Decomposition and nitrogen release of understorey plant residues in biological and integrated apple orchards under field conditions in New Zealand

In grassed-down apple orchards in New Zealand, the understorey vegetation is usually mown and the plant residues are returned to the orchard floors as a source of nutrients. It is, therefore, important to determine the decomposition pattern and the rate of N release from understorey plant residues. In this study, the decomposition and N release of surface-placed understorey plant residues were determined in the field and compared across treatments of grassed-down biological (BFP) and integrated fruit production (IFP) orchards in two different locations (Lincoln and Clyde) in New Zealand using the litterbag technique. At Lincoln, the field experiment was a randomised complete block design with three different treatments (two BFPs, one IFP) each with three replicates; while at Clyde, the field experiment consisted of non-replicated apple orchard plots with three treatments (two IFPs, one BFP). A comparison was also made between surface-placed and soil-buried understorey plant residues in a BFP orchard at one location. Samples of understorey plant residues collected from orchard mowings in the respective treatments were returned to the same treatment plot in litterbags and retrieved at intervals of 90 days for 360-450 days. Results showed that the single exponential decay model, Y=A₀ e^-kt, accounted significantly (PА.001) for 97-99% of the variation in the decomposition and N release patterns, which ranged from 6.0᎒^-3-9.6᎒^-3 day^-1 and 7.0᎒^-3-13.0᎒^-3 day^-1, respectively. Half-lives for C and N of residues were approximately 70-120 days and 50-110 days, respectively. Soil-buried plant residues showed more rapid decomposition and N release compared with those of surface-placed plant residues (80% vs. 54% in 90 days). In general, plant residue decomposition and N release were significantly more rapid in IFP than in BFP treatments (13.0᎒^-3 vs 7.0᎒^-3 day^-1 for N release). Overall, differences in plant residue decomposition and N release rates related to understorey plant residue quality and treeline management practices rather than the orchard system as a whole.