Organic Resource Quality and Soil Texture Interact in Influencing Short-Term Aggregate C and N Dynamics.

Aggregation and stabilization of soil organic C (SOC) and N are highly dependent on soil texture, and the addition and quality of organic resources (ORs). A mecocosm study was conducted on a clayey soil and a sandy soil in central Kenya to determine the influence of soil texture, OR quality and N-fertilizer on aggregation and SOC and N. Four Mg C ha-1 Tithonia diversifolia (high quality), Calliandra calothyrsus (medium quality) and Zea mays (maize; low quality) labeled with 14N or 15N were applied to soil compared to no input control. Each treatment was fertilized with 120 kg 14N or 15N ha-1 as (NH2)2CO, or not fertilized. Soil samples were collected at installation and after 8 months. Soils were separated into different aggregate fractions and analyzed for SOC and N. On average, 20% and 70% of SOC and N was in the macroaggregates in the sandy and clayey soils, respectively. While Tithonia resulted in the greatest concentrations of SOC and N in the sandy soil, in the clayey soil, there were no differences among OR qualities but all ORs resulted in greater SOC and N than the control. However, proportions of OR derived N in the macroaggregates, mostly in the microaggregates-within-macroaggregates was greater with sole applied maize residues in the clayey soil. When N-fertilizer was applied with maize, soil N and macroaggregate N and OR derived N were greater than when maize was applied alone. In the sandy soil, Calliandra, which had greater polyphenol concentration, resulted in greater OR derived N than Tithonia (i.e., 5% compared to 2% of N applied) in the coarse particulate organic matter. Thus, preservation of organic residue derived N is affected by the chemical recalcitrance of the residues in sandy soil, whereas macroaggregate protection, and not OR quality, is the major factor in clayey soils.