Sugarcane residue management and grain legume crop effects on N dynamics, N losses and growth of sugarcane

To reduce greenhouse gas emissions farmers are being encouraged not to burn sugarcane residues. An experiment was set up in NE Thailand, where sugarcane residues of the last ratoon crop were either burned, surface mulched or incorporated and subsequently the field left fallow or planted to groundnut or soybean. The objectives of the current experiment were to evaluate the residual effects of these treatments during the following new sugarcane crop on (i) microbial and mineral N dynamics, (ii) performance of sugarcane and (iii) effectiveness of recycled legume residues compared to mineral N fertilizer on N use efficiencies, ¹⁵N recovery in the system and in soil particle size and density fractions (using ¹⁵N labelled legume residues and fertilizer). The millable cane and sugar yield were positively affected by sugarcane residue mulching and incorporation compared to burning suggesting microbial remobilization of previously immobilized N. Residual effects of legumes increased sugarcane tillering and yield (127 and 116 Mg ha⁻¹ for groundnut and soybean, respectively) compared to the fallow treatment without N fertilizer (112 Mg ha⁻¹). Soybean residues of higher C:N ratio (33:1) and lignin content (13%) compared to groundnut residues (21:1 C:N, 5% lignin) decomposed slower and improved N synchrony with cane N demand. This led to a better conservation of residue N in the system with proportionally less ¹⁵N losses (15-17%) compared to the large losses from groundnut residues (50-57%) or from mineral N fertilizer (50-63%). ¹⁵N recoveries in soil were larger from residues (41-80%) than from fertilizer (30%) at final harvest. Recycled legume residues were able to substitute basal fertilizer N application but not topdressing after 6 months.