Impacts of agronomic measures on crop, soil, and environmental indicators: A review and synthesis of meta-analysis

Sustainable agricultural management implies optimization of resources for crop production while minimizing adverse impacts on the environment. This requires a better understanding of the synergies and trade-offs of agronomic management while accounting for the controlling effects of site-specific factors (covariates). We systematically evaluated 113 meta-analytical studies assessing impacts of crop management measures (rotation, cover cropping, residue retention), soil and water measures (irrigation, tillage), soil amendments (enhanced efficiency, biochar), fertilizer use (organic, mineral, combined organic-mineral) and “4R'” fertilizer strategies (right source, rate, timing, placement) on sustainability indicators. These indicators include crop yield, crop N and P (content, uptake, and use efficiency), soil quality indicators (soil organic C, N and P contents, compaction), soil emissions of ammonia (NH₃) and greenhouse gases (CO₂, N₂O), and nutrient losses to water (N and P surplus or leaching). Nutrient management, including 4R practices as well as enhanced efficiency amendments, had the largest impact, increasing crop yields and N uptake while reducing N₂O and NH₃ emissions as well as N surplus, whereas effects on CO₂ emissions were variable. Although all measures positively impacted soil C, the largest effect was due to biochar, followed by organic fertilizer input. Biochar positively impacted crop yield, diminished N₂O and NH₃ emissions as well as N surplus, and increased CO₂ emissions. Within crop management, only cover cropping had a significant positive effect on crop yield, while both cover crops and rotation slightly enhanced N uptake and the sequestration of C and N in soil, thus reducing N₂O emissions and N surplus. Minimal tillage practices generally increased SOC, while results for crop yield, N surplus and N₂O emissions were variable. Site-specific factors had substantial impacts on the evaluated impacts of measures, most importantly climate, crop type, soil texture, soil pH, soil organic C, N dose, and experimental duration. Considering the variation among meta-analytical protocols followed, we recommend that field studies and meta-analytical work adhere to harmonized guidelines with respect to the reporting of site-level data, experimental design, and the statistical procedures used. This will ensure data comparability between studies, improve the quality of meta-analysis results, and give better insights into currently uncertain or unknown impacts of agronomic measures.