Enhanced clay mineral–mediated ammonium fixation–release turnover under no–tillage with straw retention: An effective buffering mechanism for nitrogen retention and supply in agroecosystems of Northeast China

Abstract

The fixed ammonium (NH₄⁺–N) pool can serve as a stable transitional reservoir for nitrogen (N) retention and supply in agroecosystems. However, the dynamics of clay mineral–mediated NH₄⁺–N fixation (A_NH4) and subsequent release (R_NH4ads) processes during the agricultural transition from conventional intensive cultivation to conservation tillage (CT) practices, particularly no–tillage with straw retention (NTS), remain insufficiently characterized. This study integrated ¹⁵N–tracing field micro–plot experiments with laboratory incubations to quantify the dynamics of clay mineral–mediated NH₄⁺–N fixation–release turnover in response to 9 years of NTS management in Northeast China. Results revealed that 27.8 % of the applied fertilizer–N was transiently retained as newly fixed ¹⁵NH₄⁺–N in the 0–40 cm soil layer during the maize seedling stage. Compared with traditional ridge tillage (RT), NTS significantly enhanced the proportion of newly fixed ¹⁵NH₄⁺–N relative to the total applied fertilizer–N, indicating improved N retention capacity during the maize seedling stage. Notably, more than 93.9 % of newly fixed ¹⁵NH₄⁺–N was released within the current growing season, with 85.7 % exhibiting rapid release before the tasseling stage, and NTS amplified this pulse release by 6.4 % compared with the RT treatment. In addition, gross transformation rates of both A_NH4 and R_NH4ads increased by 72.8 % and 101.8 % under the NTS treatment respectively. However, net NH₄⁺–N fixation rates remained statistically comparable among treatments, suggesting intensified clay mineral–mediated NH₄⁺–N fixation–release turnover under CT agroecosystems. Our findings highlight that enhanced clay mineral–mediated NH₄⁺–N fixation–release turnover serves as an effective buffering mechanism by simultaneously improving N retention (9.7 % increase in fertilizer–N use efficiency) and synchronizing soil N supply with crop demand (12.2 % yield enhancement), thereby providing dual benefits of productivity and sustainability in CT agroecosystems of Northeast China.