Factors controlling soil carbon levels in New Zealand grasslands: Is clay content important?

Soil organic matter is a major component of biogeochemical cycles and is important in maintaining soil quality. We investigated relationships between soil organic C and various soil and site properties that may influence long-term soil C accumulation across a range of soil orders in New Zealand. We used pedon and climatic data for 167 pedons under permanent grass, and carried out regression analysis between soil C (0-200 mm) contents (t ha(-1)) or concentrations (g kg(-1)) and climatic and soil properties, namely, precipitation, temperature, and contents or concentrations of sand, silt, clay, pyrophosphate-extractable Al (Al-py), Fe oxide, and allophane. Soil clay content or concentration explained little of the variation in soil C across all soils (R-2 < 0.05) and within each soil type. Likewise, mean annual precipitation and temperature explained little variation in soil C content or concentration (R-2 < 0.15 for precipitation, R-2 = 0.04 for temperature). Allophane content or concentration was unrelated to soil C in the soils of volcanic origin; Al-py, however, correlated strongly with both soil C content and soil C concentration across all soil types (R-2 = 0.55 and 0.60, respectively). When all factors were combined in a multiple regression analysis, the combination of Al-py and allophane contents explained the greatest amount of variation in soil C content (R-2 = 0.57), whereas the combination of Al-py, Fe oxide, allophane, and clay concentrations explained the greatest amount of variation in soil C concentration (R-2 = 0.67). Our results suggest that in New Zealand soils, chemical stabilization of organic matter is the keg process controlling soil C accumulation, and that clay content relates poorly to long-term soil organic C accumulation.