Applying Biochar With Fertilisers Decreases Nitrogen and Phosphorus Leaching and Improves Phosphorus Availability: A Meta‐Analysis

Nitrogen (N) and phosphorus (P) are two of the most essential nutrients for plants, but overapplication of N and P to cropping systems can lead to eutrophication and groundwater contamination through leaching. Biochar has been used to improve N and P use efficiency, reducing leaching, and the effects of biochar on N and P leaching and availability have been widely studied. Biochar is often applied with fertilisers; however, combined effects of biochar with fertilisers on N and P leaching and availability have not been adequately addressed. This meta‐analysis synthesised 1798 data points from 48 peer‐reviewed articles to evaluate the effects of applying biochar with fertilisers on N and P leaching and availability. Our results showed that applying biochar with fertilisers decreased NO 3 − , NH 4 + and P (as PO 4 3− ) leaching by 13.2% ± 0.1%, 1.5% ± 0.1% and 16.5% ± 0.7%, respectively (Bootstrap 95% CI). Method of biochar application emerged as the primary factor affecting N and P leaching. For NO 3 − leaching, time after biochar application was the next most important predictor, whereas NH 4 + leaching was predicted by the rate of biochar application and soil organic C concentration. P leaching was moderated by the feedstock type and pyrolysis temperature used to make biochars. Applying biochar with fertilisers did not affect NH 4 + and NO 3 − availability but increased P availability by 21.9% ± 0.5%. This study provides strong empirical evidence that co‐applying biochar with fertilisers is an effective management practice that can reduce nutrient leaching and minimise environmental impacts of cropping systems.