4.2 N and P fertilization and GRSP
Human activities, such as grazing and nutrient addition, cause environmental changes (Zhang et al., 2015). Herein, grazing changed environmental conditions and soil GRSP content. Also, for five years, N and P addition increased the available N and P content and activities of some enzymes (Sun et al., 2016). However, the addition did not affect SOC and GRSP contents. Previous reports indicated that N fertilization(10g/m2)for 0.3-1.6 years does not significantly affect GRSP (Garcia et al., 2008). Zhong et al. (2017) also found that N addition (0–300 kg N ha-1 y-1) for one year does not significantly influence SOC in any size aggregate fractions. Herein, N and P fertilization for four years showed a similar effect. This may be due to the relatively long turnover of GRSP and the shorter duration of fertilization, which usually has a delayed effect. Therefore, a longer duration of nutrients fertilization is required to detect changes in GRSP concentrations. In contrast, Zhang et al. (2015) indicated that continuous N addition for five years increases T-GRSP in the top 5 cm soil by 30%, but it does not affect EE-GRSP content. Other studies have shown that GRSP contents increases after N addition (Sun et al., 2018) and manure input (Xie et al., 2015). These inconsistencies may be due to the initial soil nutrient element content (Treseder et al., 2007) and nutrient element addition level and duration (Garcia et al., 2008; Sun et al., 2018).