Legume-based catch crops for ecological intensification in organic farming

Summary

Overwintering legume-based catch crops (LBCCs) may play an important role in ecological intensification of agricultural production, especially in low-input systems like organic farming. The emerging concept of ecological intensification (or sustainable intensification) calls for new approaches to produce sufficient food with little environmental impact. Aimed at environment-friendly food production, organic farming may be a good candidate approach to meet the challenges of ecological intensification. Nitrogen (N) is a major constraint of crop yield in organic cropping systems due to restrictions of inorganic fertiliser input. LBCCs are capable to take up N from both the soil and the atmosphere (i.e., via biological N fixation (BNF)), and thus may act as an important source of N in crop rotations, enhancing crop yields but maybe also emissions of nitrous oxide (N2O, a potent greenhouse gas), in particular after soil incorporation of residues, which needs further evaluation.
The main focus of this thesis was to investigate the effects of contrasting catch crops (i.e., legume-based vs. non-legume-based) on N2O emissions and N availability to succeeding organic spring barley. Therefore, a one-year field plot study was performed to monitor the N2O fluxes and to investigate the N supply as influenced by three LBCCs compared with two non-LBCCs and a bare fallow control on a loamy sand soil in Denmark. Catch crops may be harvested before winter for other purposes, e.g., used as winter fodder or in anaerobic digestion, which may emphasise the importance of root residues in N supply. Hence, the consequences on N supply and N2O emissions of autumn harvest of catch crops were also compared with incorporation of whole-plants by spring tillage in this study. Along with the field study, an in-situ 15N labelling experiment in microplots was conducted to quantify the biological N fixation (BNF) in LBCCs and the recovery of the labelled residue N in spring barley with separate manipulation of top and root residues. Moreover, the period after residue incorporation is a “hot moment” for N2O emissions from the soil. In a laboratory incubation study, N2O evolution from simulated incorporation of catch crop residues was studied under controlled environment with addition of 15N-enriched nitrate. By this way, the contributions of denitrification and nitrification to N2O fluxes were estimated, which is necessary to better understand the underlying processes of N2O emission and to develop specific mitigation strategies.
Compared to non-LBCCs, LBCCs accumulated significantly more N, of which more than half was derived from BNF as shown by 15N dilution. The higher amounts of N accumulation in LBCCs increased the yield of the following spring barley markedly, which was negatively affected by autumn harvest of catch crops. The macro-roots at 0–18cm of the five catch crops tested accounted for 31–50 % of the total plant N, which was a substantial source of N and usually underestimated. The results of separate turnover of 15N-labelled tops and roots indicated that the root of LBCCs alone increased N supply of the following spring barley by ca. 20 kg N ha–1 compared with non-LBCCs or bare fallow, and a similar effect can be expected with the top residues of LBCCs.
With respect to the annual emission of N2O, it was comparable among all catch crop treatments, of which the highest emission was from fodder radish with 74–84 % of the annual emissions observed during winter in this treatment. In comparison with spring incorporation, autumn harvest of catch crops did not influence the annual emissions of N2O, but tended to induce greater emissions in winter and lower emissions after spring tillage. These results highlight the importance to monitor N2O fluxes over extended time periods to cover seasonal variations, in particular when including winter catch crops in a crop sequence. Although the 28-day laboratory study clearly showed a faster and greater release of N and N2O after incorporation of LBCC residues than that of perennial ryegrass, we did not observe greater annual emissions from LBCCs in the field. With addition of 15N-enriched nitrate, the incubation study revealed that denitrification was the dominant source of N2O after residue amendment at water-filled pore space of 40–60 %, and the emissions were affected by the interaction between residue quality and soil inorganic N status.
In summary, LBCCs have the potential to replace partly the effect of manure application in organic rotations to promote crop production without increased environmental impact in terms of N2O emissions, and thus contribute to ecological intensification in organic farming. The 15N-labelling study in microplots showed similar or even higher capacity of soil N extraction by LBCCs than non-LBCCs, indicating potential effectiveness on reduction of N leaching loss. However, direct evidence needs to be documented. Moreover, further studies of the complex underlying processes and controlling factors of N2O emissions after incorporation of organic residues are important. Researches on different soil types and over longer periods are also necessary before a wider adoption of using LBCCs. Species selection and management of catch crops would affect the N availability and N2O emissions, which also relies on smart decisions by farm managers to suit local conditions of soil, climate and resources.