Ploughing effects on soil organic matter after twenty years of conservation tillage in Lower Saxony, Germany

Abstract

Conservation tillage may concentrate organic matter and carbon in the soil, thus improving soil quality and counteracting CO 2-increase in the atmosphere. In parts of Germany however, continuous conservation tillage can cause problems in soil and crop management, resulting in a need to shift to short-term conventional tillage, such as mouldboard ploughing. The objective of the present research was to follow the fate of soil organic matter, when soil is ploughed after long-term minimum tillage in the temperate climate of Lower Saxony. In minimum tillage, shallow cultivation was restricted to stubble cleaning and seedbed preparation, using a rotary harrow or rototiller. After 20 years of shallow cultivation, soil organic carbon, soil nitrogen and microbial biomass carbon were concentrated in the top 5 cm of a loess-derived silt loam (Orthic Luvisol). In the 50 cm soil profile, mass of soil organic carbon tended to be higher by about 5 Mg ha −1 as compared to conventionally ploughed soil, which contained roughly 65 Mg ha −1. In the ploughed soil, soil nitrogen amounted to about 6.8 Mg ha −1, whereas in the minimum tilled soil it was roughly 1.0 Mg ha −1 higher. Total microbial biomass carbon fluctuated between 800 and 1300 kg ha −1, the differences between tillage systems being less distinct. Ploughing the old minimum tilled land destroyed the stratification of soil organic matter. Moreover, during the winter months (November–March) the surplus of soil organic carbon and nitrogen masses, enriched by conservation tillage, was completely decomposed, presumably a consequence of its labile quality. Inverting the minimum tilled soil did not increase concentrations of organic carbon and nitrogen in the lower part of the A p-horizon, but it did increase concentration of microbial biomass carbon. We conclude that organic matter stratification and accumulation as a result of long-term minimum tillage were completely lost by a single application of inversion tillage in the course of a relatively mild winter.