Abstract

One of the basic requirements for sustainable management of soils is to ensure that soil fertility is maintained in a productive state and conditions so as to enable the soil to continue to provide viable economic yields with minimum degradation of soil quality and quantity. The practice of supplying nitrogen to fruit trees from biological nitrogen fixation by pasture legumes in the understorey vegetation of orchards is a sustainable means of maintaining soil fertility.

Quantitative field measurements of amounts of biological nitrogen fixation and biomass production by three different kinds of understorey vegetation in an organic apple orchard in Canterbury New Zealand was conducted over a period of two years. Results obtained showed that biological nitrogen fixation varied from 118 to 126 kg N ha^-1 over the period of two years and herbage production varied from 8 to 12 t ha^-1. Nitrogen fixation was significantly correlated with clover dry matter production. Results were affected by seasons and understorey management practices. Implications of these results are discussed with respect to management strategies for enhancing biological nitrogen fixation in organic orchards.

Introduction

A wide range of orchard floor soil management systems are practised in different parts of the world depending on many factors such as soils, climate and accepted cultural practices. In New Zealand, traditionally, apple orchards were grown under clean cultivation but this practice has recently been largely superseded by grassing down as an understorey vegetation of orchards in their early life, often with the use of a narrow herbicide strip of variable widths along the row (Haynes 1980). In England and other parts of western Europe, fruit trees are usually grown in weed-free strips of bare soil, separated by grassed alleyways (White and Atkinson 1984). In recent times, there has been a move towards increasing the area sprayed with herbicide and in some instances, an overall herbicide system has been adopted (Johnson and Samuelson 1990).

An orchardist generally adopts a particular soil management practice for an orchard for two main reasons: to facilitate day-to-day operations and to enhance the growth and productivity of the tree crop. The adopted soil management system has considerable effect on tree performance and growth, fruit yield and fruit quality. This influence is through many complex interactions including effect on soil physical conditions, nutrient content and availability, soil moisture status and water availability.

The use of a permanent orchard floor vegetation is the most common orchard soil management system used (Hogue and Neilsen 1987). Some of the potential benefits of a plant cover include reduced soil erosion, improved soil structure, increased water infiltration, reduced surface runoff and reduced soil temperature fluctuations (Butler 1986; Hogue and Nielson 1987; Glenn and Welken 1989). Although the importance of including legumes in the understorey vegetation of orchards as a source of nitrogen has been recognised (Goh and Haynes 1983; Goh and Malakouti 1992), this contribution has not been quantified. Furthermore, the practice of supplying nitrogen (N) to fruit trees in organic orchards based on biological N fixation from legumes in the understorey vegetation is a sustainable means for maintaining soil fertility. The quantification of biological N fixation provides the opportunity to manipulate and apply agronomic practices for maximising biological N fixation.

The main objective of the present study is to measure in the field amounts of biologically fixed N and biomass production by three different kinds of understorey vegetation over a two-year period in an organic apple orchard in Canterbury, New Zealand.

Materials and Methods

The organic apple orchard used in the present study is situated in Canterbury near Ashburton at the Winchmore Research Station, New Zealand. The 4.0 ha orchard was established 8 years ago using conventional design and management. It was converted to organic management for 3 years and has been registered as a ‘Bio-gro’ production unit, designed to produce organic apples for export and the local markets. Full details of the orchard designs and layouts have been published elsewhere (Daly and Thomas 1992; McCarthy 1993).

In December 1989, three separate 1 ha orchard blocks were converted from turf grass (Lolium species) to three different understorey vegetation types; namely, red clover (Trifolium pratense), ryegrass (Lolium perenne) and mixed herb ley (prairie grass, Branus catharticus; timothy, Phleum pratense; red clover, Trifolium pratense; chicory, Cichorium intybus; sheeps burnet, Sanguisorba minor and sulla, Hedysarum coronarium). These large single blocks with different understorey herbage treatments were used for biological N fixation measurements over a two-year period.

The biological N experiment was conducted by dividing each 1 ha block into 2 equal blocks, thus giving a total of 6 replicates. A 3 x 2 x 6 split plot, randomised complete block design was used, consisting of 3 main plots (red clover, ryegrass, herb ley understorey vegetation), 2 subplots (herbage removed, herbage returned) and 6 replicates. Nitrogen fixation plots were sited at about 0.3 m on one side of the tree row. Each plot was rectangular area of 1.5163 m² (1.285 x 1.18 m) with a microplot of 0.2 m² (1 x 0.2 m) in the centre in which a 30% ¹⁵N-enriched ammonium sulphate solution was applied at the rate of 3.65 kg N ha^-1 year^-1. The boundary plot surrounding the microplot received the same amount of unlabelled ammonium sulphate solution.

Plots were harvested 5 times (winter, 1991 May-October; early spring, October-November; late spring, November-December; summer, December-1992 January; autumn, February-May) in year 1 (1991-92) and four times (winter and early spring, 1992 May-November; late spring, November-December; summer, December-1993 January; autumn, February-May) in year 2 (1992-93). For comparisons between years, data obtained for winter and early spring harvests of year 1 were combined as winter harvest. At each harvest, herbage was cut to approximately 5 cm above the ground using a manual or electric hand clipper.

Figure 1: Daily amounts of biological nitrogen fixed in different understory vegetation treatments at different periods of herbage sampling for (a) year 1 (1991-92) and (b) year 2 (1992-93)

Herbage samples for total N and ¹⁵N analyses were taken from a central strip (10 cm wide) of the microplot. These samples were dissected into grass and clover components, dried in the oven at 60°C for 48 h. The dry matter content was determined and each sample was ground through a Cyclotec mill to pass through a 0.5 mm sieve before analysis. Herbage from the remaining area of each rectangular plot was harvested and used for total dry matter yield (DMY) determination. The clover DMY was calculated from the proportion (%) of clover obtained in the microplot data and the total DMY.

Total N and ¹⁵N content of herbage were determined using a commercial continuous flow C-N analyser connected to an isotope ratio mass spectrometer (Grewal et al. 1991). The amount of biological N fixation was estimated using the ¹⁵N enrichment technique (Goh et al. 1978; Cookson et al. 1990; Hamilton et al. 1991).

Statistical analysis

Analysis of variance of the data obtained was conducted using the GENSTAT statistical package in accordance with the split plot randomised block design. The significance of the difference between means of treatments was determined using the Least Significance Difference (LSD) values at 5% probability (P<0.05).

Results

Amounts of biological nitrogen fixation

Over the two-year period, total amounts of biological N fixation varied from 118 to 126 kg N ha^-1 year^-1 (Table 1). Significantly (LSD P<0.05 = 30.9) less N was fixed in year 2 than in year 1. Differences between understorey vegetation types were not significant in year 1 but significant in year 2, when the herb ley treatment fixed considerably less N (about 62%) than other treatments. On the whole, herbage removal and return treatments showed no significant effect on N fixation except in the ryegrass treatment of year 1 (data not presented).

Table 1: Annual amounts of biologically-fixed nitrogen in different understorey vegetation treatments over a two-year period in an organic orchard at Winchmore, Canterbury, New Zealand.

Biomass production

Total dry matter yield (DMY) of herbage varied between 8 to 12 t/ha over the two-year period (Table 2). A reduction of between 25 to 40% of annual total DMY occurred in year 2 compared with year 1.

Table 2: Annual total and clover dry matter yield of different understorey vegetation treatments over a two-year period in an organic orchard at Winchmore, Canterbury, New Zealand.

Annual clover DMY accounted for between 10 to 79% of total DMY. It decreased significantly (18 to 32%) in year 2 compared with year 1. As expected, the red clover treatment produced the highest annual clover DMY but this treatment yielded the lowest annual total DMY compared with other treatments.

On the whole, herbage removal and return treatments showed no significant effect on biomass production (data not presented).

Relationships between nitrogen fixation and biomass production

Amounts of N fixed is strongly correlated with clover DMY (r = 0.44 to 0.98) at all seasons (Table 3). This is higher than the correlation obtained between N fixation and total DMY which occurred in all but two seasons (r = 0.20 to 0.57). These data suggest that clover DMY is a better indicator of amount of N fixed by the understorey vegetation than total DMY.

Table 3: Correlation coefficient (r) between biologically-fixed nitrogen and total dry matter yield (DMY) or clover DMY of understorey vegetation at different periods of harvest in an organic apple orchard at Winchmore, Canterbury, New Zealand.

Seasonal effects on biological nitrogen fixation and biomass production

Maximum biological N fixation occurred in early spring, between October and November (Figures 1a and 1b) for both years. In general, highest N fixation occurred in the red clover treatment in late spring (November-December) but this became the lowest in winter (July-August).

Biomass production showed similar seasonal patterns to that of N fixation (Figures 2a, 2b, 3a, 3b) although the trends were less consistent between different understorey treatments. Daily clover DMY was consistently highest in the red clover treatment than in other treatments at all seasons, except winter (Figures 2b, 3b).

Figure 2: Amounts of (a) daily dry matter yield (DMY) and (b) clover DMY of different understorey vegetation treatments at different sampling periods in year 1 (1991-92)

Discussion

Results obtained in the present study showed that significant amounts of N were fixed by the different understorey vegetation over a two-year period (118 to 126 kg N ha^-1 2 years¹) (Table 1). However, these differed considerably between years 1 and 2. The difference was as much as 98% in the herb ley treatment. These results suggest that biological nitrogen fixation can become insignificant with increasing age of the understorey vegetation.

The main reason for the reduction in N fixation in year 2 is due largely to the significant decline in the clover component of the understorey vegetation (Table 2), as N fixation was significantly correlated with clover DMY (Table 3). The largest decline occurred in the herb ley treatment in year 2 (Table 2) probably due to the vigour of the prairie grass and other herb species shading out the clover.

As expected, seasonal effects on N fixation and biomass production (Figures 1a, 2a, 2b, 3a, 3b) are largely due to increasing temperature in the early and late spring periods and low temperature in the winter. In addition, soil moisture may have an effect, as only the tree rows were irrigated. Clover tended to dominate during the spring period and this explains high N fixation occurring during this period.

Figure 3: Amounts of (a) daily dry matter yield (DMY) and (b) clover DMY of different understorey vegetation treatments at different sampling periods in year 2 (1992-93)

Conclusion

Present results show that biological N fixation in the understorey vegetation of an organic apple orchard can be an important source of N for apple trees.

Annual amounts of N fixed (12 to 105 kg N ha^-1 year^-1, Table 1) may be sufficient to replace N removed by the fruit crop and leaching losses, which accounted for about 58 kg N ha^-1 year^-1 for a mature conventional orchard in Canterbury, New Zealand (Goh and Haynes 1983). Losses of N in organic apple orchards have not been measured.

However, amounts of N fixed varied with the kind of understorey vegetation and its management. The major factor is the proportion of the clover component in the understorey vegetation. Thus management strategies for increasing N fixation should be aimed at encouraging clover growth such as regular defoliation, oversowing with clover, adjusting soil pH and adding phosphate, molybdenum and other fertilisers to favour the growth of clover. As biological N fixation provides not only a relatively cheap but also a renewable source of N to fruit trees, this source of N is a sustainable means for maintaining soil fertility.

Acknowledgements

We wish to thank Lesley Hunt, AgResearch, New Zealand Pastoral Agriculture Research Institute Ltd, Lincoln and Dr J.R. Sedcole, Lincoln University, for providing assistance in statistical analysis and Neil Smith, Roger Cresswell and Anne Jordan for technical assistance and AgResearch, Winchmore, for providing the climatic data.

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