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

The emissions of nitrous oxide (N2O) and leaching
of nitrate (NO3) 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 N2O emissions and NO3
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), N2O emissions and NO3 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 N2O 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 N2O fluxes. N derived from soil organic matter
(background emissions) does, however, seem to be the
most important driver for N2O emission from organic arable
crop rotations, and the correlation between yearly total Ninput
and N2O emissions is weak. Incorporation of N-rich
plant residues or mechanical weeding followed by bare fallow
conditions increases the risk of NO3 leaching. In contrast,
strategic use of deep-rooted crops with long growing
seasons or effective cover crops in the rotation reduces NO3
leaching risk. Enhanced recycling of herbage from green manures,
crop residues and cover crops through biogas or composting may increase N efficiency and reduce N2O emissions
and NO3 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 NO3 leaching as monocultures
of non-legume species. Continued regular use of cover
crops has the potential to reduce NO3 leaching and enhance
soil organic matter but may enhance N2O 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 NO3 leaching and N2O emissions.