Rootstock-dependent soil respiration in a citrus orchard
Soil respiration is a major contributor to the carbon flux in apple orchards, but very little data exist for citrus. Day-time soil respiration, grass respiration, and grass photosynthesis were recorded in a 'Nules' clementine (Citrus reticulata Blanco) orchard on 'Carrizo' citrange or 'X639' rootstock in Stellenbosch, Republic of South Africa, under typically variable spring weather conditions such as cool to hot, and clear to cloudy days. The amount of CO2 contributed by the citrus roots and the Oakleaf soil type to the carbon fluxes in the orchard, was evaluated. Soil respiration rate in the citrus rows varied between 0.1 and 0.4 g CO2 m-2h-1 and increased with temperature, with a Q10 of about 1.4. After rainfall or irrigation, soil respiration was higher, especially at elevated temperatures. The rootstocks 'Carrizo' and 'X639' were characterised by slightly different rates of soil respiration. The difference was more pronounced after irrigation, and soil respiration of 'X639' exceeded that of 'Carrizo' by 0.03 to 0.04 g CO2 m-2 h-1. Grass respiration between the rows ranged from 0.7 to 1.4 g CO2 m-2 h-1 and grass net photosynthesis exceeded 1.0 g CO2 m-2 h-1, indicating that the grass strips between the tree rows act as a sink for CO2 during the light period. It was concluded that soil respiration represents a major source of CO2 in a citrus orchard.