Legumes add not only nitrogen (N), but also carbon (C) to soils, so their effects on the soil microbial community may be different from those of chemical fertiliser. Soil microbes often compete with plants for N when excess C is applied due to their increased N immobilisation potentials and denitrification. In the present study we evaluated the effects of the 9-year use of a green manure legume (hairy vetch; Vicia villosa) in a greenhouse tomato system on soil microbial community structures as well as on the decrease of nitrate when rice straw was incorporated into the soil. Soil microbial community structures and their diversity were altered by the long-term use of legumes. The ratios of Acidobacteria, Gemmatimonadetes and Proteobacteria increased in the hairy vetch soils. The rates of decrease in nitrate were similar in soils with a history of chemical fertiliser and hairy vetch, following the addition of rice straw. In addition, during incubation with added rice straw, the difference between the two soil microbial community structures became less clear within 2 weeks. Thus, we conclude that even though growing a green manure legume changed soil bacterial community structures, this did not result in relatively faster loss of available N for plants when rice straw was added to the soils.

IntroductionNitrogen is a key nutrient for plants. Often less than 50% of the applied nitrogen fertilisers is acquired by crops and nitrogen can be easily lost into the environment causing environmental pollution. Thus, to make agriculture more sustainable, it is important to investigate which factors determine nitrogen use efficiency (NUE). We investigated whether NUE was higher in organically managed soils compared to conventionally managed soils.Materials and MethodsTo test this, we carried out a pot experiment in a greenhouse using soils from 16 fields. The soils were collected from conventionally (eight fields) or organically managed fields (eight fields). In addition, plants received two different 15N enriched N sources (mineral 15N or an organic fertiliser source, namely 15N enriched plant litter). Plants were harvested at three time points, and growth and nitrogen uptake were assessed at each time point.ResultsNUE depended on management type and harvest time and the higher NUE of organically managed soils became more evident towards the second and third harvest. The average NUE at the end of the experiment was 93% and 55% for mineral fertiliser and litter application, respectively. This indicated that mineral fertilisers were immediately acquired by the plants, while nutrients in organic amendments had a lower availability and probably would be supplied later but steadier. Further, NUE was positively linked to microbial biomass, soil organic carbon content, and aggregate size, indicating that enhanced soil quality and soil health leads to a more efficient use of fertilisers.ConclusionOur results indicate that organic management and soil health promote a more efficient use of nutrients and contribute to a more sustainable agriculture.

In recent decades, the rate of milk production per unit land area and per cow has increased with the intensification of the dairy system. The possible environmental risks arising from nutrients surpluses, such as nitrogen (N), are often evaluated using the N balance approach. In Hokkaido, the biggest dairy farming area in Japan, many dairy farms have started introducing a new dairy farming system called the total mixed ration (TMR) and biogas system. Feed and manure are managed at a community scale in these systems while each farm focuses primarily on milking cows. Thus, calculating the N balance for this system is complicated. Therefore, this study aimed to evaluate the N surplus and use efficiency (NUE), focusing mainly on the community-based dairy farming system, as described above. We investigated twenty dairy farms comprising a TMR centre (TMR-based farms) and nineteen conventional dairy farms (conventional farms). The Hokkaido dairy farms had a smaller N surplus and higher NUE than farms in other countries. The whole farm N surplus and NUE ranged from −163 to 701 kg N ha−1 and from 20% to 171% with median values of 40.5 kg N ha−1 and 69.5%, respectively. One of the possible reasons for the smaller N surplus and higher NUE is a lower stocking rate (averaged 1.3 cows ha−1) on Hokkaido dairy farms. There were strong relationships between feed N and N surplus because the studied dairy farms depended on purchased feed. In the comparison between the TMR centre and conventional dairy farms, the milk production level per cow and stocking rate tended to increase, and variations between farms decreased on the TMR-based farms. Increasing the amount of home-grown feed with pasture management is essential to decreasing N surplus for the new dairy farming systems.