4.5 The effect land-use changes and pedo-climatic factors on the response ratio of SOC storage
In addition to the potential method uncertainties (i.e., sample depths and calculation methods), there are still some uncertainties in estimating SOC and TN storages (Fig. 6). In our meta-analysis, all available data are integrated and analyzed to study the relationship between their factors and SOC response to LUCC (we only discussed SOC). The uncertainties were mainly raised from four aspects: soil physicochemical characteristics (BD, TN, pH, TP, TK, clay content, soil water content), topographic factors (slope, altitude), climatic factors (average temperature, average rainfall) and the year of land-use change.
In this study, there was a significant correlation between soil characteristics and soil SOC storage (Fig. 6). Except for the soil BD, other factors were positively correlated with SOC storage in the deforestation (Fig. 6). Soil pH (r = 0.50, P < 0.01,n = 93) and total N storage (r = 0.80, P < 0.01,n = 69) were positively correlated with SOC storage. Taken together, we highlight that soil properties have strong effects on SOC storage based on the collected database.
We focused on two topographic variables, altitude and slope. It is generally believed that elevation could induce temperature changes and thus the SOC and TN storage, showing a positive correlation with SOC storage (Tashi et al., 2016). Recent studies have shown that the contribution of elevation to different vegetation systems may be due to rock exposure or human activities (Hu et al., 2018). On the other hand, the effect of slope on the distribution of SOC storage may be associated with soil erosion, microbial biomass and activity, evaporation and infiltration (Moffet et al., 2005). Different vegetation types will be formed with the combination of altitude and climate factors, resulting in different types and quantities of plant residues entering the soil, making SOC estimation more uncertain (Oskar et al., 2017). Temperature and precipitation are another main climatic variable considered in this study. These two variables not only affect the net primary productivity, but also the soil microbial properties, and thus the SOC and TN dynamics (Sotta et al., 2010). Besides, duration of LUCC is another important factor impacting SOC storage. In the early stage of LUCC, the changes of SOC is relatively higher, and gradually decreased with times (Morris and Sven, 2007; Wang et al., 2016;)
Taken together, it is a challenge to quantify the contribution of environmental factors to estimating SOC and TN storage. To reduce the estimated bias of SOC/TN storage, the interaction between human activities and climatic factors should be comprehensively considered in future work.