Influence of Land Management and Soil Type on Soil C Pool Turnover.

Soil organic matter is a key indicator for soil quality, both for agricultural functions (i.e. production and economy) and for environmental functions (e.g., C sequestration and air quality). Soil organic matter helps stabilize soil particles, thus decreasing erosion, improves soil structure and workability, enhances aeration and water penetration, increases water-holding capacity, and stores and supplies nutrients for growth of both plants and soil micro-organisms As a result, lower nutrient inputs are needed to sustain productivity. In addition, an increase in soil organic matter levels can help reduce atmospheric CO2 levels that contribute to climate change by sequestering otherwise labile C in the soil. Future sustained productivity and environmental quality will require a better understanding of how various agricultural management practices affect carbon dynamics, soil carbon content and various soil biological functions.

Our research shows that, when considering only total quantities, most of the SOC stocks can indeed be found in this top layer, irrespective of soil type. However, not all forms of SOC are equally valuable as long-term, stable stores of carbon. SOC associated with small soil aggregates and the clay fraction is more stable and therefore represents a long-term carbon store. Our research shows that most of this stable carbon is located at depths below 30 cm (42% of subsoil SOC is located in microaggregates and silt and clay, compared to 16% in the topsoil), specifically in soils that are subject to clay illuviation.

Similarly land-use and land management had a large influence on the distribution of C between different aggregate sizes with a larger proportion of microaggregates (both free and macro-aggregate –associated microaggregates, while for minimum tillage and cover crop systems were observed to have a larger proportion of macroagggregate C compared to inversion tilled systems or systems with no winter cover. Long-term isotope studies indicated that macroaggregate C had a residence time of 4±7 years, microaggregate C 47±21 years and silt/clay 139±65 years. Soil type, land management and land use can all influence not only bulk soil C but the quality of C sequestered.