Edaphic adaptation of gorse (Ulex europaeus L.)

Abstract

Aspects of the edaphic adaptation of gorse (Ulex europaeus L.) were investigated using field and glasshouse experiments. Variables compared included P and N supply, soil pH, frequency of cutting and stage of soil development. Particular emphasis was placed on soil chemical properties of most importance. to legumes growing on low fertility soils in New Zealand (available P arid acidity) and on the ability of gorse to fix nitrogen and thus thrive under conditions of low available soil N. Gorse was contrasted with the high soil fertility demanding pasture legume, white clover (Trifolium repens L.).

Under dryland conditions in Canterbury unfertilized gorse in its second growth season, grown on a soil with very low available P and cut frequently (twice yearly) had similar annual dry matter production (50S3 kg ha⁻¹) to pasture (5870 kg ha⁻¹) fertilized with 250 kg ha ⁻¹ year ⁻¹ superphosphate. Infrequently cut, unfertilized gorse or gorse which was P-fertilized (and either frequently or infrequently cut) produced more than 18500 kg dry matter ha ⁻¹. On a river terrace sequence in North Westland, established gorse grown on unfertilized soils with very low to low available P concentrations, and some of which had physical constraints to root growth, produced similar amounts of dry matter (7210 to 14 600 kg ha ⁻¹ year ⁻¹) as pasture (10920 kg ha ⁻¹ year ⁻¹) fertilized with 1000 kg ha ⁻¹ year⁻¹ of 33% potassic, cobaltised, lime reverted superphosphate (giving about 42 kg P ha⁻¹, 165 kg K ha⁻¹ and 56 kg S ha⁻¹).

Gorse gave dry matter responses to applied P, but was less responsive than white clover, both in the field (where the two species were grown together) and in pots (where the two species were grown separately). Gorse was able to take up more P than did white clover from soils with very low available P concentration both in pots and in the field.

The P response curves for gorse were quadratic in shape, suggesting a tendency for decline in yield at high rates of P, whereas those for white clover were exponential in shape with increases in dry matter yield diminishing at the greatest rates of P used but showing no tendency for decline.

Shoot P concentrations of field grown gorse were less than those of white clover at all rates of applied P (except for the nil rate at one sampling time) and the critical shoot P concentration for gorse (0.19%) appeared to be less than that for white clover (0.35%), indicating that gorse used P more efficiently in the processes of growth than white clover.

In the field infrequently cut gorse (at the end of 2 years growth) produced about twice as much dry matter as frequently cut gorse (cut twice yearly), but the total P contents of the two cutting treatments were similar, indicating similar capacities for P uptake. Critical P concentrations in young tissue were also similar for both cutting treatments. The ability of infrequently cut gorse to out-yield frequently cut gorse appeared to result from the greater potential for P transfer from old to young tissue in infrequently cut plants combined with their greater leaf area index. The P contained in harvested shoots was lost to the frequently cut plants, whereas infrequently cut plants appeared to make more efficient use of the P taken up by transferring it from old to new tissue.

Gorse grown in pots was less sensitive to soil acidity (0.02 mol 1⁻¹ CaCl₂-extractable Al in particular) than was white clover and was also less responsive to applied lime. Nitrogenfixing (acetylene reducing) activity did not appear to be any more sensitive to soil acidity than host plant growth, for either gorse or white clover.

Gorse did not have a noticeable soil acidifying effect when grown in pots, but white clover did.

Gorse grown in sand culture was responsive to increasing nutrient solution nitrate concentration within the range found in natural and agricultural soils (0-10 mmol 1⁻¹). In terms of dry matter production, gorse was not significantly less responsive to increasing nitrate concentration than white clover, but was less responsive than white clover in terms of total N content. Gorse reached 90% maximum dry matter yield at a lower solution nitrate concentration (1.2 mmol 1⁻¹) than white clover (2.9 mmol 1⁻¹). Unlike white clover, gorse growth and N accumulation were depressed at the greatest nutrient solution nitrate concentration used (20 mmol 1⁻¹) which is about the top of the range which can temporarily occur in highly fertilized soils. Similarly to white clover, gorse was able to use available mineral N by increasing its nitrate reductase activity of its roots and shoots.

N concentration of gorse tissue was less than that of white clover tissue at all solution nitrate concentrations and the critical N concentration for gorse (2.79%) was less than that for white clover (4.62%), indicating that gorse used N more efficiently in dry matter production than white clover did.

The symbiotic nitrogen-fixing system of gorse appeared to be able to meet the needs of the plant in the field where the application of 200 kg N ha⁻¹ gave no dry matter response. In sand culture the reduction in nodule weight and nitrogen-fixing activity with increasing nutrient solution nitrate concentration was less for gorse than white clover. This suggests that the N₂-fixing system of gorse may be able to recover more readily than that of white clover following depression by applied N.

Dry matter yield of established gorse on a river terrace soil sequence in North Westland was not clearly linked with soil fertility but appeared to be more affected by physical factors affecting rooting volume and rooting depth.

It was confirmed that gorse is relatively tolerant of low soil fertility conditions, hence its significance as a pest in low input pastoral agriculture (low fertilizer inputs and low stocking rates).

Under conditions of high soil fertility, it should be possible to control gorse at the seedling stage when its growth is slow relative to high fertility-demanding pasture species. However, when it is mature, gorse has the potential to grow very rapidly and responds to applications of P and N within the range normally given to pastures.

High forage yields of gorse compared with pasture are possible under conditions of low soil fertility. However, yields were greatest from uncut plants. Therefore, if gorse is used as forage it will be important to achieve an appropriate balance between browsing by animals and the maintenance of sufficient photosynthetic tissue to maintain rapid growth.