Nitrogen uptake dynamics and biological nitrogen fixation in a silvopastoral system.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Forestry Science
Two sets of field experiment were conducted at the Lincoln University's agroforestry trial. The first experiment was to study nitrogen (N) uptake by radiata pine and pasture, and soil total N changes with time. The second experiment was to assess the magnitude of input from biological nitrogen fixation (BNF) and factors affecting BNF. Lucerne was found to be the most severe competitor with trees. It reduced tree height, root collar diameter and diameter at breast height, and occasionally reduced fascicle dry weight and foliar N content. However, lucerne had a high dry matter yield (DMY), nitrogen concentrations, nitrogen yields, and amounts of nitrogen fixed. It had lower percentage of N derived from atmosphere (%Ndfa) than clover which resulted in a high N removal from the lucerne plot, when the herbage was removed as silage. Clover has high %Ndfa during spring and summer ranging from 83 to 97%. Radiata pine did not affect total N concentration of pastures and %Ndfa of the legumes. However, radiata pine reduced seasonal DMY of the pastures and seasonal and annual DMY of legumes, which led to the reduction of N yield and amount of N fixed. Clovers in ryegrass/clover, cocksfoot/clover and phalaris/clover were estimated to fix 134,71, and 75 kg N ha ⁻¹ year ⁻¹ which were lower than lucerne which was estimated to fix 230 kg N ha ⁻¹ year ⁻¹. The variations of amounts of N fixed by clover in different grass/clover mixtures were due to the persistence and productivity of the clover in pasture mixtures. Nitrogen balance in all pasture treatments was negative showing that N removal in herbage exceeded N input from BNF. Similarly, the total N in the soil decreased with time. Biological nitrogen fixation was important to stabilise N balance in pasture by minimising soil N removal and to ensure a high pasture productivity. Soil moisture and N were likely to be the important resources competed for by pasture plants and the trees. However, the effect of competition was more apparent on altering N status of the trees than that of the pastures. The N status of radiata pine grown with pastures was occasionally marginal. Soil moisture content close to the row of trees was lower than that at the midway between two rows of trees. Rain shadow effect from trees further lowered the moisture content of soil to the north side of trees. Overall the use of ¹⁵N isotope dilution technique for measuring %Ndfa and percentage of grass N derived from transfer (%Ndftrans) has given satisfactory results. Nitrogen transfer from clover to ryegrass/clover was considered as insignificant (<1.5 g m ⁻² annually). The atom % ¹⁵N enrichment in the soil decreased with time.