ORGANIC-MATTER AND NITROGEN DYNAMICS IN CONVENTIONAL VERSUS INTEGRATED ARABLE FARMING

Carbon and nitrogen cycling and crop yields in integrated (INT) arable farming, with 35% ofN fertilisation by organic manures, were compared with those in conventional (CONV) management, using only mineral-N fertiliser, in a long-term field trial on a calcareous silt loam soil. In fields with a relatively high initial soil organic matter (SOM) content, crops, especially potatoes, seemed to benefit from the increased soil N supply. Under INT management, original SOM levels were maintained in fields with 2.2% and 2.8% SOM in the 0-25 cm layer; under CONV management these levels decreased to 2.15% and 2.6% SOM in 6 years. In situ, periods ofN immobilisation as well as ofN mineralisation were found, while in vitro only net N mineralisation occurred. Model calculations of C and N turnover in soil were used to estimate the N mineralisation pattern and the pool sizes of microbial biomass and young humus. Mineral-N balance calculations for potatoes suggested N losses of 0-100 kg ha(-1) over the 1989 growing season. For the rotation cycle (1988-1991), calculated N losses to the environment were less than 300 kg ha(-1) and 170 kg ha(-1) at the low and the high SOM level, respectively. High losses may be partly explained by high levels of rainfall In 1988 and 1990, with denitrification as a likely cause of N loss. The highest (risk of) N loss in spring is for potatoes and sugar beet, where high nitrate levels are present in soil and N uptake starts relatively late. Calculated average efficiencies of N inputs were 88% and 72% for CONV management started on soil with 2.8% and 2.2% SOM, respectively; for INT management these values were 82% and 65%. Values ofN efficiencies and N losses are still preliminary because steady state had not yet been attained.