2.1 Site description and sample collection
The sampling site is located in Yanqing District (40o29’ N, 11 5 o 56’ E), Beijing, China, with a temperate continental monsoon climate. The annual temperature and precipitation was 8oC and 400-500 mm, respectively. Most of the rainfall occurs between July and September. The soil is classified as aquic inceptisol with a bulk density of 1.31 g cm-3, and a sandy loam texture. There is a local dairy farm sitting within this area with a total of over two hundred cattle, producing around 4,000 t a-1 livestock waste loading into the maize fields near the daily farm (Figure 1). Therefore, three maize fields each with over 100 x 100 m size were selected as three experiment plots, and each received cattle manure with 4x 104 kg ha-1 without any other fertilizers every spring for ten years. Soil samples were also collected from three adjacent woodlands with secondary forest developed from abandoned cropland for over ten years receiving no dairy manure or chemical fertilizers.
Sampling along a 0-100 cm profile including 0-20, 20-40, 40-60, 60-80 and 80-100 cm depth took place in May and October in 2018 just after maize seeds were sown and maize harvested, respectively. For each plot, 5 soil cores from each plot were collected with a hand auger and homogenized into a soil sample for each depth. Therefore, a total of 60 soil samples were obtained from two land uses for two seasons. After removing large pieces of rocks and plant material, soils collected from fields were transported to the laboratory immediately, and passed through a 2.0-mm sieve. Samples were preserved at 4 oC and -20 oC for further soil properties and molecular analysis, respectively.