Burning crop residues under no-till in semi-arid land, Northern Spain - effects on soil organic matter, aggregation, and earthworm populations

Stubble burning has traditionally been used in semi- arid land for pest and weed control, and to remove the excess of crop residues before seeding in no- tillage systems. We compared differences in soil properties in a long- term ( 10 years) tillage trial on a carbonated soil in semi- arid north- east Spain under no- tillage with stubble returned and stubble burnt, with the conventional tillage system ( mouldboard plough, stubble returned) as a reference. Differences in total soil organic C and C in particulate organic matter, mineralisation potential, soil physical properties ( bulk density, penetration resistance, and aggregate size distribution and stability), and earthworm populations were quantified. The effect of stubble burning was absent or insignificant compared with that of tillage in most of the parameters studied. The most significant effect of stubble burning was the change in soil organic matter quality in the topsoil and penetration resistance. No- till plus stubble burning stocked an amount of organic C in the soil similar to no- till without burning, but the particulate organic matter content and mineralisation potential were smaller. Earthworm activity was similar under the 2 no- till systems, although a trend towards bigger earthworms with increasing penetration resistance was observed under the system with burning. Our results indicate that the role of burnt plant residues and earthworms in organic matter accumulation and soil aggregation in Mediterranean carbonated soils under no tillage is of major importance, meriting further attention and research.