Soil respiration, nitrogen mineralization and uptake in barley following cultivation of grazed grasslands

Soil tillage was studied as a strategy to synchronize N mineralization with plant demand following ploughing of two types of grazed pastures [ryegrass/ white clover (Lolium perenne/Trifolium repens) and pure ryegrass]. The swards were either rotovated and ploughed or ploughed only. Soil respiration, as determined by a dynamic chamber method, was related to net N mineralization and to plant N uptake in a subsequent spring barley crop (Hordeum vulgare). Diurnal variations in temperature were important for the CO2 flux and care must be taken that temperatures during measuring periods are representative of the daily mean. Soil tillage increased the CO2 flux considerably compared with untilled soil with total emissions of 2.6 and 1.4 t C ha(-1), respectively, from start of April to end of June. Sward type or rotovation did not markedly influence accumulated emissions. Rotovation significantly increased the content of nitrate in the soil until 43 days after rotovation, showing that net N mineralization occurred rapidly during this period, in spite of low soil temperatures (5-10 degreesC). Rotovation increased barley grain yield by 10-12% and N-uptake by 14%. For both sward types, rotovation caused an extra N-uptake in harvested plant material of about 12 kg ha(-1). The availability of soil inorganic N at the early stages of barley was important for the final yield and N-uptake. The results indicated that soil biological activity was not enhanced by rotovation and that the yield effect of rotovation was mainly caused by quicker availability and better synchrony between N mineralization and plant uptake due to earlier start of decomposition.