Reviews and syntheses: Review of causes and sources of N<sub>2</sub>O emissions and NO<sub>3</sub> leaching from organic arable crop rotations

Abstract

The emissions of nitrous oxide (N 2 O) and leaching of nitrate (NO 3 ) from agricultural cropping systems have considerable negative impacts on climate and the environment. Although these environmental burdens are less per unit area in organic than in non-organic production on average, they are roughly similar per unit of product. If organic farming is to maintain its goal of being environmentally friendly, these loadings must be addressed. We discuss the impact of possible drivers of N 2 O emissions and NO 3 leaching within organic arable farming practice under European climatic conditions, and potential strategies to reduce these. Organic arable crop rotations are generally diverse with the frequent use of legumes, intercropping and organic fertilisers. The soil organic matter content and the share of active organic matter, soil structure, microbial and faunal activity are higher in such diverse rotations, and the yields are lower, than in non-organic arable cropping systems based on less diverse systems and inorganic fertilisers. Soil mineral nitrogen (SMN), N 2 O emissions and NO 3 leaching are low under growing crops, but there is the potential for SMN accumulation and losses after crop termination, harvest or senescence. The risk of high N 2 O fluxes increases when large amounts of herbage or organic fertilisers with readily available nitrogen (N) and degradable carbon are incorporated into the soil or left on the surface. Freezing/thawing, drying/rewetting, compacted and/or wet soil and mechanical mixing of crop residues into the soil further enhance the risk of high N 2 O fluxes. N derived from soil organic matter (background emissions) does, however, seem to be the most important driver for N 2 O emission from organic arable crop rotations, and the correlation between yearly total Ninput and N 2 O emissions is weak. Incorporation of N-rich plant residues or mechanical weeding followed by bare fallow conditions increases the risk of NO 3 leaching. In contrast, strategic use of deep-rooted crops with long growing seasons or effective cover crops in the rotation reduces NO 3 leaching risk. Enhanced recycling of herbage from green manures, crop residues and cover crops through biogas or com-Published by Copernicus Publications on behalf of the European Geosciences Union. 2796 S. Hansen et al.: Review of N 2 O emissions and NO 3 leaching from organic arable rotations posting may increase N efficiency and reduce N 2 O emissions and NO 3 leaching. Mixtures of legumes (e.g. clover or vetch)and non-legumes (e.g. grasses or Brassica species) are as efficient cover crops for reducing NO 3 leaching as monocultures of non-legume species. Continued regular use of cover crops has the potential to reduce NO 3 leaching and enhance soil organic matter but may enhance N 2 O emissions. There is a need to optimise the use of crops and cover crops to enhance the synchrony of mineralisation with crop N uptake to enhance crop productivity, and this will concurrently reduce the long-term risks of NO 3 leaching and N 2 O emissions.

Articulating the effect of food systems innovation on the Sustainable Development Goals

“…To capture the benefit of less nitrogen loss from the use of inorganic fertiliser, the system needs to use any residual nitrogen in roots and residues remaining after harvest. 68 Lower prices could, however, also increase the demand for both food and feed, leading to increased livestock production, reducing (potentially even entirely offsetting) the direct environmental savings (figure 1D).…”

The Biology of Legumes and Their Agronomic, Economic, and Social Impact

“…Despite the fact that legumes contribute to nitrogen enrichment of soil BNF, it is worth mentioning that their over-frequent use of these plant species can also lead to nitrate leaching (De Notaris et al 2018;Hansen et al 2019). Thus, it is important to optimize the use of legumes (e.g., appropriate crop rotation sequences, mixtures of legumes, and nonlegumes) in order to reduce the risk of nitrate leaching (Hansen et al 2019;Rakotovololona et al 2019).…”

Nitrous oxide emissions from cassava fields amended with organic and inorganic fertilizers

“…Though more N was applied in the organic fertilizer‐based treatment, it appears most of the N was in organic form and would have had to undergo mineralization to be available for processes such as nitrification and denitrification that are responsible for N 2 O production (Hansen et al., 2019). Our results suggest that there may have been good synchrony between N demand and supply in the organic fertilizer treatment.…”