Nitrogen loss from croplands is a major environmental problem with human health and ecosystem consequences. Cover crops are planted during the fallow period between cash crops to provide a number of ecosystem services and are a popular tool for nitrogen reduction. Unfortunately, on-farm adoption of cover crops in the USA is limited to less than 2 % of land area in most regions. Naturally occurring weeds, if left unmanaged between cash crops (i.e., "weedy fallow"), may provide similar ecosystem services (e.g., nitrogen reduction) as cover crops. To test this hypothesis, a metaanalysis of 17 studies was conducted to compare potential nitrogen loss (inorganic soil nitrogen or leachate nitrogen) from fallow annual cropping systems managed with cover crops, weeds, and bare soil. A potential nitrogen loss response ratio (e.g., leachate nitrogen from bare soil relative to weedy fallow) was determined for independent paired observations in each study, and factors influencing nitrogen loss across all studies were determined with 95 % bootstrap confidence intervals and meta-regression analysis. Results suggest that potential nitrogen loss from croplands is 60 % greater in bare soil compared to weedy fallow fields. Cover crops further reduced potential nitrogen loss by 26 % compared to weedy fallow, and the nitrogen reduction potential of cover crops (relative to weedy fallow) likely increases with greater biomass accumulation of the cover crop. While cover crops were more effective in mitigating potential nitrogen loss, weedy fallow may provide greater net benefits on a regional scale if there were fewer barriers to farmer adoption. The weedy fallow strategy for nitrogen reduction has several potential pitfalls (e.g., propagation of noxious or herbicide-resistant weedy species), but this meta-analysis demonstrates that fallow weeds provide an important ecosystem service and policy-makers should consider revising conservation incentive programs accordingly. Keywords Cover crops. Conservation agriculture. Nitrogen leaching. Water quality. Winter weeds Contents 1. Introduction 2. Meta-analysis methods 2.1. Article search and selection criteria 2.2. Data extraction and analysis 3. Results and Discussion 3.1. Weedy fallow and cover crops reduce potential nitrogen loss 3.2. Increasing vegetative biomass may reduce potential nitrogen loss 3.3. Options for increasing the benefits of weedy fallow 3.4. Limitations of weedy fallow 3.5. Policy implications and potential impact of weedy fallow 4. Conclusion * Sam E. Wortman
