Surrounding plant community and soil measurements

A detailed description of the field experiment is presented in the article. At the end of August 2011, plant community measurements were made in each plot, to estimate the structural complexity of the community. For each plant community, the percentage cover of plant species was recorded in two 1 m2 quadrants along a diagonal transect within each plot. The total percentage cover can exceed 100% because plants in a community can overlap. The height of the vegetation was measured using the vertical drop disc method (Stewart et al. 2001). The disc weighed 200 g, had a diameter of 300 mm, and was released from a 1.5-m height. The height was measured at 10 random locations within each plot. One week after plant sampling, soil cores of 15 cm depth and 2.5 cm diameter were collected from each experimental plot at five random positions. The soil samples were pooled per plot and used for chemical analysis. Mineral N content (NH4+ and NO3) in soil samples was determined colorimetrically in the CaCl2 extraction using a Traacs 800 autoanalyser (TechniCon Systems Inc, USA). The C:N-ratio in soil samples was measured on a FlashEA 1112 Series NC soil analyser (Thermo Scientific). pH was measured in 2:5 dry soil : water suspensions. The percentage organic C was determined according to Nelson and Sommers (1982) and available P according to Olsen et al. (1954) and measured at 720 nm.