Contrasting effects of crop straw and green manure amendments on soil gross N transformations in a soil-maize system: a short-term ¹⁵N incubation case study

Organic amendments directly affect soil N transformations, while the direction and magnitude of these effects remain uncertain. Most previous studies through laboratory incubation experiments without plants likely neglected the feedback interactions of plant, thereby limiting the applicability in field conditions. This study aims to explore the effects of organic amendments on soil gross N transformations with consideration of plant feedback.

The ¹⁵N tracing pot experiments were performed using a soil-maize system with two types of organic amendments—crop straw (wheat straw, CS) and green manure (Chinese milk vetch, GM) to determine soil gross N transformation processes and rates by using the Ntrace_plant model.

Green manure amendments significantly increased soil gross N mineralization and nitrification rates compared to crop straw treatment and the control. In contrast, crop straw incorporation did not enhance gross N mineralization rates and even significantly decreased soil gross nitrification rates relative to the control. Both green manure and crop straw amendments significantly increased soil microbial ammonium (NH₄⁺) immobilization rates compared to the control. However, green manure amendments significantly enhanced soil microbial nitrate (NO₃⁻) immobilization rates only in the presence of maize, with no significant effect observed in the absence of maize. Meanwhile, crop straw incorporation significantly decreased soil microbial NO₃⁻ immobilization rates.

Our findings indicate that green manure and crop straw amendments have contrasting effects on soil gross N transformations, with green manure demonstrating a more pronounced positive impacts, particularly in the presence of plants.