Restricting tillage to shallow-mixing operations as conducted in many conservation tillage systems occasionally causes problems such as compaction of the formerly ploughed layer or severe weed and slug infestation. To overcome these constraints, one moulboard ploughing operation is carried out and, subsequently, cultivation is shifted back to non-inverting shallow-mixing tillage. This study investigated the effect of one single ploughing action conducted after several years of conservation tillage on the amount of organic matter in the soil profile. It was hypothesized that soil organic carbon (SOC) and soil nitrogen (SN) stocks decrease very rapidly during the first few months after ploughing. The study was carried out in temperate climate on three adjacent fields with loess-derived soil (Stagnic Luvisol) located in southern Lower Saxony, Germany. The crop rotation included sugar beet (Beta vulgaris L.), winter wheat (Triticum aestivum L.), winter barley (Hordeum vulgare L.), and white mustard (Sinapis alba L.) as a catch crop. Conservation tillage started in 1992 on all fields and consisted of 30 cm deep non-inverting tillage combined with shallow-mixing (10 cm deep) operations. Mouldboard ploughing (30 cm deep) was conducted once in each field in August 1998-2000 prior to sowing of mustard. Thereafter, shallow conservation tillage (10 cm) was established. Soil samples were taken once before and at three dates (1-6 months, 7-9 months, 1.5-2.5 years) after ploughing and masses of SOC and SN were determined. After 7-9 years of conservation tillage, SOC and SN were concentrated in the top 10 cm layer of the soil. One ploughing operation resulted in a substantial loss of organic matter. At 0-30 cm soil depth, losses of 0.26 kg m−2 SOC and 0.046 kg m−2 SN occurred within 1-6 months after ploughing, accounting for 4 and 7 % of the total initial masses, respectively. At the soil layer of 0-45 cm, these early losses were much lower (2 % and 5 % of the initial masses of SOC and SN per unit of soil area, respectively), caused by an increase of masses in the 30-45 cm depth. Further losses of SOC and SN from the 0-30 cm layer were small, while from the lower layer a second pronounced loss of organic matter occurred. After 1.5-2.5 years after ploughing, losses from the 0-45 cm depth accounted for 6 % and 10 % of the initial total mass of SOC and SN, respectively. In temperate climate, one ploughing operation after several years of conservation tillage obviously causes a partial loss of organic matter within a few weeks after ploughing.
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