Search strategy and eligibility criteria
Two reviewers will search the literature independently. The keywords and their combination will be used to identify proper studies and the search strategy will be listed as follows:
#1 soil mineral-associated carbon or soil mineral-associated carbon matter or MAOC or MAOM or MOM
#2 enrichment or addition or application or input or amend
#3 labile organic C or fresh organic C or external organic C or exogenous organic C or litter or straw or residue or substrate
#3 content or distribution or respiration or mineralization
#4 decomposition or respiration or CO2 or mineralization or distribution or content
#5 #1#2 #3 and #4
#6 #5 NOT (review or meta-analysis [Title])
#7 #6 NOT (meeting or abstract or letters or other [Publish Type])
Then we will select the final studies manually after #7 based on the following criteria:
ⅰ) The study was conducted in terrestrial ecosystems, or the soils used for laboratory incubation were collected from terrestrial ecosystems.
ⅱ) The study separated MAOC from POM through the method of particulate size or density fractions or their combination, and reported MAOC-derived CO2 in the incubation experiment or MAOC content in the field or incubation experiment.
ⅲ) The study included the control treatment (without C additions), and treatment (C additions).
ⅳ) Means, standard deviations (SDs) or standard errors (SEs), and the number of replicates for control and treatment groups could be obtained from text, tables or digitized graphs.
The three databases will use a similar search strategy. A flow diagram demonstrating the entire study selection process is shown in Fig. 1.
Data collection and analysis
We will collect information from each included study, including first author, publication date, study site and its basic information (longitude, latitude, MAT, MAP, and soil type), ecosystem, the forms and rates of C additions, experiment duration, and the means and SD or SE of MAOC-CO2 and MAOC content and the replicates from treatments and control. we will categorize them into topsoils (0–20 cm) and subsoils (below 20 cm depth) when studies include soils from multiple layers at the same site. All unit of MAOC content or MAOC-CO2 will be converted to be the same during data collection. We will use mean difference as an effect size to indicate the effects of C additions on MAOC decomposition. The random effect model will be chosen when the heterogeneity is above 50%.
To test the general effects of C additions on MAOC content and priming decomposition, we will calculate the mean effect size and its corresponding 95% confidence interval (CI) using the mixed-effects model. The subgroup analyses and meta-regression analyses will be used to explore sources of heterogeneity when the heterogeneity is observed. Subgroup and meta-regression analyses will be carried out on ecosystems, the rates and the forms of C additions, surface and subsurface soils, experiment duration, soil properties including the content of carbon, nitrogen, the ratio of carbon to nitrogen, clay, silt and sand. We will carry out the sensitivity analysis to evaluate the bias. In addition, we will explore potential publication bias using funnel plots, trim and fill method, and the Egger test. All calculations will be performed using Review Manager (version 5.3) and STATA (version 13.1) software.