Effects of grassland afforestation on soil nutrient dynamics and availability in New Zealand

Continued expansion of plantation forestry in New Zealand is expected to alter soil chemical, biochemical and biological properties which will have important implications for long-term sustainable land management. The major objectives of this study were to investigate the impacts of grassland afforestation on the dynamics of soil organic matter and associated phosphorus and sulphur with respect to chemical, biochemical and biological processes. This study involved a combination of field and glasshouse experiments. A paired-site study of a 19-year old mixed conifer forest stand (Pinus ponderosa, Pinus nigra) and adjacent unimproved grassland was conducted at Craigiebum, Waimakiriri catchment, Canterbury. It was found that concentrations of organic carbon, phosphorus and sulphur were significantly lower, but levels of labile inorganic phosphorus and sulphur were higher in topsoil under forest compared with grassland. These findings supported earlier research indicating that afforestation caused enhanced mineralization of organic matter and associated nutrients. On the other hand, concentrations of microbial biomass carbon, phosphorus and sulphur and microbial and enzyme activities were lower in soils under forest compared with grassland. It is possible that the apparent enhanced mineralization of organic matter and associated nutrients in soil under forest may be due to elevated levels of microbial and enzyme activity during the earlier stages of forest development. A seasonal study at the Craigieburn site further confirmed that afforestation enhanced mineralization of organic matter and organic phosphorus. Moreover, seasonal changes in soil moisture and temperature influenced the biological and biochemical processes involved in phosphorus recycling. Results showed that phosphorus recycling was primarily driven by plant demand and sustained mainly by inputs of root litter under grassland and needle litter under forest. Microbial biomass played a pivotal role in phosphorus recycling, and phosphorus turnover through the microbial biomass was higher under forest than under grassland. Results from a short-term glasshouse experiment using 15 different grassland soils showed that perennial ryegrass (Lolium perenne) and radiata pine (Pinus radiata) caused redistribution of inorganic phosphorus from slowly exchangeable to rapidly exchangeable pools. Once again, soil organic phosphorus mineralization was consistently greater under radiata pine compared with ryegrass. Higher mineralization was accompanied by higher concentrations of water soluble organic carbon, microbial biomass carbon and carbon dioxide respiration under pine. This supported the previous contention that enhanced mineralization of organic phosphorus occurred during the early stages of forest development. Enhanced mineralization of organic phosphorus under radiata pine may also be partly attributed to higher levels of root phosphatase activity. The importance of microbial and enzyme activities in soil organic phosphorus mineralization was confirmed by results from a detailed study of rhizosphere processes in two soils under ryegrass and radiata pine. The precise role of root exudates (particularly organic acids) in the mineralization of soil organic phosphorus under conifer warrants further investigation