Study area and experimental design
This study was conducted at the Wulongchi Experiment Station (32°45′N, 111°13′E; 280-400 m a.s.l) in the Danjiangkou Reservoir area, central China (Cheng et al., 2013). This area is a subtropical monsoon climate, with cold-dry winters and hot-humid summers. The mean annual precipitation is 749.3 mm, with 70% - 80% of the rain falls between April and October. The mean annual temperature is 15.7℃, with monthly maximum average temperature of 27.3℃in July and average minimum temperature of 4.2℃ in January (Wu et al., 2016). The soil is a yellow brown soil (according to Chinese soil classification system) with 11% sand, 41% silt, and 48% clay in the top 30 cm (Zhu et al., 2010). Human activities, such as tillage and deforestation, have caused water pollution, soil erosion and soil nutrient element losses around this reservoir (Zhu et al., 2010). Since 1980s, lager areas of uncultivated land around the reservoir have been converted to a woodland plantation of coniferous trees (Platycladus orientalis (Linn.) Franco) and a shrubland plantations (Sophora davidii (Franch.) Skeels) (Zhu et al., 2010). Cron (Zea mays (Linn.)) and rape (Brassica campestris (Linn.)) have been grown in rotation in cropland by conventional agricultural practices, including artificial weeding and chemical weeding, mineral fertilizations (approximately urea 375 kg ha-1 and urine ammonium 200 kg ha-1) and plowing to a depth of 0.4 m. The aboveground biomass of the corn straw and the rape was removed through harvesting.
The experimental design was a completely randomized complete block design containing four blocks/sites. Each block was approximately 3 ha (600 m×50 m) with 100 m buffer rows between two adjacent blocks. Four adjacent land types i.e., woodland, shrubland, cropland and uncultivated land (without any input of organic matter from trees and /or shrubs, i.e., the control) were included at each block. A comprehensive survey of soil and vegetation were conducted in October, 2016 to ensure the comparability (such as similar soil types and topographies) of the soil sampling plots among the four land types.