Long‐term conservation tillage in organic farming maintains sandy loam soil functioning despite increased penetration resistance

Abstract

Conservation tillage is an attractive practice in organic farming; however, it requires more trafficking for weed management than conventional tillage. This scenario can lead to soil compaction below the tillage working depth. However, it is not clear whether long￢ﾀﾐterm conservation tillage practices impair soil functions. Therefore, this study investigated whether long￢ﾀﾐterm conservation tillage causes soil compaction and impairs water retention, gaseous exchange capability and microbial activity of the soil below the tillage working depth. A long￢ﾀﾐterm (16￢ﾀﾉyears) organic farming experiment consisting of conservation tillage (VST, shallow tillage to 7￢ﾀﾉcm depth) and conventional tillage (MP, mouldboard ploughing to 30￢ﾀﾉcm depth) was conducted on a sandy loam soil. Soil penetration resistance (PR), saturated hydraulic conductivity (SHC), water retention, gaseous exchange properties and microbial activity were determined in the 20￢ﾀﾓ30￢ﾀﾉcm soil layer. The PR did not significantly differ between the tillage treatments at any depth; however, VST recorded 30%￢ﾀﾓ63% higher PR values in the 10￢ﾀﾓ30￢ﾀﾉcm soil layer. Among the measured soil properties, only microbial activity could yield a statistical difference between the two tillage practices. The water retention behaviour, plant available water capacity, SHC and relative gas diffusivity were very similar under both tillage treatments. The mean effective air￢ﾀﾐfilled porosity (E￢ﾀﾐAFP) at pF 1.5 was 21% lower under VST than that under MP; however, the difference was reduced to 3% at pF 2. At field capacity, the E￢ﾀﾐAFP was approximately 14%￢ﾀﾔabove the critical limit (10%) for plant growth￢ﾀﾔunder both tillage practices. Air permeability was 76% and 57% higher under VST at pF 1.5 and 1.7, respectively, compared with MP. The microbial activity was 56% lower (p￢ﾀﾉ=￢ﾀﾉ.04) under VST than that under MP. Likewise, 31%, 65% and 34% higher microbial biomass carbon, microbial biomass nitrogen and dissolved organic carbon, respectively, were observed under MP compared with VST. These results indicated that despite higher penetration resistance, long￢ﾀﾐterm conservation tillage did not limit water retention and aeration capability of soil in the 20￢ﾀﾓ30￢ﾀﾉcm soil layer. However, long￢ﾀﾐterm conservation tillage may induce vertical stratification of organic matter and microbial activity, which have implications for crop production.