Soil fungi helped break down nutrient-rich forest litter 13%–28% faster, but slowed nutrient-poor litter by 19%–20% over 120 days. When leaves and plant debris had more nitrogen, the fungi boosted microbial activity and helped release nutrients back into the soil faster. Trees growing with the fungi also grew 20%–30% larger in the richer litter conditions, showing how fungi help forests recycle nutrients and support plant growth.