Abstract. 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 N-input 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.
Grain legume-based rotations managed under conventional tillage need cover crops to mitigate soil organic matter losses