Effects of litter and woody debris quality on decomposition and nutrient release in exotic forests in New Zealand

Short rotation plantation forestry based on exotic tree species (principally radiata pine (Pinus radiata)) is a major land use in New Zealand (1.7 million ha, 7% of total land area). The maintenance of primary production in such a plantation forest ecosystem depends upon a number of factors including replenishment of available nutrients removal in harvested biomass. Litterfall is a major pathway of nutrient return to the forest floor. In addition, understorey biomass and woody debris from thinning and pruning also contribute to nutrient and organic matter cycling in a managed forest. Needle litter, understorey litter and woody debris undergo decomposition to release nutrients and thereby maintain the integrity of the forest ecosystem. Substrate quality is an important factor that regulates the decomposition of litter and woody debris materials. Decomposition and associated nutrient release from selected above ground litter and woody debris components from radiata pine forests were investigated using a combination of controlled environment (microcosm) and field experiments. The principle objective of the study was to determine the influence of litter and woody debris quality on decomposition and associated nutrient release. The impact of green needle litter on senescent needle litter decomposition and nutrient turnover in the forest floor was investigated under controlled conditions (microcosm) and in the field. The results obtained indicated that green needle addition did not significantly affect organic matter and nutrient turnover. However, addition of green needles did affect short-term nutrient availability. Green needles lost 69,87,75,72, and 79 % of the original nitrogen, phosphorus, calcium, magnesium, and potassium, respectively after 10 months, while the corresponding data for senescent needles was 22, 71,31,4, and 38 %, respectively. The effect of selected understorey litters on decomposition and nutrient release from senescent pine needles was investigated in a microcosm experiment. After 10 months the weight loss of senescent pine needles (64%) was significantly greater than bracken (Pteridium aquilinum-26%), gorse (Ulex europaeus-30%), broom (Cytisus scoparius-30%) and lotus (Lotus pedunculatus-40%) litter. Net release of all nutrients (nitrogen, phosphorus, calcium, magnesium and potassium) occurred from decomposing senescent pine needles, although net immobilization of potassium was observed for all understorey litters. Mixing pine and understorey litter significantly influenced decomposition rate and associated nutrient release. Pine needles mixed with understorey litter decomposed slower than needles alone, but mixed understorey litter (except bracken) decomposed faster than understorey alone. Nutrient availability (especially magnesium and potassium) was found to be as important as energy availability in determining the overall rate of decomposition. A laboratory study on the biodegradability of dissolved organic carbon from radiata pine litter indicated that this was influenced by its quality as determined by the stage of decomposition. The aqueous extract of the litter (L) layer decomposed faster (38%) than that of the fermented/humus (FH) layer (15%) during 3 months of incubation. The inhibitory effects of polyphenols on the biodegradability of dissolved organic carbon was confirmed in this study. The ¹³C NMR technique was used to describe the DOC degradation. The NMR results complemented the results obtained through the proximate analyses and it gave better insights in to the pathways of Land FH DOC breakdown. A chronosequence study of woody debris decomposition over 13 years showed that the overall half- life was 14 years and decomposition of different components was clearly related to quality. Thus, log wood contained greater concentrations of carbohydrates (soluble and holocelluose) and decomposed faster than log bark and side branches. ¹³C NMR technique was used to study the decomposition of wood, bark and sidebranches over 13 years. The NMR spectra confirmed the results obtained through proximate analysis and proved to be an useful technique in understanding the process of litter decomposition. The slower decomposition rate of log bark was attributed to the higher polyphenol concentration, while higher lignin concentration and an unfavorable microclimate were the main reasons for slow decomposition of side branches. The mass and volume of woody debris was much lower than that of natural conifer forests due to the young age of woody debris (thinning slash) and net mineralization of nutrients from the decomposing woody debris was apparent during decomposition. Even after 13 years, woody debris contained considerable amounts of carbon (5.5 t ha⁻¹) and nutrients (41, 2,41,8,3 kg ha⁻¹ of nitrogen, phosphorus, calcium, magnesium and potassium, respectively). Therefore, management of woody debris (thinning residues) may be important for the long-term sustainability of forest soils.