Impact of Management and Texture on Soil Organic Matter Fractions

Abstract

Growing concerns about elevated levels of atmospheric CO2 and associated climate change have increased interest in soil C. While general increases in the adoption of conservation management practices may result in C sequestration, efficient utilization of soil as a C sink will require identification of soils with high potential for sequestration and improved methods of monitoring soil C. The objectives of this research were to: 1) evaluate the historical roots, experimental validation and subsequent impact of the C saturation relationships proposed by Jan Hassink, 2) evaluate the effects of management and texture on aggregation and C fractions using soil from two long term experiments, 3) develop new methods of structural disruption and physical fractionation that address shortcomings in existing methods, 4) evaluate the impact of antecedent C on C and aggregate dynamics and 5) evaluate the simplified MnoxC method proposed by Weil et al. (2003). Collectively, the literature we reviewed did not support broad application of simple C saturation relationships such as those proposed by Hassink but did support selective use of fine mineral content as an indicator of C storage capacity. Results from two incubation experiments demonstrated the modulating effect of antecedent C on soil C and aggregate dynamics following structural disruption and residue addition. Positive effects of residue and structural disruption on aggregation were greatest in soil with low antecedent C. Residue decomposed more rapidly in soil with high antecedent C but had a greater priming effect in soil with low antecedent C. Addition of a 15N labeled nitrate source revealed that immobilization of nitrate-N within microaggregates is a minor process irrespective of structural disruption and antecedent C. Carbon contained in microaggregates within stable macroaggregates from an organic transition experiment was sensitive to C input regime but unrelated to fine mineral content. Strong tillage system effects on C fractions, aggregation and texture (tillage intensity↑ = ↓C, aggregate stability and sand content) were identified in soil from a long term tillage system study. Permanganate oxidizable C (Weil method) was found to be a sensitive indicator of management effects on soil C particularly after correction for non-linearity.