Table 2 Systematic review eligibility criteria using the PICOS framework

Populations (P):

Soil carbon stocks in various ecosystems and land uses

Included: All ecosystem types and land uses are acceptable if there is documentation of the study site and description of method used to determine carbon concentration and/or stock. Experiments that report results of multiple systems that can be deemed independent of another (e.g., different climatic region, soil type, cropping system) will be designated as a separate study.

Excluded: Laboratory and greenhouse experiments will not be included. Container/pot setups in field conditions will not be included.

Interventions (I):

Land application of biosolids

Included: Field studies of at least 24 months in duration that investigates biosolids/sewage sludge or domestic sewage/human waste that has undergone a recognized treatment practice to clean the waste [19]. Biosolids treatments can be mixed with other materials (e.g., compost, sand, sawdust, etc.), however biosolids must be at least 50% (on a dry weight basis) of the mixture. Liquid sewage sludge will be included. The purpose of the application of sewage sludge/biosolids should be for soil value and not irrigation value. Digestate and biochar derived from human fecal matter/domestic sewage will be included. Must include amount of biosolids carbon/organic matter added or provide enough details to calculate biosolids carbon/organic matter added to experiment (e.g., dry weight of biosolids added and total carbon content of biosolids).

Excluded: Municipal waste is defined as waste collected and treated by or for municipalities (e.g., food waste, glass, metals, paper, plastics, yard trimmings, etc.) [20]. Industrial wastes (e.g., papermill sludge, tannery sludge, etc.) will not be included in this study. Wastewater (i.e., effluent) will be excluded. Biosolids that are spiked by researchers (e.g., additional metals or other contaminants) will not be included as this represents a manipulated media outside of standard wastewater treatment processes for biosolids/sewage sludge/domestic waste. Studies less than 24 months.

Comparators (C):

Non-amended control

Synthetic fertilizer control

Included: Must include at least one or both control types (i.e., non-amended or synthetic fertilizer). Must include “before-and-after” soil organic carbon/matter measurements for both a control and biosolids populations (i.e., soil organic carbon/matter measurement prior to experiment and after intervention) OR a final control and biosolids soil organic carbon/matter measurement.

Excluded: Studies that do not have a control “final” measurement comparator.

Outcomes (O):

Changes in soil organic carbon/matter

Changes in biomass

Changes in soil biological, chemical, physical parameters

Included: Must include primary research that documents the change in soil organic carbon/matter concentration/stock. Other measurements will be documented if reported (i.e., change in plant biomass, soil bulk density, etc.).

Excluded: If there is more than one primary research article on the same experiment, the latest reported measurement values will be used.

Study type (S):

Included: Before-After-Control-Impact (monitors control and impact groups from before and after impact occurred), Control-Impact (lacks pre-impact data), and Randomized Controlled Trials (though these are not typical study designs and do not expect them).

Excluded: Before-After (lacks control) and After (lack pre-impact and a control). Personal views and perspectives, and model predictions.