potassium. Burning temperature also altered the distribution of P fractions in residues.

The second experiment compared the changes in soil properties (pH, electrical conductivity and Colwell P), microbial biomass P, phosphomonoesterase activity and P distribution in soil with addition of burnt or unburnt crop residues under laboratory incubation conditions. Phosphorus distribution in different soil fractions was affected by temperature of residue burning as well as the quality of residues. Colwell P and microbial biomass P increased initially following the application of residues and decreased towards the end of incubation. Phosphomonoesterase activity in soil was depressed by the addition of inorganic P in burnt-residue treatments.

In the following experiment, biomass production and P content in wheat after 6 weeks of growth in soils amended with inorganic (P fertiliser) or organic sources (unburnt and burnt crop residues) at two rates (2 and 20 mg P kg^-1 soil). Soil properties (pH, electrical conductivity, Colwell P and microbial biomass P) and distribution of P fractions were also determined at harvesting. Phosphorus applied at low rate was insufficient to increase growth and P content in wheat, but significantly affected the distribution of P fractions in soil. Shoot and root biomass and P content in wheat at high rate of P application was greater in legume residues (lucerne forage and pea straw) burnt at 300⁰C and in cereal residues (wheat straw and oat hay) burnt at 550⁰C. Burning temperature had variable effects on distribution of P fractions in soil at high rate of P application. The highest biomass production was observed in the inorganic fertiliser amended soil, and was comparable to that in the wheat straw and oat hay burnt at 550⁰C.

In conclusion, temperature of burning had a variable effect on total and water-soluble elemental composition and distribution of P fractions in different crop residues. Phosphorus availability may be decreased with burning compared to the unburnt residues. Incorporation of unburnt residues increased microbial biomass P and phosphomonoesterase activity in soil, whereas the burnt residues increased soil pH and the concentration of Colwell P. Distribution of P fractions in soil was influenced mainly by the quality of residues and their variable effects on soil pH. Wheat growth and P content were adversely affected even at high rate of P application due to immobilisation of N by soil microorganisms. Burnt residues not only increased growth and P uptake by wheat, but also maintained higher concentration of available P in soil. Further research is required to compare the long-term effects of residue incorporation and burning on crop growth and distribution of P fractions in soil.