Labile Fractions of Soil Organic Matter as Indices of Soil Change Associated with Management Practices.

Many studies have shown that adoption of conservation practices (e.g. no-till, cover crop, manure) in cropping systems generally results in increasing levels of soil organic matter (SOM) with associated additional benefits such as increased fertility, aggregation, and water holding capacity. The most common measurements used to assess changes in SOM due to changes in management are total soil organic carbon (SOC) and total soil organic nitrogen (SON) contents. However, detectable changes in levels of total SOC and SON due to changes in management practices are usually restricted to at least 5 to 7 years of continuous adoption of such practices. This happens because several years are needed for transferring the C and N left on the soil surface as crop residue or manure application into stable fractions of SOM and, hence, to observe differences in total SOC and SON levels. To overcome this usual lack of differences in the short-term, the use of labile (active) fractions of SOM has been proposed as more sensitive indices of changes in management practices. The objective of this work was to evaluate particulate organic matter (POM) and Illinois soil nitrogen test (ISNT) as sensitive indices of changes in C and N in soil due to a cropping system in Valley region of Virginia. The experiment was a Split-Split-Plot design with crop rotation as whole-plot treatment factor, tillage as sub-plot treatment factor, and cover crop management as sub-sub-plot treatment factor. POM and ISNT were sensitive to changes in soil due to management practices while total SOC and SON were not. Between the sensitive fractions, C-to-N ratio of POM was the most sensitive while ISNT was the less sensitive. For all sensitive fractions the crop rotation by tillage by cover crop management interaction was significant with cover crop management being the most influential treatment factor.