Combined deep sampling and mass-based approaches to assess soil carbon and nitrogen losses due to land-use changes in karst area of southwestern China

Yecui Hu,Guichun Li,Qibing Wang,Zhangliu Du

doi:10.5194/se-7-1075-2016

Abstract

Abstract. The conversion of natural vegetation to human-managed ecosystems, especially the agricultural systems, may decrease soil organic carbon (SOC) and total nitrogen (TN) stocks. The objective of present study was to assess SOC and TN stocks losses by combining deep sampling with mass-based calculations upon land-use changes in a typical karst area of southwestern China. We quantified the changes from native forest to grassland, secondary shrub, eucalyptus plantation, sugarcane and corn fields (both defined as croplands), on the SOC and TN stocks down to 100 cm depth using fixed-depth (FD) and equivalent soil mass (ESM) approaches. The results showed that converting forest to cropland and other types significantly led to SOC and TN losses, but the extent depended on both sampling depths and calculation methods selected (i.e., FD or ESM). On average, the shifting from native forest to cropland led to SOC losses by 19.1, 25.1, 30.6, 36.8 and 37.9 % for the soil depths of 0–10, 0–20, 0–40, 0–60 and 0–100 cm, respectively, which highlighted that shallow sampling underestimated SOC losses. Moreover, the FD method underestimated SOC and TN losses for the upper 40 cm layer, but overestimated the losses in the deeper layers. We suggest that the ESM together with deep sampling should be encouraged to detect the differences in SOC stocks. In conclusion, the conversion of forest to managed systems, in particular croplands significantly decreased in SOC and TN stocks, although the effect magnitude to some extent depended on sampling depth and calculation approach selected.

Highlights

Land-use change, like deforestation has become a significant concern in terms of environmental degradation and global climate change (Harris et al, 2012; Mukhopadhyay et al, 2016; Wiesmeier et al, 2015)
For the 0–40 cm depth, the averaged soil organic C (SOC) concentration was in the order of Native forest (NF) > UG > Secondary shrub (SS) = EF, and SS > Sugarcane field (SF) = Corn field (CF), indicating deforestation significantly reduced SOC level, converted into the croplands (Table 2)
Our study demonstrates that the conversion of native forest into cropland and other managed systems led to SOC and total nitrogen (TN) losses

Summary

Introduction

Land-use change, like deforestation has become a significant concern in terms of environmental degradation and global climate change (Harris et al, 2012; Mukhopadhyay et al, 2016; Wiesmeier et al, 2015). The large-scale conversions of natural ecosystems to croplands and other managed ecosystems have already resulted in historically large emissions of C into the atmosphere (as higher as 320 Pg C), since the dawn of settled agriculture (Lal, 2010). Most soil C studies have focused only on the surface layers (i.e., ≤ 30 cm) to clarify the SOC in response to land-use changes (Baker et al, 2007; Post and Kwon, 2000; Wei et al, 2014; West and Post, 2002). Because more than one-third of roots and more than one-half of soil C are stored below 20 cm depth (Jobbágy and Jackson, 2000), the conversion of land use change may well influ-

Methods

Results

Discussion

Conclusion