Assessing spatial variability of soil enzyme activities in pasture topsoils using geostatistics Tayfun Aşkın a, * ,Rıdvan Kızılkaya b a Department of Soil Science, Faculty of Agriculture, Karadeniz Technical University, Ordu, Turkey b Department of Soil Science, Faculty of Agriculture, Ondokuz Mayıs University, Samsun, Turkey Received 10 August 2005; accepted 6 February 2006 Available online 28 February 2006 Abstract The purpose of this study was to assess the spatial variability of the activity of three hydrolytic enzymes, i.e. urease activity (UAc), alkaline phosphatase activity (APAc), and arylsulfatase activity (ASAc), in pasture topsoils using geostatistics. Enzyme activities along a transect in a 1.35-ha pasture were determined using 77 soil samples from the upper 20 cm of soil. UAc varied from 101.0 to 182.7 μgNg -1 soil h -1 ; APAc varied from 1.56 to 3.62 μg p-nitrophenol g -1 soil h -1 ; and ASAc varied from 1.50 to 3.26 μg p-nitrophenol g -1 soil h -1 . The linear models fit the best semivariogram models for UAc, APAc, and ASAc. Semivario- grams for enzyme activities exhibited spatial dependence with ranges of influence of approximately 124.7 m. © 2006 Elsevier Masson SAS. All rights reserved. Keywords: Spatial variability; Urease activity; Alkaline phosphatase activity; Arylsulfatase activity 1. Introduction Soil enzymes are potential indicators of soil quality because of their relationship to soil biology, ease of measurement, and rapid response to changes in soil management [3,10–12,20,45]. Such indicators integrate chemical, physical, and biological characteristics and may be used to monitor the effects of soil management on long-term productivity. Some researchers already include extracellular enzyme activity as a biological in- dicator when discussing soil quality indicators [13]. Enzymes catalyze biochemical reactions and are an integral part of nutrient cycling in the soil. Soil en- zymes may be of microbial origin [22] or derived from plants and animals [42]. They are usually associated with viable proliferating cells, but enzymes can be ex- creted from a living cell or released into the soil solu- tion from dead cells [42]. A free enzyme complex with humic colloids may be stabilized on clay surfaces and organic matter [6], and hydrolytic extracellular soil en- zymes make nutrients available to plants and microor- ganisms by converting nutrients from inassimilable to readily assimilable forms [38]. In the present study, soil enzymes representative of the main nutrient cycles (nitrogen, phosphorus, and sul- fur) were examined. Urease is involved in the hydro- lysis of urea-type substrates to NH 3 and CO 2 ; its origin http://france.elsevier.com/direct/ejsobi European Journal of Soil Biology 42 (2006) 230–237 * Corresponding author. Tel.: +90 452 230 0556; fax: +90 452 225 1261. E-mail address: tayfuna@ktu.edu.tr (T. Aşkın). 1164-5563/$ - see front matter © 2006 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ejsobi.2006.02.002