Decomposition and nitrogen release of understorey vegetation residues in biological and integrated apple orchards in Canterbury and Central Otago, New Zealand

In grassed-down biological and integrated fruit production (BFP and IFP) orchard treatments at Lincoln, Canterbury and Clyde, Central Otago, New Zealand, the decomposition and N release of orchard understorey residues were determined and compared across the treatments using the litterbag (100 x 100 mm, 1mm mesh size) method. At Lincoln, the experimental site was a randomised complete block with 3 orchard treatments (BFP1, BFP2 and IFP) each with 3 replicates, while at Clyde the experiment site consisted of non-replicated plots of IFP1, IFP2 and BFP orchard treatments. Litterbags of respective orchards were laid on the treelines of the orchards and retrieved every 90 days over a period of 360 - 450 days. In addition, understorey vegetation herbage dry matter (DM) production as returns to the orchard treelines, its species composition, soil fertility and soil biological properties were determined. Results obtained showed that the decomposition and N release of understorey residues followed the single exponential decay model, Y = A0 e⁻kt, (R² = 0.97 - 0.99), with decomposition and N release rate constants ranging from 6.0 x 10⁻³ - 9.6 X 10⁻³ day⁻¹ and 7.0 x 10⁻³ - 13.0 X 10⁻³ day⁻¹, respectively. The C and N half-lives of understorey residues ranged from approximately 70 - 120 days and 50 - 110 days, respectively. In general, the results showed that the residue decomposition and N release were significantly most rapid in IFP compared with BFP treatments (e.g. N release: 13.0 x 10⁻³ vs.7.0 x 10⁻³day⁻¹). In addition, the burying of residues in the soil significantly increased understorey residue decomposition and N release compared with surface placement (80% vs. 54 % in 90 days). At Clyde, differences in the rates of decomposition of understorey residues were related to the residue quality. Herbage DM return through frequent mowings was approximately 18.0 t ha⁻¹ yr⁻¹ and 7.0 – 10 t ha⁻¹ yr⁻¹ at Lincoln and Clyde, respectively, returning substantial amounts of N (503 - 572 kg N ha⁻¹ yr⁻¹ at Lincoln and 225 - 310 kg N ha⁻¹ yr⁻¹ at Clyde) back to the orchards. Herbage DM return and proportions of legume species increased in the spring summer season across all treatments. At Lincoln, in spring 1999 significantly higher amounts of soil total N and available Ca, K, Mg and P were found in treelines of BFP1 compared to BFP2. In general, soil microbial biomass C showed no significant differences between BFP and IFP treatments at both Lincoln and Clyde. At Lincoln, however, earthworm number was significantly higher in BFP1 and IFP compared with BFP2 treatment. Two incubation experiments were conducted concurrently in the laboratory at 25°C for 110 days. Experiment I was a randomised complete block design using 3 orchard understorey residues and 6 other residues (barley straw, pine bark, pea straw, spent mushroom compost, sawdust and clover residues) to determine the relationships between decomposition rates and residue quality parameters. Experiment II was a randomised complete block factorial design simulating orchard treeline surface effects of bare soil, pea straw and pine bark mulches on the rate of understorey residue decomposition. Results obtained showed that the significantly rapid decomposition of understorey residues compared to pea straw, barley straw, spent mushroom compost and pine bark was related to the high quality of orchard residues. The decomposition rate constant, k, was comparable to clover residues. In simulated treeline surfaces, the understorey residue decomposition was significantly most rapid on bare soil compared to mulched surfaces (k = 0.36 - 0.40 vs. 0.12 - 0.16 day⁻¹). The highest negative effect on the decomposition rates of understorey residues was shown by the pea straw mulch surface. It was concluded that substantial amounts of N were returned to the orchard soil annually through frequent mowing of the understorey vegetation. In general, differences observed in residue decomposition and N release rates between BFP and IFP treatments were largely due to treatment components related to treeline mulches at Lincoln and understorey residue quality at Clyde rather than the orchard system as a whole