2.1 Data collection
For the datasets used by meta-analysis, we selected peer-reviewed
literature on the effects of LUCC on SOC and TN storage in southwest
China from 1990 to 2018 by searching major academic databases, e.g.,
CNKI, CAB Abstracts, Elsevier, China Knowledge Resource Integrated
Database, and by sorting the reference lists. The keywords and phrases
used for the literature search were: ‘Karst area of China’, ‘land
use/land cover change’, ‘soil organic carbon’, ‘total nitrogen’, ‘soil
quality’ in title, abstract, or keywords. In addition, each study was
further screened for integrity, relevance and scientific merits based on
the following steps: (1) Select an appropriate classification for each
study: defined as the experimental group and the control group. In the
vegetation destruction mode, the land use mode with the minimum human
disturbance intensity was selected as the control group (usually forest
land and grassland). In the model of returning farmland to forest and
grass, farmland was selected as the control group; (2) Ensure that each
study was independent and published after 1990. We excluded studies
before 1990 because land use patterns did not change much before that
year; (3) Make sure the soil sampling depth was more than 10 cm (the
minimum soil stratification in this study), and the same study cannot be
classified as both vegetation destruction and conversion of farmland to
forest and grass (unless clearly stated in the study); (4) Make sure all
publications specified the results of SOC or TN concentrations. If the
data was graphically represented, the
GetData
Graph Digitizer (Russian Federation version 2.22) was utilized to obtain
the data.
In total, 89 published papers and 939 sets of data were included,
consisting of 471 sets of SOC data and 468 sets of TN data (SOC or TN
concentration and storage, and soil mass). More specifically, 253 sets
of SOC data and 240 sets of TN data belonged to deforestation; 218
groups SOC data and 228 groups TN data belonged to land restoration.
According to the land use status classification criteria issued by the
Ministry of Land and Resources on November 1, 2017 (GB/T21010-2017), the
land cover in the Karst area was categorized into native forest,
grassland (including natural grassland, artificial grassland and
abandoned farmland), secondary forest, shrubby land, cropland (including
dry land and paddy fields), plantation and fruit tree. It is noted that
DF-others mainly refers to the transformation of grassland into
farmland, while LR-others mainly refers to the transformation of
grassland into woodland. Spatial distribution of the study sites where
the datasets were collected is shown in Fig. 1.
In this study, we classified the collected data into three categories:
(1) different patterns of LUCCs (vegetation destruction; returning
farmland to forest and grass); (2) SOC and TN storages accumulated in
different soil layers (0-20, 20-30, 30-40, 40-60, and 60-100 cm) in
response to LUCCs; (3) four specific impact factors as:
climate factors (annual average
temperature and annual precipitation), geographical factors (slope and
elevation), the duration of LUCCs, soil physicochemical properties
(initial SOC/TN storage, BD, TN, total phosphorus, total potassium and
pH, clay and water content), which were selected in order to further
explain the cumulative SOC/TN storage changes affected by LUCC.