Effects of fencing on contents of soil ammonium nitrogen and nitrate nitrogen in the Loess Plateau, northern China

A study was conducted in fenced grassland (F, fenced for 7 years) and grazed grassland (G) on the Loess Plateau to investigate the seasonal variation of the contents of soil ammonium nitrogen(N-N) and nitrate nitrogen (N-N). Redundancy analysis was also used to study the effects of soil water content, above-ground green biomass and microbial biomass carbon on N-N and N-N contents. The results showed that N-N was the main component of soil available nitrogen in both fenced and grazed grasslands. In the 0-10 cm soil layers of the two sites, soil N-N and nitrate nitrogen were mainly controlled by above-ground green biomass. In the 10-20 cm soil layers, soil N-N and nitrate nitrogen were mainly controlled by both of soil water content and above-ground green biomass in fenced grassland, while soil N-N and N-N were mainly controlled by above-ground green biomass in grazed grassland. There was significantly positive correlations (P<0.05) between N-N and N-N in the two sites. The results indicated that the higher above-ground biomass after fencing for 7 years increased litter inputs and soil organic matter, thus increased substrate for nitrogen mineralization. In addition, soil water content significantly increased in most seasons (P<0.05) and all of these factors promoted the nitrogen mineralization in the soil.