Increasing Straw Addition Induces a Relay Effect of Forces Driving Fixed Ammonium Release

The release of fixed ammonium (NH₄⁺) is likely related to the concentration of exchangeable NH₄⁺ and is regulated by various microbial processes, which are affected by the changes in soil carbon (C) and nitrogen (N) sources that are caused by straw addition. This increases the need to clarify the dose effects of straw on the release of fixed NH₄⁺ and the fate of fixed NH₄⁺-derived N. In this study, the fixed NH₄⁺ pool of an Alfisol was labelled with ¹⁵N under chloroform fumigation, and then the labelled soil (fumigated) was mixed with untreated soil (unfumigated) and incubated for 288 days with four rates of straw addition: S0 (no straw), S4 (4 t ha⁻¹), S8 (8 t ha⁻¹), and S12 (12 t ha⁻¹). The addition of a low amount of straw (4 t ha⁻¹) promoted the release of labelled fixed NH₄⁺, whereas greater amounts of straw (8 and 12 t ha⁻¹) resulted in inhibition. By the end of incubation, 63% of the fixed NH₄⁺-derived N had been transformed into nitrate-N in S0, whereas this percentage significantly decreased in straw treatments. The percentage of the fixed NH₄⁺-derived N that transformed into organic N increased from 23% to 61% with increasing straw addition. The soil total C content was the primary factor influencing the release of fixed NH₄⁺ in S0, and nitrification was responsible for this in S4. For S8 and S12, microbial immobilisation, succeeding nitrification, became the dominant driving factor for fixed NH₄⁺ release. These results indicated that there is a relay effect between soil C source, nitrification, and microbial immobilisation on fixed NH₄⁺ release with increasing straw addition. These results are helpful for improving the understanding of the relay effect of factors that drive fixed NH₄⁺ release in Alfisols with increasing straw addition and provide a basis for optimising the straw management.