Review of causes and sources of N2O emissions and NO3 leaching from organic arable crop rotations

Sissel Hansen*, Randi Frøseth, Maria Stenberg, Jaroslaw Stalenga, Jorgen E Olesen, Maike Krauss, Pawel Radzikowski, Jordi Doltra, Shahid Nadeem, Torfinn Torp, AV Pappa, CA Watson

*Corresponding author for this work

Research output: Contribution to journalReview articlepeer-review

12 Citations (Scopus)
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Abstract

The emissions of nitrous oxide (N2O) and leachingof nitrate (NO3) from agricultural cropping systems haveconsiderable negative impacts on climate and the environment.Although these environmental burdens are less perunit area in organic than in non-organic production on average,they are roughly similar per unit of product. If organicfarming is to maintain its goal of being environmentallyfriendly, these loadings must be addressed. We discussthe impact of possible drivers of N2O emissions and NO3leaching within organic arable farming practice under Europeanclimatic conditions, and potential strategies to reducethese. Organic arable crop rotations are generally diversewith the frequent use of legumes, intercropping andorganic fertilisers. The soil organic matter content and theshare of active organic matter, soil structure, microbial andfaunal activity are higher in such diverse rotations, and theyields are lower, than in non-organic arable cropping systemsbased on less diverse systems and inorganic fertilisers. Soilmineral nitrogen (SMN), N2O emissions and NO3 leaching are low under growing crops, but there is the potential forSMN accumulation and losses after crop termination, harvestor senescence. The risk of high N2O fluxes increases whenlarge amounts of herbage or organic fertilisers with readilyavailable nitrogen (N) and degradable carbon are incorporatedinto the soil or left on the surface. Freezing/thawing,drying/rewetting, compacted and/or wet soil and mechanicalmixing of crop residues into the soil further enhance therisk of high N2O fluxes. N derived from soil organic matter(background emissions) does, however, seem to be themost important driver for N2O emission from organic arablecrop rotations, and the correlation between yearly total Ninputand N2O emissions is weak. Incorporation of N-richplant residues or mechanical weeding followed by bare fallowconditions increases the risk of NO3 leaching. In contrast,strategic use of deep-rooted crops with long growingseasons or effective cover crops in the rotation reduces NO3leaching risk. Enhanced recycling of herbage from green manures,crop residues and cover crops through biogas or composting may increase N efficiency and reduce N2O emissionsand NO3 leaching. Mixtures of legumes (e.g. clover or vetch)and non-legumes (e.g. grasses or Brassica species) are as efficientcover crops for reducing NO3 leaching as monoculturesof non-legume species. Continued regular use of covercrops has the potential to reduce NO3 leaching and enhancesoil organic matter but may enhance N2O emissions. Thereis a need to optimise the use of crops and cover crops to enhancethe synchrony of mineralisation with crop N uptake toenhance crop productivity, and this will concurrently reducethe long-term risks of NO3 leaching and N2O emissions.
Original languageEnglish
Pages (from-to)2795-2819
Number of pages25
JournalBiogeosciences
Volume16
Issue number14
Early online date17 Jul 2019
DOIs
Publication statusFirst published - 17 Jul 2019

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