Transformations of N derived from straw under long-term conventional and no-tillage soils: A 15N labelling study

Understanding wheat straw nitrogen (N) transformations in soil depending on tillage practices is necessary to increase the efficient N use and to protect diyland farming environments. Labile organic N pools are crucial for N mineralization but are less investigated because of their fast dynamics. The objectives of this study were to determine the effects of winter wheat straw N (N-15-labelled) on soil labile organic N transformations under long-term (27 years) no-tillage and conventional tillage. Four treatments were established under controlled conditions: no-tillage without straw (NoTill); no-tillage with N-15-labelled wheat straw (NoTill+Straw); conventional tillage without straw (ConvTill); and conventional tillage with N-15-labelled straw (ConvTill +Straw). Straw application increased the particulate organic matter N (POMN) and microbial biomass N (MBN) contents, especially in NoTill +Straw, but decreased the dissolved organic N (DON) content. This reflects intensive microbial DON immobilization at the background of plant residues with a high C/N ratio. The 40% to 80% increase in the NH4+ content after straw addition reflects fast straw and soil organic matter (SOM) mineralization, while NO3- declined by 60% to 93% over 56 days, especially in ConvTill soil. N-15 recovered in POMN after 14 d was greatest in NoTill +Straw and ConvTill +Straw, reaching 20 and 18 mg N-15 kg(-1), respectively. Overall, the straw N recovered as NO3- was 72% and 48% greater than that in NH4+ in ConvTill +Straw and NoTill +Straw, respectively, reflecting ac-celerated SOM mineralization and N mining in the presence of straw. The straw N recovered as POMN was greater through the incubation period (56 days) than that in DON and MBN, especially in NoTill +Straw, indicat-ing that the no-tillage practice reduced straw mineralization. N-15 in DON and N-15 in NO3- were higher in NoTill +Straw than in ConvTill+Straw, implying that N limitation was alleviated to a certain extent under no-tillage conditions. In conclusion, wheat straw N remained mainly in POMN after 56 days, especially under no-till condi-tions. and only a minor part was incorporated into DON and microbial biomass. This provides a theoretical basis for straw N use efficiency in agroecosystems. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Understanding the nitrogen transformations in soil with different tillage practices is crucial for efficient nitrogen use and environmental protection. This study found that application of wheat straw significantly increased organic nitrogen content in soil and promoted fast nitrogen mineralization. Under long-term no-tillage conditions, the straw N remained mainly in the particulate organic matter nitrogen pool, providing a theoretical basis for improving straw N use efficiency.