Organic Amendments Tighten Nitrogen Cycling in Agricultural Soils: A Meta-Analysis on Gross Nitrogen Flux

Authors: Breza, Lauren; GRANDY, STUART - University Of New Hampshire

Submitted to: Frontiers in Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/3/2025
Publication Date: N/A
Citation: N/A
DOI: https://doi.org/10.3389/fagro.2025.1472749

Interpretive Summary: Title: Organic Amendments Tighten Nitrogen Cycling in Agricultural Soils: A Meta-Analysis on Gross Nitrogen Flux Authors: Lauren C. Breza and A. Stuart Grandy Interpretive Abstract This study examines the effects of different nitrogen (N) inputs—specifically organic (plant residues and animal manures) and synthetic amendments—on N transformation processes like mineralization, immobilization, and nitrification. We conducted a systematic review of existing literature that measured gross rates of nitrogen transformations using 15N isotope pool dilution methods. The findings from our analysis show that synthetic, manure, and residue amendments significantly increased mineralization rates by 60%, 135%, and 214%, respectively, compared to unamended controls. Plant residues promoted both higher immobilization and enhanced ammonium (NH4+) pool sizes, suggesting a tighter coupling of the carbon and nitrogen cycles due to the higher C/N ratio substrates provided to soil microbes. In contrast, manure accelerated mineralization but did not significantly affect NH4+ pool size or immobilization, potentially increasing nitrogen loss risks. These results underscore the importance of balancing nitrogen inputs to enhance both short-term nitrogen availability and long-term retention in soils, which is critical for maintaining soil fertility and improving nitrogen use efficiency in agricultural systems.

Technical Abstract: Title: Organic Amendments Tighten Nitrogen Cycling in Agricultural Soils: A Meta-Analysis on Gross Nitrogen Flux Authors: Lauren C. Breza and A. Stuart Grandy Technical Abstract Understanding how nutrient inputs affect nitrogen (N) transformations and storage is vital for developing sustainable agroecosystems. Organic N inputs can accelerate N cycling by increasing mineralization and immobilization rates to provide crops with more opportunities to intercept N as it moves through bioavailable pools. We aim to understand how organic (plant residues and animal manures) and synthetic, inorganic soil amendments inhibit or promote N mineralization, immobilization, and nitrification rates. Our literature synthesis targeted studies using 15N pool dilution methods to assess N transformation rates in response to different soil amendments. Our findings indicate that adding synthetic, manure, and residue soil amendments increased mineralization by 60%, 135%, and 214%, respectively, relative to the unamended controls. While manure and residue (organic amendments) produced similar mineralization rates, residue amendments induced significantly higher immobilization rates than synthetic and manure amendments. Furthermore, only residue N amendments enhanced the NH4+ pool size, while synthetic and manure amendments resulted in no change in NH4+ pool size. These results suggest that residue amendments might encourage tighter coupling of the carbon and nitrogen cycles by delivering higher C/N ratio substrates to soil microbes, resulting in faster mineralization-immobilization processes and larger NH4+ pools. Manure amendments strongly accelerate mineralization and thus can provide crops N but without enhanced immobilization recycling this N back into organic forms it may also be vulnerable to loss. Balancing N production and consumption is critical for managing soil fertility and our analysis improves our understanding of N transformations under organic and synthetic N inputs.