Teatime for Triticum: (how) can the presence of plants slow down decomposition?

Decomposition of organic matter is a key process in soils contributing to carbon and nutrient cycling. To identify management strategies for agroecosystems that reduce nutrient losses while maximizing plant growth, it is important to understand which parameters determine decomposition rates. This study therefore investigated how the presence of winter wheat (Triticum aestivum var. Asory) affects decomposition in a controlled Ecotron setup with two soil types with varying organic matter content across three simulated climates (2013, 2068, 2085). Using the tea bag index, interstitial soil pore water analyses, microbial biomass quantification, bacterial and fungal gene abundance, and soil respiration measurements, we tested the hypotheses that plant exudates would enhance decomposition rate and microbial biomass, while plant nitrogen uptake would deplete soil nitrate, potentially mitigated by fertilization. Contrary to expectations, decomposition rates were lower in planted than in unplanted soils, suggesting resource competition between plants and microbes. No significant differences were observed in microbial biomass or respiration due to plant presence, and fertilization effects on nitrate or microbial mineralization were undetectable, likely due to rapid turnover of organic molecules including uptake by plants and microbes. Mechanistically, fungi and soil humidity were more important for decomposition than bacteria or temperature. The findings corroborate climate impacts on decomposition but also indicate microbial resilience and highlight the potential of management strategies like cover crops, adjusted planting dates and crop residual management which can contribute to healthy soils by sustaining carbon and nutrient cycling.