Soil respiration and microbial biomass in a pecan - cotton alley cropping system in Southern USA

Little information is available on soil respiration and microbial biomass in soils under agroforestry systems. We measured soil respiration rate and microbial biomass under two age classes (young and old) of a pecan (Carya illinoinensis) - cotton (Gossypium hirsutum) alley cropping system, two age classes of pecan orchards, and a cotton monoculture on a well-drained, Redbay sandy loam (a fine-loamy, siliceous, thermic Rhodic Paleudult) in southern USA. Soil respiration was quantified monthly during the growing season from May to November 2001 using the soda-lime technique and was corrected based on infrared gas analyzer (IRGA) measurements. The overall soil respiration rates ranged from 177 to 776 mg CO2 m-2 h-1. During the growing season, soil respiration was higher in the old alley cropping system than in the young alley cropping system, the old pecan orchard, the young pecan orchard, and the monoculture. Microbial biomass C was higher in the old alley cropping system (375 mg C kg-1) and in the old pecan orchard (376 mg C kg-1) compared to the young alley cropping system (118 mg C kg-1), young pecan orchard (88 mg C kg-1), and the cotton monoculture (163 mg C kg-1). Soil respiration was correlated positively with soil temperature, microbial biomass, organic matter, and fine root biomass. The effect of alley cropping on soil properties during the brief history of alley cropping was not significant except in the old systems, where there was a trend of increasing soil respiration with short-term alley cropping. Over time, different land use and management practices influenced soil properties such as soil temperature, moisture, microbial biomass, organic matter, and fine root biomass, which in turn affected the magnitude of soil respiration. Our results suggest that trees in agroforestry systems have the potential to enhance soil fertility and sustainability of farmlands by improving soil microbial activity and accreting residual soil carbon.