Factors influencing the molecular composition of soil organic matter in New Zealand grasslands

Few studies have attempted to simultaneously link multiple environmental factors to the composition of soil organic matter (SOM). This knowledge gap challenges the ability to predict the response of SOM to environmental changes and make recommendations for effective land-based C sequestration strategies. In this study, topsoil samples from the New Zealand National Soils Archive, of different soil orders and obtained from different climatic regions, land-uses, and positions in the landscape, and taken at two sampling times (one from 1960s to early 1990s and the other after 2004) were analysed for SOM composition using Pyrolysis-GC/MS after being treated with HF to minimise the interference of the soil mineral fraction. Principal component analysis and redundancy analysis were used to screen out major environmental factors that impact on SOM composition. Ordination of SOM composition generally followed two gradients (canonical axes):decomposition and fresh input, and was significantly explained by the C to N ratio and total C (Monte Carlo permutation test, P < 0.01) with other factors, such as average annual temperature, average annual rainfall rate, total N and HF-removable C being somehow redundant with total C. Soil order, landscape type and land-use type further contributed to the explanation of the variability of SOM composition, although they were largely redundant with the above-mentioned factors. Overall, sampling time showed little effect on SOM composition. Given the large variation of SOM composition for different soils (e.g. Allophanic soils having a larger fraction of decomposed organic matter compared to other soil orders) and different mechanisms governing SOM stabilisation, effective C sequestration strategies should be case-specific for New Zealand grasslands.