Changes in microbial enzyme activity in response to long- term dry season nitrogen inputs in semi-arid shrublands

Abstract

Atmospheric nitrogen (N) deposition has the capacity to alter soil microbial activity and organic matter decomposition. β-glucosidase, N-acetylglucosaminidase (NAGase), phosphatase, and peroxidase activity was measured in soils exposed to experimental N inputs for 13 years. We hypothesized that soils exposed to high N would have lower extracellular enzyme activity. Surface (0-10 cm) soil was sampled from the Sky Oaks Field Station and Santa Margarita Ecological Reserve in the spring of 2003, 2005, 2007, 2009, 2011, 2013, and 2015. The experimental design consisted of eight-10 x 10 m plots arranged in a paired-design. N plots have been fertilized annually since 2003 with 50 kgN/ha of dry N fertilizer, which is applied during the summer and fall of each year.  β-glucosidase and phosphatase activity declined over time in added N plots at SOFS but not SMER, while peroxidase (Fig. 5) activity increased in added N plots at SOFS but not SMER. NAGase activity was not affected by N addition at either site. We found that added N stimulated peroxidase activity but inhibited b-glucosidase and phosphatase activity over time for SOFS, which is contrary to other experiments in deciduous forest. Decomposition and enzyme activity are affected by organic matter quality. Decomposition of more recalcitrant organic matter (SOFS) may be inhibited by N while more labile litter (SMER) may not be affected by added N.