Sunlight and soil biota accelerate decomposition of crop residues in the Argentine Pampas

The benefits of maintaining soil organic matter in agroecosystems have long been recognized, although there are many open questions with respect to the controls on crop residue decomposition and the consequences for carbon and nutrient cycling. The large extension of intensively cropped agriculture in the Argentine Pampas of soybean, sunflower, wheat, and maize motivates the need for a broader understanding of these controls. In modern no-till agriculture, post-harvest crop residues may remain upright as standing or on the soil surface for extended periods of time; nevertheless, the effects of sunlight exposure and its interaction with soil biota on decomposition of crop residues have not been evaluated. We established a manipulative experiment using soybean, sunflower, wheat and maize crop residues with treatments of full or attenuated sunlight exposure and presence or absence of soil biota. Species identity was significant in determining rates of leaf residue decomposition with mass loss of soybean > sunflower > wheat > maize. Sunlight exposure significantly accelerated decomposition of leaf resi-dues in the absence of soil biota, while soil biota significantly contributed to increased leaf residue decompo-sition with and without sunlight exposure. In contrast, stem residue decomposition was modest under all conditions and differences in decomposition were determined by species identity. Surface area for leaf and stem residues across species was the strongest litter quality predictor for the variation in crop residue decomposition. Our results suggest that sunlight exposure, duration of fallow period with or without rotation or cover crops, and the proportion of leaves vs. stems in crop residues interacting with soil biota may be fundamental elements in determining carbon turnover in these intensively cropped agroecosystems. There would be great value in incorporating the impacts of sunlight exposure and unexplored aspects of crop residue quality in models for sustainable agriculture in the region and globally.