Nitrogen dynamics in agroforestry systems. A review

• Published in: Agronomy for sustainable development Vol. 42; no. 4
• Main Authors: Kim, Dong-Gill, Isaac, Marney E.
• Format: Journal Article
• Language: English
• Published: Paris Springer Paris 01.08.2022; Springer Nature B.V; Springer Verlag/EDP Sciences/INRA
• Subjects: Agriculture; Agrochemicals; Agroforestry; Agronomy; Ammonia; Biomedical and Life Sciences; Continuous cropping; Crop diversification; Crop yield; Crops; Emissions; Fertilizers; Fluxes; Leaching; Life Sciences; Mineralization; Mitigation; Nitric oxide; Nitrogen; Nitrogen fixation; Nitrogenation; Nitrous oxide; Organic matter; Pools; Review Article; Riparian buffers; Runoff; Soil erosion; Soil Science & Conservation; Subsoils; Sustainability; Sustainable Development; Trees; Nitrogen use efficiency; Internal nitrogen cycling; Ecosystem services; Diversified farming systems; Nitrous oxide emissions; Nitrogen cycling
• ISSN: 1774-0746; 1773-0155
• DOI: 10.1007/s13593-022-00791-7

Agroforestry is an agricultural diversification practice that purposefully grows trees together with crops and/or animals. Nitrogen (N) plays a pivotal role in the sustainability and productivity of these systems. In this study, we synthesize our current understanding and data availability of N dynamics in agroforestry systems, providing the first comprehensive synthesis on major N pathways in agroforests. We review five categories of N pools and fluxes: N input, internal N cycle, N sequestration, N loss, and N output. Within these categories, we compile data from worldwide measurements of N input and N gaseous emissions. The N inputs among agroforestry systems are from N 2 fixation through leguminous trees (global average: 240 kg N ha –1 yr –1 ), organic amendments and inorganic fertilizers (36 kg N ha –1 yr –1 ), and, possibly, undocumented N deposition. Various pathways are identified in internal N cycles: direct transfer of fixed N from tree to crop, N mineralization of organic matter originating from trees, pumping up subsoil N by tree roots, and ammonia captured by trees. Agroforestry practices almost consistently show reductions in N loss, via soil erosion, runoff, and leaching. Agroforestry systems release gaseous N as nitrous oxide (N 2 O) (~4.7 kg N 2 O–N ha –1 yr –1 ) and nitric oxide (NO). While N 2 O emissions in agroforestry are either higher (improved fallows) or lower (riparian buffers) in comparison to monocropping, NO emissions are similar in agroforestry (improved fallows) and monocropping. The N sequestration rates in agroforestry soil pools (98–154 kg N ha −1 yr −1 ) are three times higher than biomass pools (36–43 kg N ha −1 yr −1 ). Since agroforestry practices can increase crop yield and N content in crops, this translates into higher N output in agroforestry systems. Studies are required to accurately quantify sub-components of N pools and fluxes and identify mitigation measures to manage N loss in agroforestry systems.