Changes in microbial biomass and community composition, and soil carbon and nitrogen pools after incorporation of rye into three California agricultural soils

Abstract

The effects of long-term agricultural management on active soil organic matter (SOM) and short-term microbial C and N dynamics were investigated. Short-term changes in chemical and biological variables after incorporating fresh rye shoots were measured in intact soil cylinders from three contrasting agricultural systems. Two of the soils were from organic or conventional 4-yr rotations which had been in place for 6 yr as part of the University of California at Davis Sustainable Agriculture Farming Systems (SAFS) project and the third was from a double-cropped, intensive vegetable production system in the Salinas Valley of California. Microbial biomass (MB) and respiration, numbers of organisms in several trophic groups, soil inorganic N, dissolved organic C and recoverable rye were measured before and during the 6 weeks following rye incorporation. Active soil organic matter, expressed as the ratios of microbial biomass C or N to total soil C or N, respectively, appeared to be related to long-term management. These ratios increased in proportion to increased organic inputs and reduced tillage or periods of fallow. In all soils, MBC increased and decreased rapidly following rye incorporation, but MBN was fairly constant. Significant differences among the soils in MBC and MBN were maintained over the 6 week experiment. Following rye incorporation, fluorescein diacetate (FDA) active counts of bacteria and bacterial-feeding nematodes increased rapidly, whereas changes in FDA active fungal hyphal lengths and fungal-feeding nematodes were less pronounced. The rates of rye decomposition, respiration and net N mineralization were highest the first week after incorporation, coincident with increases in MBC and numbers of active bacteria in all three soils. There were significant differences among soils in numbers of organisms in the trophic groups on some sample dates, but changes in soil respiration and inorganic N and the rate of rye decomposition remained similar in all three soils. The SAFS organic soil had a somewhat lower ratio of bacterial to fungal biomass and lower ratio of respiration to MBC throughout the experiment than the SAFS conventional soil. Despite long-term differences in agricultural management and differences in active SOM contents among the three soils, the rates of rye decomposition and C and N mineralization were similar. Rye incorporation produced a short-term burst of microbial growth and activity of similar magnitude in all three soils although the initial MB contents in the three soils were different. Variations among the soils in FDA active counts of fungi and numbers of bacterial- and fungal-feeding nematodes indicated that microbial community composition was more responsive to rye incorporation than were changes in soil C and N pools.