Abstract

This work deals with the behaviour of oilseed rape residues in soil and their impact on soil inorganic N dynamics after harvest. Potential decomposition was measured under optimal conditions during soil incubations by using ¹³C¹⁵N labelled oilseed rape residues (roots, stems, pod walls and leaves). Decomposition was rapid, 30 to 45 % of residue-C being mineralised during 168 days at 15°C. C mineralisation rates were influenced by the biochemical composition of residues but was not affected by the C:N ratio of residues. All residues induced soil inorganic N immobilisation during the first step of decomposition. Elsewhere, the labelling of residues with ¹⁵N showed that 15 to 27% of residues-N were mineralised during the incubation period, depending of organs. Decomposition of high-N and low-N content residues in soil during a field experiment showed that about 50% of residue-C were mineralised during the first two months following their incorporation. Residues decomposition induced soil inorganic N immobilisation which intensity and duration was related to the N content of residues. Finally, 269 days after their incorporation in soil, only the high-N content residues induced in a mineral N surplus in the soil, equivalent to 9 kg N ha^-1, by comparison to a control soil without application of residues.

Introduction

N fertilisation and growth conditions of oilseed rape crops influence the total dry matter production, the proportion between organs (i.e. ratio between the root system and the aerial parts) and the N content of the different organs (Rahn et al., 1992). Furthermore, at harvest, crops residues represent heterogeneous materials (roots, pods walls, leaves, stems) with different N contents and biochemical characteristics (Trinsoutrot et al., 1999a). Few laboratory (Janzen and Kucey, 1988) or field experimental work (Franzluebbers et al., 1995; Thomsen and Christensen, 1996; Jensen et al., 1997) has been done to study C and N cycling after incorporation of oilseed rape crop residues in soil. Aim of this work was to study the decomposition of oilseed rape residues in soil and its impact on the soil inorganic N dynamics by using laboratory and field experiments.

RESIDUES DECOMPOSITION UNDER OPTIMAL LABORATORY CONDITIONS

The decomposition of ¹³C¹⁵N labelled oilseed rape organs (stems, pod walls, roots and leaves), which was obtained in a growth chamber and which characteristics are given in Table 1, were studied during laboratory incubation under optimal conditions (ground residues, 15°C, soil moisture = pF 2.7, non-limiting N) (Trinsoutrot et al., 1999b).

Table 1: Characteristics of ¹³C¹⁵N labelled residues studied during soil incubations

Plant organs Stems Pod walls Roots Leaves

N content % d.m. 1.1 1.4 1.7 1.6

Soluble compounds % residue-C 35.7 35.1 31.1 64.7

Hemicelluloses % residue-C 17.4 19.3 20.7 16.2

Cellulose % residue-C 37.9 37.1 30.5 16.9

Lignin % residue-C 8.8 8.7 17.6 2.2

Fig 1: C decomposition of ¹³C¹⁵N labelled oilseed rape organs in soil.

Fig. 2: Soil inorganic N dynamics
after incorporation of ¹³C¹⁵N
oilseed rape organs.

Carbon decomposition of residues was rapid, differences between organs being essentially due to the biochemical characteristics of residues (Fig. 1): the lowest decomposition was obtained for roots which have the highest lignin content and the highest for leaves which have the higher soluble compounds content. C mineralisation was not affected by the residues C:N ratio since sufficient inorganic N was added in soil at the beginning of incubation to avoid the limitation of decomposition by N availability.

Fig. 3: Net N mineralisation of
¹³C¹⁵N labelled oilseed rape
organs in soil.

By comparison with a control soil without residues, incorporation of oilseed rape organs induced soil inorganic N immobilisation at the beginning of the decomposition (Fig. 2). Both amount of N immobilised and N dynamics were related to the organ C:N ratio. The use of ¹⁵N labelled organs allowed to calculate of the net N mineralisation of residues (Fig. 3). After 168 days of incubation at 15°C, depending of organs, 15 to 27% of organic residues N was mineralised in soil.

RESIDUES DECOMPOSITION UNDER FIELD CONDITIONS

The field experiment was set up in an highly calcareous rendosol located in the Champagne area (Northeast of France) (Trinsoutrot et al., 1999c). Whole crop residues were obtained from an experiment involving two N fertilisation levels: no N fertilisation (N0 residues) and 270 kg N ha^-1 (N270 residues). These residues differed essentially by their organic N content: 0.4 and 0.9% for N0 and N270 residues, respectively. Three treatments were considered (control soil without incorporation of residues, soil with N0 residues and soil with N270 residues). Residues decomposition were studied in bare soil by using two experimental designs: plots and PVC cylinders inserted into the soil profile which constitute a semi-controlled system where soil and residues quantities and residues spatial distribution in soil are well controlled and known.

Fig. 5: Inorganic N dynamics in the soil of cylinders with or without residues.

Carbon decomposition, measured for residues incorporated in the PVC cylinders was rapid (Fig. 4). A large part of the mineralisation of the residues took place during the two months following their incorporation in soil, where 48 and 43 % of N0 and N270 residues-C were respectively mineralised. These small differences in decomposition were attributed to the similarity of the biochemical characteristics of the two residues.

Fig. 4: C decomposition of oilseed rape residues in soil under field conditions.

During the 2 first months, in the 0-10 cm layer, inorganic N amounts increased only for the control soil (Fig. 5). For the 10-28 cm layer, inorganic N amounts increased for all treatments, but this increase was more important for the control soil. Those results suggested that the incorporation of N0 and N270 residues lead to an immobilisation of soil inorganic N.

Net N mineralisation for the three comparable treatments in the plot experiment was calculated using the LIXIM model (Fig. 6a). This model calculates N leaching and N mineralisation in bare soil by taking into account evolution of N profiles between two dates, climatic data and soil characteristics (Justes et al., 1999). The evolution of soil inorganic N content was equally simulated for the soil with incorporation of N270 residues for the cylinder experiment (Fig. 6a). 269 days after the incorporation of the residues, the total amounts of N mineralised, simulated by the model, were in good agreement between the two experimental designs: 95 and 102 kg N ha^-1 were respectively mineralised for the N270 residues in field and in cylinders. Net effect of the residues on soil inorganic N dynamics (Fig. 6b) was calculated by difference between simulated cumulative mineralised N in soil with residues and control soil. Both N0 and N270 residues induced net N immobilisation during the first month after the incorporation in soil. The amounts of N immobilised by the two residues were similar and equal to 20 kg N ha^-1. Net immobilisation was followed by net N mineralisation. At the end of the experiment, the incorporation of N270 and N0 residues induced respectively a positive (+ 9 kg N ha^-1) and a negative (-2 kg N ha^-1) net effect by comparison with the control soil.

Fig. 6: Simulated cumulative N mine-ralisation in soil and net effect of residues calculated by compa-rison with the control soil.

Fig. 7: Comparison of soil inorganic N
evolution after incorporation of
residues studied in field or during
soil incubation.

Results obtained in the field experiment with N0 and N270 residues were compared with soil incubation data obtained with the same residues by using the concept of normalised days (Rodrigo et al., 1997): it allows to compare data obtained under various soil moisture and temperature conditions by using standardised conditions (normalised days) with reference temperature and soil moisture conditions. Important differences were observed for the decomposition of N0 and N270 residues in field and during soil incubations (Fig. 7). Amounts of N immobilised in the field experiment were less than those immobilised during soil incubations. In laboratory experimental data were obtained under optimal conditions (non-limiting N conditions, optimisation of contact between soil and residues), while N availability was less in field and heterogeneity of the localisation and size of residues resulted from the agricultural practices. Indeed, crop residues decomposition in soil are influenced by inorganic N availability (Recous et al., 1995) and the contact between soil and residues which result from the localisation of the residues in the soil profile (Darwis et al., 1994) and the size of residues (Fruit et al., 1998).

Conclusions

N content of oilseed rape crop residues may vary between 0.4 to 1.3% d.m., depending on N fertilisation and preceding crop; consequently residues-N present at harvest represent from tens to more than 100 kg N ha^-1. Elsewhere, potential N mineralisation of residues is comparable to other crop residues and represent less than 30% of residues-N during the first year after incorporation. In fact, these potential decomposition values in field conditions will be affected by climatic conditions, non-limiting inorganic N conditions and contact between soil and residues. Nevertheless, to avoid N leaching during the between crop period following oilseed rape, it is necessary to catch the nitrate produced by the mineralisation of residues-N or soil organic N by favoring oilseed rape volunteers or growing a cover crop.

Acknowledgments

This work was supported by ADEME (Angers, France), CETIOM (Grignon, France) Europol’Agro (Reims, France) and INRA.

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