Microbial responses to simulated tillage in cultivated and uncultivated soils

Tillage is known to have long-term effects on organic matter and labile pools of nutrients in soil, but the short-term changes in microbial dynamics and activity after tillage are less well understood. We investigated the immediate effects of simulated tillage on microbial community structure as determined by phospholipid fatty acid (PLFA) profiles, microbial activity, and carbon (C) and nitrogen (N) pools. Intact cores were obtained from Chualar sandy loam soils under grassland and vegetable production. The top 15 cm of soil was sieved to simulate tillage, then the cores were incubated in the greenhouse. Sampling took place 1 day before the tillage simulation and throughout the next 2 weeks. In the grassland soil, multivariate analysis showed changes in PLFA profiles within hours, indicating rapid changes in microbial community structure. Specific PLFA markers indicated a reduction in microeukaryotic biomass as well as an increase in a microbial stress marker after sieving. Respiration (as determined by soil incubation in sealed containers) decreased immediately after sieving and continued to decline through the next 14 days. Sieving was followed by a continuous accumulation of nitrate. In the vegetable soil, the changes in PLFA profiles were slow and gradual. The PLFA stress indicator rose only slightly. Microbial activity and biomass were low, and only small changes occurred in most variables. A decline in respiration and an increase in nitrate occurred several days after sieving. In both soils, decreased soil moisture may have contributed to changes in soil responses after sieving. Short-term responses to tillage may be less pronounced in soils with a long history of cultivation because of a relatively resilient microbial community and/or because lower initial microbial biomass and nutrient pools preclude a strong response to disturbance.