SOIL RESPIRATION IN AN APPLE ORCHARD

To investigate why the largest CO2 concentration occurred before sunri se at 0.6 m below an apple tree canopy, seasonal and diurnal fluctuati ons in soil and grass respiration were recorded in an apple orchard ne ar Bonn, Germany between May 1994 and April 1995 with a portable, batt ery-driven infrared gas analyser and a soil respiration chamber. Respi ration from soil on raised nursery beds with fresh organic matter was compared with herbicide treated, flat soil in tree rows and the grass between the rows to study the contribution of soil and grass respirati on to the CO2 balance of a fruit orchard. The following results were f ound. 1. Soil and grass respiration ranged from 0.18 to 24 mu mol CO2 m(-2) s(-1). 2. On cold winter days, with soil temperatures below 0 de grees C, soil and grass respiration during the day remained below 0.6 mu mol CO2 m(-2) s(-1) and had the smallest diurnal variation. 3. The largest diurnal variations were in late spring with diurnal temperatur e differences of 15 degrees C, resulting in respiratory values between 3 in the morning and 5-8 mu mol CO2 m(-2) s(-1) in the afternoon. 4. Soil and grass respiration (up to 40 kg CO2 ha(-1) hr(-1)) exceeded th e CO2 exchange of the apple trees per unit surface area, contributed s ignificantly to the CO2 flux in a fruit orchard, and were a pertinent source of CO2 for tree photosynthesis. 5. The temperature coefficient, Q(10), ranged from 4 below 0 degrees C, to 2.3 above 0 degrees C, 1.7 above 10 degrees C and 1.3 above 20 degrees C.