Soil carbon and nitrogen cycling following afforestation with mixed-species tree plantings.

Mixed-species restoration tree plantings are being implemented increasingly throughout the world as they may contribute to mitigate climate change, as well as providing other ecological benefits such as increasing biodiversity. However, the response of soil C after afforestation remains unclear. In order to assess the soil’s C sequestration potential of afforested pastures, we need to understand the factors and processes driving C sequestration. This thesis-by-publication consists of four data-chapters, contributing to answering two main questions that are important when assessing the C sequestration potential of afforested pastures: 1) Can including nitrogen(N)-fixing trees species in plantings increase C sequestration? and 2) How will a drying climate affect the relative C sequestration potential of mixed-species plantings and pastures? A meta-analysis presented in Chapter 2 showed no substantial changes in soil C or N across three decades after afforestation of pastures, in the Mediterranean climate of Australia. However, C stocks under remnant woodlands were significantly higher compared with C stocks in afforested pasture land, suggesting that afforestation may have the potential to increase soil C over longer time scales. The selection of tree species is an important aspect when designing tree plantings and may help increase soil C sequestration rates. Soil C and N contents were assessed under N-fixers and non-N-fixers. Overall, there were higher levels of soil C and N under N-fixing trees compared with non-N-fixing trees. At the individual planting scale, the patterns were less clear with both large increases and decreases occurring across the range of sites. The results indicate that the inclusion of N-fixers may help to increase soil C and N, but that the response may be site- and tree species-specific. Differences between N-fixing tree species were confirmed in Chapter 4. Two N-fixers were found to be substantially different in terms of C and N addition to the soil, as well as microbial community composition beneath them. There were also indications that fixed atmospheric N was utilized by the non-N-fixing trees, most likely through tight root connections, as opposed to via the decomposition of N-fixers litter. This indicates that even in dry environments, where litter decomposition is slow, the inclusion of N-fixing tree species can be beneficial in the early development of a tree planting. Carbon cycling in dry environments are largely driven by wetting and drying cycles of the soil. As precipitation frequencies are predicted to decrease, it is important to understand the response of soil C and N dynamics to different frequencies of wetting and drying cycles. In Chapter 5, it was found that while the concentration of soil C was similar in pasture and tree planting soils, respiration was significantly lower in pasture soil. Although there was little difference in the composition of the soil microbial community in any of the soils or wetting treatments, differences in the levels of potentially mineralized N (PMN) may indicate a difference in microbial activity. Cumulative CO2 emission was significantly lower in the reduced wetting treatment compared with the historical wetting treatment. The size of the reduction was the same for both land uses, indicating that land use change did not affect the response of soil to a reduction in wetting frequency. Finally, the findings in this thesis were discussed in the context of the two questions posed above and recommendations for future research were given