Long‐Term Application of No‐Tillage‐Induced Greater Risk of Poor Topsoil Aeration Along a European Pedoclimatic Gradient

ABSTRACT This paper assesses the effect of long‐term contrasting tillage practices on topsoil structural characteristics critical for nitrous oxide (N 2 O) emissions and carbon sequestration across a pedoclimatic gradient. The hypotheses tested are that: (i) aeration is greater in the topsoil of ploughed (to 0.20–0.30 m depth) than in no‐till soils and (ii) the effect of tillage practice on soil functionality depends on the context, and thus varies between sites with different pedoclimatic conditions. We evaluated the topsoil characteristics of seven long‐term tillage experiments, spread along a 2600‐km transect in Europe. A total of 576 soil cores (100‐cm 3 ) were sampled from 0 to 0.10 and 0.10 to 0.20 m depths in mouldboard‐ploughed and no‐tillage treatments after harvest. The soil water content at −30, −60, and −100 hPa matric potential was measured as well as air permeability ( k a ) and relative gas diffusivity ( D s /D o ) at −100 hPa, from which soil bulk and gas transport characteristics were derived. Despite large variations in the characteristics among sites, tillage did significantly affect the characteristics across sites. The degree of compactness was less and total pore volume was greater in the ploughed than in the no‐till treatments. Still, thresholds indicating suitable conditions for root growth were largely met under both practices. The ploughed soils showed vertical stratification, with a better aeration of the 0–0.10 m soil layer compared to the 0.10–0.20 m layer. No differences were observed between the ploughed 0.10–0.20 m and no‐till layers, which were attributed to soil settlement after ploughing. While the D s /D o at 0.10–0.20 m depth was favourable for promoting N 2 O emissions, the water‐filled pore space was below suggested thresholds. Impacts of tillage on soil structural and functional characteristics were both significant and generalisable but also deviated locally. For example, D s /D o and k a generally increased with the air‐filled pore volume ( ε a ), yet sites with greater ε a did not necessarily have higher D s /D o and k a . Existing models explaining D s /D o and k a with ε a were fitted to the measured data and performed best when both depths and tillage practices were assessed altogether. Despite the limited differences at −100 hPa, anoxic conditions may in reality prevail for a longer period under no‐till than ploughing.