From Physical Architecture to Ecosystem Function: Tillage Exerts Indirect Control on Nitrogen Transformation by Restructuring Preferential Flow Paths

How long-term tillage drives nitrogen (N) function succession by degrading vadose zone preferential flow paths remains unclear. This study investigated croplands across multiple tillage chronosequences by integrating dye tracing, ¹⁵N isotope tracing, and metagenomic techniques. The study shows that tillage-induced degradation of preferential flow structures homogenizes the vadose zone and reshapes N-cycling communities, shifting denitrifiers from nirK- to nirS-dominated assemblages. Consistent with this community turnover, gross nitrification rates in shallow preferential paths are 30.8% higher than in the adjacent matrix in short-term tillage, whereas in deep layers matrix denitrification and DNRA rates exceed those in preferential paths by 37.9 and 76.1%, and anammox appears only in the matrix at medium- and long-term tillage, indicating a concentration of reductive N processes in the matrix. Path analysis further shows that tillage-driven alterations in hydraulic properties (path coefficient = -0.91, p < 0.01), together with these community and process-rate shifts, redirect the dominant N pathway from hotspot-driven nitrification in preferential channels to matrix-driven reductive N loss, converting a nitrate-leaching-prone system into one with higher gaseous emission potential. This establishes a mechanistic link linking soil hydraulic degradation to N functional succession and supports targeted, stage-dependent farmland nitrogen management.