Short-term dynamics of soil carbon, microbial biomass, and soil enzyme activities as compared to longer-term effects of tillage in irrigated row crops

Biol Fertil Soils (2009) 46:65–72 DOI 10.1007/s00374-009-0400-0 SHORT COMMUNICATION Short-term dynamics of soil carbon, microbial biomass, and soil enzyme activities as compared to longer-term effects of tillage in irrigated row crops Daniel Geisseler & William R. Horwath Received: 6 March 2009 / Revised: 1 May 2009 / Accepted: 5 May 2009 / Published online: 25 August 2009 # The Author(s) 2009. This article is published with open access at Springerlink.com Introduction The effects of disturbance on soil quality are difficult to determine because soil is inherently variable and physical and chemical soil properties change too slowly to reflect recent management history. Microbial and biochemical soil properties have been suggested as early and sensitive indicators of changes in soil quality as they manifest themselves over shorter timescales and are central to the ecological function of a soil (Karlen et al. 1994; Bandick and Dick 1999). Soil enzyme activities in particular are increasingly used as indicators of soil quality because of their relationship to decomposition and nutrient cycling, ease of measurement, and rapid response to changes in soil management (Dick 1994; Dilly et al. 2003). In a long-term study, Kandeler et al. (1999) found that enzyme activities were significantly increased in the top 10 cm of the profile after only 2 years of minimum and reduced tillage compared to conventional tillage. In contrast, significant effects of tillage treatments on microbial biomass, nitrogen (N) mineralization, and potential nitrification were not observed until after 4 years. Soil disturbance, however, is only one of the many factors affecting soil microbial and biochemical properties. Seasonal fluctuations in soil moisture, temperature, and substrate availability can also have large effects on microbial biomass and activity. Franzluebbers et al. (1994) found that soil microbial biomass carbon (C mic ) changed significantly during the cropping season in all crop sequences D. Geisseler (*) : W. R. Horwath Department of Land, Air, and Water Resources, University of California, Davis, PES Building, 1 Shields Ave, Davis, CA 95616, USA e-mail: djgeisseler@ucdavis.edu and tillage regimes under investigation. Bausenwein et al. (2008) also reported significant effects of sampling date on C mic and enzyme activities under minimum tillage. These observations raise the question of whether microbial and biochemical properties are affected by too many factors and fluctuate too much during the course of a season to be sensitive indicators of tillage-induced effects on soil quality. In general, conservation tillage (CT) practices leave a significant amount of plant residue on the soil surface. This results in an increased soil organic matter content in the topsoil, which in turn leads to higher microbial biomass and activity. This increase in organic matter content in the topsoil however is often offset with a decrease in lower soil layers (Dick 1992; Omidi et al. 2008). Franzluebbers (2002) suggested using changes in soil organic C with depth rather than the total amount of soil organic C in the profile as indicators of tillage-induced effects on soil properties. The stratification ratio, calculated by dividing the value for a soil property in the topsoil by its value in the subsoil, could not only be a more sensitive way to measure tillage-induced changes, but it also normalizes for differ- ences in climatic conditions and soil types between study sites. Because surface organic matter is essential to erosion control and water infiltration, the degree of stratification is directly linked to soil quality (Franzluebbers 2002). The objective of this study was to compare short-term dynamics of soil C, microbial biomass, and soil enzyme activities with longer-term effects of tillage in irrigated row crops in order to determine how environmental factors affect the use of these properties as sensitive indicators of tillage-induced changes in soil quality. A field trial was designed to test the following hypotheses: (1) enzyme activities and microbial biomass N (N mic ) respond more rapidly to differences in tillage treatments than total soil C.