Effect of Burning Cereal Straw on Soil Properties and Grain Yields in Saskatchewan

Abstract

In western Canada cereal straw is sometimes burned to facilitate seedbed preparation. Evidence on the consequence of this practice is limited. This study was initiated to determine the long- and short-term effects of burning cereal straw on wheat (Triticum aestivum L.) yields and soil properties. It was carried out on chernozemic soils at Melfort (orthic Black), Indian Head (orthic Black), and Swift Current (orthic Brown). Heat from burning straw barely penetrated the soil below 1 cm with maximum temperatures reaching between 338 and 422°C on the straw mulch itself. Between 32 and 76% of the straw weight and 27 and 73% of the N were lost in the burn, but no P was lost. Bacterial and fungal populations decreased immediately and substantially only in the top 2.5-cm of soil upon burning, with the fall burn being more detrimental than spring burn. Repeated burnings in the field permanently diminished the bacterial population by more than 50% but the fungi appeared to recover. Soil respiration measurements also confirmed a permanent reduction in total biological activity from repeated burning. However, for single burns, soil respiration rates increased temporarily to rates considerably above those in unburnt soils. Burning immediately increased the exchangeable NH4-N and the bicarbonate-P content, but there was no buildup of nutrients in the soil profile over the years. Long-term burning reduced total soil N and C and potentially mineralizable N in the 0- to 15-cm soil segment but did not affect humic and fulvic acids. Its effect on the soil surface tension was not conclusive, but burning seemed to increase the susceptibility of the soil to water erosion, reduced the permeability of the Melfort soil, and increased the compaction of the Indian Head soil. Burning stubble for up to 20 consecutive years had no significant effect on grain yields, perhaps because of the high initial fertility of the test soils. However, there are sufficient danger signals to indicate that this practice should be discouraged on the prairies.