Nitrous oxide and N-leaching losses from agricultural soil: Influence of crop residue particle size, quality and placement

Incorporation of crop residues provides a source of readily available C and N, and previous works indicate that farming strategies where crop residues are used for soil fertility purposes may lead to increased emissions of N2O. Information on the importance of different residue management on the potential for N2O emissions, however, is missing.

The objectives of this work were to determine the short-term effects of crop residue particle size and spatial distribution on soil-atmosphere fluxes of N2O. Implications for leaching losses of inorganic N were also assessed. The work included an experiment with lysimeters incubated in the field and an experiment with soil incubated under controlled conditions.

The results show that finely ground pea material (<3 mm) evolved 50 % more N2O (33.8 mg N m(-2)) than coarse particles (25 mm) of pea material (22.7 mg N m(-2)) and twice as much N2O as residue-free soil (16.5 mg N m(-2)). Barley material, on the other hand, did not influence N2O emissions regardless of particle size (10-17 mg N m(-2)).

The lack of N2O evolution with barley residue was likely due to N-limitations whereas with N-rich pea material the particle size obviously controlled N-availability. Carbon dioxide evolution increased about three-fold both with barley and pea residue, but apart from a transient initial depression in CO2 evolution with <3 mm particles there was no overall effect of particle size on CO2 evolution.

Very likely the grinding to <3 mm was inadequate to achieve soil physical protection of the crop residue material against microbial attack. Leaching of N tended to be reduced about 40 % with barley and 20 % with pea, but the numbers were not significantly different from residue-free soil, which leached 4.7-4.9 g N m(-2).

When wheat and alfalfa residues were mixed into the soil N2O emissions increased 6.5 and 1.6 times, respectively, compared with residue placed in a layer. Wheat residue in a layer evolved 3.4-times less N2O than alfalfa in a layer, whereas when mixed the two residue types evolved similar amounts of N2O.

This difference was probably due to N-limitations in localised zones around the layered wheat, The results from this study should be extrapolated to the Field situation only after very careful consideration. Nevertheless, the study emphasizes the potential for residue management to restrain N2O emissions from agricultural soils.

From a N2O mitigation point of view, incorporating of residues with low N-contents is advantageous over a homogeneous mixing of N-rich materials into the soil.