Factors Controlling Soil Carbon Levels in New Zealand Grasslands Is Clay Content Important?

Abstract

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, 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 ( , 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 , whereas the combination of Al py , Fe oxide, allophane, and clay concentrations explained the greatest amount of variation in soil C concentration Our results suggest that in New Zealand soils, chemical stabilization of organic matter is the key process controlling soil C accumulation, and that clay content relates poorly to long‐term soil organic C accumulation.