In farm soils, the steady breakdown of dead leaves, roots, and other organic matter releases nutrients that feed the next crop. Scientists have long assumed that living plants would speed this process up, because their roots leak sugary compounds that microbes love to eat. A new study finds the opposite; the presence of growing wheat plants slows decomposition down.
Researchers at the University of Liège and colleagues grew winter wheat (Triticum aestivium var. Asory) in a set of climate-controlled chambers that mimicked meteorological conditions of the years 2013, 2068, and 2085 in Central Europe. They used two real farm soils, one low in organic matter, one high, and left half the chambers unplanted as controls. To track how fast organic material broke down, they buried identical tea bags (fast-decomposing green tea and slow-decomposing rooibos) eight centimetres deep for two weeks during the wheat’s tillering stage. The “tea bag index” gave them a standardised measure of early-stage decomposition.
“Contrary to expectations, decomposition rates were lower in planted than in unplanted soils, suggesting resource competition between plants and microbes,” the team reports in a paper in BioRxiv. The effect held across both soil types and all three climate scenarios. Planted soils also showed lower levels of glucose-like compounds and nitrate in the water between soil particles — clear signs that the wheat was taking up the very resources microbes need to do their work.
The researchers had expected plant sugars to boost microbial numbers and activity, leading to faster breakdown and more carbon dioxide released from the soil. Instead, they saw no significant change in microbial biomass, bacterial or fungal gene copies, or overall soil respiration when plants were present. “Mechanistically, fungi and soil humidity were more important for decomposition than bacteria or temperature,” the study notes. Fungal gene abundance and average soil moisture stood out as the strongest predictors of how fast the tea broke down.
The results surprised the team because warmer future climates and extra fertiliser in one scenario did not overcome the slowdown. Nitrate levels still dropped in planted soils even after nitrogen was added, and the extra warmth did not noticeably rev up microbial work. The authors write that the findings “corroborate climate impacts on decomposition but also indicate microbial resilience.”
“The slowing down of decomposition in the presence of growing plants, as demonstrated here, supports the integration of cover crops in rotations as they can prevent N-leaching and provide habitat for diverse species,” the paper concludes.
Citations
Pierre Delaplace et al. Teatime for Triticum – (how) can the presence of plants slow down decomposition?. BioRxiv. Published online March 20, 2025. DOI: 10.64898/2026.03.19.712830
