Dynamics of nitrogen after deeper tillage in arable loess soils of West Germany

Summary The depth of ploughing has increased in West Germany during the last three decades. Before the 1960s, the depth of the A_p horizon rarely exceeded 25 cm; in the early 1980s it reached about 35 cm on average but it has remained constant since that time. In 1989, the total N content of 105 plots in the southern Niedersachsen loess area was measured after deepening of the plough layers. The N content of the samples was compared with that of earlier samplings (1) before deeper tillage in the 1960s, the 1970s, and the 1980s; and (2) in 1983. Directly after the deeper ploughing, the N content of the topsoil decreased, presumably due to dilution with the subsoil material. Mass balance calculations for total N in 1989 showed that the phase of N accumulation can take 20 years or more. Within two decades, up to 2000 kg N ha^-1 was immobilized in the soil organic matter. Recent incubation experiments with disturbed soil indicated that the N mineralization capacity was reestablished in all soils and is now similar to that of the early (1960s and 1970s) and more recent (1980s) deepened plough layers. Undisturbed soil material incubated in plastic tubes showed a significantly reduced net mineralization at water contents above 65% of the waterholding capacity, particularly in the lower part (15–30 cm) of the A_p horizon. This study suggests that N accumulated in the deep plough layers cannot contribute noticeably to net N mineralization in loess soils during the growing season.